Skip Navigation


Cardiovascular Research Advance Access first published online on December 12, 2007
This version [Corrected Proof] published online on January 17, 2008
Cardiovascular Research, doi:10.1093/cvr/cvm098
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
77/4/619    most recent
cvm098v2
cvm098v1
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Davis, J. P.
Right arrow Articles by Tikunova, S. B.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Davis, J. P.
Right arrow Articles by Tikunova, S. B.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2007. For permissions please email: journals.permissions@oxfordjournals.org

Ca2+ exchange with troponin C and cardiac muscle dynamics

Jonathan P. Davis* and Svetlana B. Tikunova

Department of Physiology and Cell Biology, The Ohio State University, 1645 Neil Avenue, 400 Hamilton Hall, Columbus, OH 43210, USA

* Corresponding author. Tel: +1 614 247 2559; fax: +1 614 292 4888. E-mail address: davis.812{at}osu.edu

Received 1 August 2007; revised 2 November 2007; accepted 4 December 2007

Time for primary review: 41 days


   Abstract
Top
 Abstract
1. Introduction
 2. Troponin C structure...
 3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
 5. Conclusions
 Funding
 References
 
Controversy abounds in the cardiac muscle literature over the rate-limiting steps of cardiac muscle contraction and relaxation. However, the idea of a single biochemical mechanism being the all-inclusive rate-limiting step for cardiac muscle contraction and relaxation may be oversimplified. There is ample evidence that Ca2+ concentration and dynamics, intrinsic cross-bridge properties, and even troponin C (TnC) Ca2+ binding and dissociation can all modulate the mechanical events of cardiac muscle contraction and relaxation. However, TnC has generally been thought to play no role in influencing cardiac muscle dynamics due to the idea that Ca2+ exchange with TnC is very rapid. This definitely is the case for isolated TnC, but not for the more sophisticated biochemical systems of reconstituted thin filaments and myofibrils. This review will discuss the biochemical influences on Ca2+ exchange with TnC and their physiological implications.

KEYWORDS Troponin; Cardiac; Muscle; Myosin; Calcium


   1. Introduction
Top
 Abstract
 1. Introduction
2. Troponin C structure...
 3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
 5. Conclusions
 Funding
 References
 
Discovered by Ebashi over 40 years ago (for review1), troponin is a hetero-trimeric complex consisting of three subunits: TnC (troponin C, the Ca2+-binding subunit), TnI (the inhibitory subunit), and TnT [the tropomyosin (Tm)-binding subunit] (for review2). Multiple protein–protein interactions between TnI, TnT, actin, and Tm anchor the Tn complex into the thin filament (for review2). Ca2+ binding to TnC leads to a series of conformational rearrangements in Tn and Tm, which reveals strong myosin binding sites on actin, allowing cross-bridges to cycle, and muscle contraction. Relaxation occurs as intracellular Ca2+ declines, Ca2+ dissociates from TnC, Tm blocks strong myosin-binding sites on actin, and cross-bridges detach (for review3,4).

Although Ca2+ must bind to TnC to allow contraction, and dissociate from TnC to allow relaxation, the contribution of TnC to the kinetics of contraction and relaxation is controversial. The heart works to meet the metabolic demands of the body. Since the body’s demands can fluctuate widely from rest to intense exercise, the heart has the capability to dramatically alter not only its contractility, but also the speed at which the heart contracts and relaxes. In addition, different species possess different ranges of working heart rates, leading to vastly different kinetics of contraction and relaxation across the animal kingdom. A recent study suggests that nature has modified Tn in different species to help tune the kinetic performance of the heart.5 Unlike skeletal muscle which can recruit different fibre types to alter speed and force, the heart relies on the ability of the molecular machinery within each heart muscle cell to keep pace. These cellular processes can become impaired in many forms of heart disease.6–8 Thus, it is essential to understand the fundamental elements that contribute to the rates of contraction and relaxation so that better treatments and preventions can be devised for systolic and diastolic dysfunction, of which TnC is a prime target.9


   2. Troponin C structure and function
Top
 Abstract
 1. Introduction
 2. Troponin C structure...
3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
 5. Conclusions
 Funding
 References
 
TnC is a member of the EF-hand super-family of Ca2+-binding proteins and consists of N- and C-terminal globular domains connected by a central {alpha}-helix. Each domain contains a pair of helix–loop–helix EF-hand metal-binding motifs (for review2,10). Although all four EF-hands of skeletal TnC are capable of binding Ca2+ ions, the first EF-hand of cardiac TnC is apparently unable to bind Ca2+ due to several loop residue substitutions.11 As a result, the second EF-hand is the only functional Ca2+-binding site in the N-terminal domain of TnC. It is generally accepted that the second EF-hand of TnC regulates muscle contraction, whereas the third and fourth EF-hands play a structural role by anchoring TnC into the Tn complex. Although these are generally accepted paradigms for the separate function of the two domains of TnC, the C-terminal domain of TnC may play more than just a structural role in cardiac muscle contraction/relaxation.12

Many studies have focused on elucidating the interactions between TnC and TnI/TnT. Earlier studies determined that TnC and TnI interact in anti-parallel orientation, with the N-terminal domain of TnC interacting with the C-terminal domain of TnI.13 The recently determined crystal structure of the core domain of the Tn complex provides a deeper insight into the interactions between TnC and TnI/TnT.14 The core domain of the Tn complex can be divided into two subdomains, the regulatory head and the IT arm (Figure 1), connected by a flexible linker. The regulatory head consists of the N-terminal domain of TnC and the switch region of TnI. The switch region is situated between the two putative actin-binding segments of TnI and forms multiple van der Waals interactions with the hydrophobic patch on the surface of the Ca2+-bound N-terminal domain of TnC. According to the ‘drag and release’ mechanism proposed for the regulation of muscle contraction and relaxation (for review15,16), Ca2+ induces the movement of the switch region towards the hydrophobic patch on the N-terminal domain of TnC, dragging along the adjacent actin-binding segments of TnI, causing their release from actin and ultimately force generation. Eventually, a decline in intracellular [Ca2+] would lead to the dissociation of the switch region of TnI from the hydrophobic patch on the N-terminal domain of TnC, resulting in muscle relaxation (for review15,16).


Figure 1
View larger version (37K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 1 Crystal structure of the troponin core domain. The figure shows the crystal structure of the core domain of the human cardiac troponin complex (PDB file 1J1E14). TnC, TnI, and TnT are coloured in grey, yellow, and green, respectively, with Ca2+ coloured in black. Brackets indicate the regulatory head and IT arm. The figure was generated using Rasmol.88

 
Numerous studies have also focused on elucidating the factors controlling metal- and protein-binding properties of TnC in biochemical systems of increasing complexity (Figure 2). A lot of important information on the conformational changes occurring in TnC upon Ca2+ or ligand binding has been obtained from spectroscopic studies such as circular dichroism, NMR, and X-ray crystallography.14,17–19 In addition, researchers have utilized either intrinsic (primarily utilized only for isolated TnC20,21) or extrinsic fluorescent probes as tools to obtain a more complete understanding of how TnC activates and deactivates the thin filament in a Ca2+-dependent manner.22–26


Figure 2
View larger version (20K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 2 Systems of increasing complexity utilized to study Ca2+ binding and exchange with troponin C.

 
It is clear that there are many biochemical, physiological, and pathophysiological factors that can influence the Ca2+ sensitivity of force development and specifically of the Tn complex (for review3,8,16,27). For instance, TnI is phosphorylated during β-adrenergic stimulation.28 The activation of protein kinase A increases the phosphorylation state of TnI (and other EC-coupling and sarcomeric proteins) and has been associated with accelerated relaxation of the heart (for review16,27). It is also becoming clear that activation of another family of kinases, protein kinase C, can either enhance or decrease the Ca2+ sensitivity of force development through differential phosphorylation of a number of Ser and Thr residues on TnI.29,30 In addition, a host of assorted naturally occurring Tn subunit mutations linked to various cardiomyopathies can enhance or decrease the Ca2+ sensitivity of the thin filament through currently unknown mechanisms that ultimately alter cardiac performance and morphology.22,23


   3. Measurements of Ca2+ exchange kinetics with troponin C
Top
 Abstract
 1. Introduction
 2. Troponin C structure...
 3. Measurements of Ca2+...
4. Rate-limiting step(s) of...
 5. Conclusions
 Funding
 References
 
It has been generally assumed that rates of Ca2+ exchange with TnC in muscle are more rapid than the kinetics of contraction and relaxation. Indeed, the rates of Ca2+ dissociation from the second EF-hand of TnC, measured in the stopped-flow apparatus utilizing the fluorescence of IAANS attached to TnC on Cys84 or Trp fluorescence of TnCF27W, were at least an order of magnitude faster than the mechanical event of relaxation.21,31 The very rapid rate of Ca2+ dissociation from the second EF-hand of TnC was also measured directly following changes in the spectroscopically sensitive Ca2+ chelators Quin-2 or Bapta (700–800 s–1 at 4°C32) and utilizing NMR spectroscopy (5000 s–1 at 30°C33). In addition, the rate of Ca2+ association to the second EF-hand of TnC, measured by the stopped-flow technique, was also reported to be extremely rapid, yet not diffusion controlled, contrary to dogma.21,31 However, the rates of Ca2+ exchange with TnC in muscle are unlikely to be the same as with isolated TnC, since TnC functions not in isolation but as a part of the thin filament-bound Tn complex.

The Ca2+-binding properties of TnC are drastically modified upon incorporation into the Tn complex. In fact, binding of TnI/TnT increases the Ca2+ affinity of the second EF-hand of TnC by at least an order of magnitude.34 This increase in the Ca2+ affinity is largely due to a decreased rate of Ca2+ dissociation, which was measured in a number of studies by the stopped-flow technique utilizing fluorescently labelled TnC (for review35). Also measured following the fluorescence of Quin-2 in a stopped-flow apparatus, the rate of Ca2+ dissociation from the second EF-hand decreases from >1000 s–1 in isolated TnC21 to ~120 s–1 upon binding of TnI, and further to ~35 s–1 upon binding of TnT at 15°C.24 Incorporation of the Tn complex into the reconstituted thin filament (actin–Tm–Tn) desensitizes the regulatory N-terminal domain of TnC to Ca2+(see Kobayashi and Solaro23 and Davis et al.24) and accelerates the rate of Ca2+ dissociation to ~105 s–1 (see Davis et al.24) (Figure 3A). Consistent with the effects of myosin binding to actin and sensitizing the thin filament to Ca2+ in cardiac muscle,36,37 addition of myosin S1 reverses the thin filament-induced desensitization, even though myosin does not directly bind to Tn. Addition of myosin S1 to the thin filament also slows Ca2+ dissociation from the second EF-hand of TnC to 13 s–1, 24 a rate comparable with that of cardiac muscle relaxation (5–11 s–1 at 15°C)38–40 (Figure 3B). Recently, the rate of Ca2+ dissociation from the regulatory domain of fluorescently labelled TnC in isolated myofibrils was measured using stopped-flow technology in the presence of ATP to activate cycling cross-bridges (although the strongly bound states under these condition of no load may be very short lived) at 20oC, which occurred at ~39 s–1.41 Since the rate of Ca2+ dissociation has not been measured in intact cardiac muscle, it may be that the reconstituted thin filament system and possibly the isolated myofibrils are the minimal biochemical systems required to adequately address the physiological rate of Ca2+ dissociation from TnC.


Figure 3
View larger version (24K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 3 Effects of actin–tropomyosin and myosin S1 on Ca2+ dissociation from troponin. (A) Incorporation of troponin into the thin filament (actin–Tm) increases the rate of Ca2+ dissociation from ~42 to ~105 s–1. (B) Addition of myosin S1 to the reconstituted thin filament (actin–Tm–Tn) slows the rate of Ca2+ dissociation to ~13 s–1. Monocysteine troponin C was labelled with IAANS on engineered Cys 53. Data were adapted from Davis et al.24

 
The mechanisms behind the changes in the Ca2+ sensitivity and exchange rates with the second EF-hand of TnC occurring upon incorporation of Tn into the thin filament are unclear. The ‘drag and release’ mechanism proposed for the regulation of muscle contraction implies that TnC and actin compete for the binding of TnI. The competition between TnC and actin for TnI might be the reason behind the decrease in the Ca2+ sensitivity and acceleration of the Ca2+ dissociation rate occurring upon Tn reconstitution into the thin filament.24 The molecular mechanisms behind the myosin S1 effect on the Ca2+ sensitivity and dissociation from the second EF-hand are also unclear. However, myosin binding to actin displaces Tm on actin further than does Ca2+ alone (for review4) and increases the affinity of Tm for actin.42 The additional movement of Tm on actin might weaken TnI binding to actin and ultimately increase the probability of TnI binding to the TnC. Increased TnC–TnI interactions could be the mechanism by which myosin binding to actin increases the Ca2+ sensitivity and decreases the Ca2+ dissociation rate from the second EF-hand of TnC.

The affinity of TnC for Ca2+ can vary depending on the species of origin. For instance, the affinity of trout TnC for Ca2+ is increased compared with that of mammalian TnC, due to the several residue substitutions in the regulatory N-terminal domain.43 Pharmacological compounds, such as bepridil, have been shown to affect the rate of Ca2+ dissociation from the second EF-hand of TnC19,44 and modify its Ca2+-binding sensitivity.45 However, the effects of bepridil or species-dependent mutations on TnC have not been measured in the more sophisticated biochemical systems. All the major thin filament proteins (TnI, TnT, Tm, and actin) can directly or indirectly affect the Ca2+ sensitivity and the kinetics of Ca2+ exchange with TnC.24 A number of studies have demonstrated that Tn subunit isoform switching, phosphorylation of TnI, and mutations in Tn subunits linked to cardiomyopathies can sensitize or desensitize TnC to Ca2+ and/or affect the rates of Ca2+ exchange with TnC. For instance, slow skeletal TnI (ssTnI; expressed in embryonic and early postnatal hearts and possibly re-expressed during disease46,47) increases the Ca2+ affinity and decreases the rate of Ca2+ dissociation from the second EF-hand of TnC in the Tn complex, on the thin filament, and in the presence of myosin S1.24 The rate of Ca2+ dissociation from the second EF-hand of TnC in the Tn complex was dependent on the TnT isoform present.48 It has been known for a long time that phosphorylation of Ser 23 and 24 of TnI by PKA (in response to β-adrenergic stimulation of the heart) decreases the Ca2+ affinity of the Tn complex and increases the rate of Ca2+ dissociation, which appears to still occur on the thin filament.26,49 The effect of mutation on the Ca2+ affinity of TnC can vary depending on the biochemical system studied. For instance, mutations of TnI linked to hypertrophic or restricted cardiomyopathy (R145G, R145W, D190H, and R192H) increased the Ca2+ sensitivities of the Tn complex on the thin filament, but not in isolation.23 Thus, it should not be assumed that the behaviour of the simple biochemical systems (TnC and Tn) will always reflect the more sophisticated biochemical systems or intact muscle.


   4. Rate-limiting step(s) of muscle contraction/relaxation
Top
 Abstract
 1. Introduction
 2. Troponin C structure...
 3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
5. Conclusions
 Funding
 References
 
Just as there are different levels of complexity to study the kinetics of Ca2+ exchange with TnC, there are many physiological systems to study the kinetics of cardiac muscle contraction and relaxation ranging from in vivo whole-heart function to isolated, single myofibrils. Tn subunit mutation, phosphorylation, and isoform variation have been implicated in affecting the contractility and kinetics of cardiac muscle contraction in vivo.8,47,50 These experiments suggest that the Ca2+-binding properties of Tn in the cardiac muscle cell are among many modulators of cardiac muscle contractility. However, it is not a simple matter to directly correlate a Tn modification to altered cardiac physiology. For instance, the heart has additional compensatory mechanisms that can also go awry or stabilize the new physiological set point potentially caused by the Tn modification, such as adjusting the Ca2+ transient, kinase/phosphatase activity, and up/down regulating other proteins.47,51–53 Perhaps the most direct technique to measure of the effect of TnC or Tn subunit modification on cardiac muscle performance is the ‘skinned’ muscle preparation (for review3). This technique reduces and simplifies the muscle primarily to the contractile apparatus by removing or ‘skinning’ the plasma and internal membrane systems with detergents. Once skinned, there are now several options to extract/exchange Tn subunits or whole Tn for the endogenous proteins to test the effects of Tn modification on cardiac muscle mechanics.5,54,55

However, since the sarcoplasmic reticulum has been destroyed by the skinning process, Ca2+ must be introduced and removed via solution exchange, which due to the thickness of the preparations interferes with the true rates of contraction and relaxation. To alleviate the problem of Ca2+ diffusion in and out of these skinned cells, ‘caged’ compounds that either rapidly release or chelate Ca2+ at rates significantly faster than the ryanodine receptor or the sarcoplasmic reticulum Ca2+-ATPase (SERCA) can be used (for review3). Another approach to minimize diffusion distances has been the recent advances in measuring the kinetics of contraction and relaxation of isolated cardiac myofibrils utilizing rapid solution exchange techniques (for review56). In addition, Brenner57 developed a technique in which a rapid shortening–re-stretch manoeuvre, applied to an isometrically contracted muscle, mechanically detaches the force producing cross-bridges. The rate at which the cross-bridges re-attach and generate force is a measure of the rate of contraction, known as the rate of force (tension) re-development (ktr), without having to alter the concentration of Ca2+. Most studies have demonstrated that the rate of force re-development, ktr, and the rate of force development caused by a rapid increase in Ca2+ are controlled by the same rate-limiting mechanism(s), and thus ktr is a good surrogate for studying the rate of contraction.58–60 However, one study found that ktr was faster than the rate of contraction induced by caged Ca2+, implying that the rate of activation of the thin filament, possibly via TnC Ca2+ binding, can influence the rate of cardiac muscle contraction.61

4.1 Influences on the rate of cardiac muscle contraction
At the cellular level, contraction of cardiac muscle involves multiple steps: (i) concentration of intracellular Ca2+ elevates; (ii) Ca2+ binds to the second EF-hand of TnC; (iii) the N-terminal domain of TnC binds to the C-terminal domain of TnI; (iv) TnI dissociates from actin; (v) Tm shifts its position on actin; (vi) weakly bound cross-bridges transition to strongly bound cross-bridges further moving Tm on actin; and (vii) force and/or muscle shortening occurs (for review3,4). In intact muscle preparations, the rise of intracellular Ca2+ significantly precedes force development and is not considered to be rate-limiting for contraction (for review3). In addition, Bell et al.62 recently demonstrated that the Ca2+-dependent structural transitions in TnC also precede force development. Furthermore, it is generally believed that steps 3 through 5 are rapid, allowing step 6 to be rate-limiting for contraction.63,64 It is also generally assumed that the rate of transition from weakly bound cross-bridges to strongly bound cross-bridges is primarily determined by the kinetics of myosin.63,64 However, the concept of a single process being the all-inclusive rate-limiting step for cardiac muscle contraction might be oversimplified.65 It could be that the rate of transition from weak to strong cross-bridge binding is not only influenced by the characteristics of myosin, but also by the Ca2+-binding properties of TnC on the thin filament, as well as the level of free Ca2+.

There are two contractile isozymes of the myosin heavy chain in cardiac muscle, {alpha} and β, at varying ratios depending on the state of health and species.66 Supporting the notion that the intrinsic properties of myosin dictate the rate of contraction, {alpha}-myosin goes through the ATPase cycle faster than β-myosin, and likewise, cardiac muscle containing primarily {alpha}-myosin contracts faster than muscle containing primarily β-myosin.59,60,67 In addition, the products of ATP hydrolysis, inorganic phosphate (Pi) and adenosine diphosphate (ADP), increase and decrease the maximal rate of myosin ATPase activity, respectively (for review3). Again, in agreement with myosin controlling the rate of cardiac muscle contraction, increases in Pi accelerate the rate of cardiac muscle contraction.60,68 The myosin isozymes and ATP byproducts modulate the rate of cardiac muscle contraction independently of TnC. However, it is well established that the rate of cardiac muscle contraction is also strongly Ca2+ dependent (for review3).

In skinned cardiac muscle studies, the rate of contraction increases ~3 to 5-fold with increasing Ca2+ concentration, reaching a maximal rate when force is maximal (for review69). It would appear that none of the steps in the cross-bridge cycle are directly modulated by Ca2+ (for review3). As discussed earlier, TnC is the Ca2+ sensor for muscle contraction. Even though the rate of Ca2+ binding to TnC is not diffusion controlled,21 the regulatory domain of TnC still binds Ca2+ significantly faster than the rate of cardiac muscle contraction.62 Thus, the rate of Ca2+ binding to TnC itself is not rate-limiting for contraction. However, it may be that as Ca2+ rises, TnC binds the Ca2+ rapidly and either concomitantly or very quickly thereafter activates the thin filament via the proposed drag and release mechanism to a state dependent on the amount of Ca2+. The thin filament is now primed to recruit weakly bound cross-bridges to strongly bound cross-bridges to generate force. The higher the free Ca2+ (up to a saturating amount), the more strongly bound cross-bridges can interact with actin and the faster the force develops. If this is true, then the rate of cardiac muscle contraction at submaximal concentrations of Ca2+ should be modulated by the Ca2+ sensitivity of TnC.

Recently, two TnC mutants were designed that increase or decrease the Ca2+ sensitivity of cardiac muscle force development.21,68 The two TnCs did not significantly alter the minimal or maximal ktr of rat’s skinned trabeculae but did increase or decrease ktr at submaximal Ca2+ concentrations in the presence and absence of Pi (Figure 4).68 Thus, it was concluded that the intrinsic properties of both myosin and TnC can affect the rate of cardiac muscle contraction. That the effects of the TnC mutants were most prevalent at subsaturating Ca2+ may be of physiological consequence considering cardiac muscle functions primarily at subsaturating Ca2+ levels, and many of the cardiac disease-associated Tn mutations alter the Ca2+ sensitivity of force development (for review8). Alternatively, it has been suggested that the Ca2+ sensitivity of TnC plays no role in the rate of muscle contraction, since the ktr vs. relative force relationship was similar for control and different conditions that sensitize TnC and myofibrils to Ca2+.60,70 However, in order to match levels of relative force production, different concentrations of Ca2+ had to be used under the different conditions, and thus the Ca2+-sensitizing techniques must have accelerated ktr at submaximal levels of Ca2+ similar to the effect of the Ca2+-sensitizing TnC mutant.68 Interestingly, when TnC is fractionally exchanged with a TnC that does not bind Ca2+ at the regulatory domain and thus does not support force development, maximal force production by the muscle decreases, but maximal ktr remains unaffected.54 These studies suggest that relative force and ktr can be dissociated, and that as long as a minimal region of the thin filament is maximally activated by Ca2+ through the remaining endogenous TnC, ktr remains maximal.


Figure 4
View larger version (24K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 4 Effects of Ca2+ sensitizing and desensitizing troponin C mutants on ktr in rat’s skinned trabeculae. The figure shows that sensitizing or desensitizing troponin C to Ca2+ increased or decreased, respectively, ktr at submaximal Ca2+. All troponin C mutants contained the F27W mutation. Data were adapted from Norman et al.68 The inset shows the crystal structure of the regulatory N-terminal domain of troponin C from the Ca2+-saturated troponin core domain (PDB file 1J1E14), with Val 44 shown in red, and Phe 20 shown in blue. The inset was generated using Rasmol.88

 
4.2 Influences on the rate of cardiac muscle relaxation
Relaxation is as critical as contraction for a normal beating heart. Impaired relaxation occurs in approximately half of heart failure patients (for review71). Cardiac muscle relaxation is a complex process involving the reversal of the seven biochemical steps listed earlier for contraction (not necessarily in the same order) as well as mechanical inhomogeneities of the sarcomeres.41 The decline in the intracellular Ca2+ transient, dissociation of Ca2+ from the regulatory domain of TnC, and cross-bridge detachment have all been hypothesized as rate-limiting steps for relaxation (for review3,63,72). There is no general agreement on the relative contribution of each of these steps to the rate of mechanical relaxation. Similar to the process of contraction, the mechanism of relaxation may not be governed by a single rate-limiting step but may involve all three processes working together.

Much of the work that supports the fall in Ca2+ as being rate-limiting for relaxation comes from isolated cardiac myocytes (for review72). These experiments have added a vast wealth of information to the cardiac field by simultaneously measuring the time course of the Ca2+ transient, with changes in contraction and relaxation (indicated by unloaded cell shortening and re-lengthening). The use of myocytes from transgenic mice has most clearly demonstrated a direct correlation between the fall of the Ca2+ transient and relaxation kinetics (for review73). For instance, overexpression of SERCA or knockout of the SERCA regulatory protein phospholamban significantly accelerates both Ca2+ sequestration and re-lengthening. On the other hand, poisoning SERCA depresses both Ca2+ sequestration and relaxation.74 These experiments indicate that the fall in Ca2+, which is controlled primarily by SERCA, may be rate-limiting for relaxation. However, these studies were conducted with unloaded myocytes. Increasing the load on the myocytes slows the time course of relaxation independent of the Ca2+ transient.75 Even though the rate of the Ca2+ transient decline is probably not rate-limiting for loaded relaxation, its duration can modulate the time course of relaxation. For instance, increased expression of SERCA or the soluble Ca2+-buffering protein parvalbumin in vitro accelerates Ca2+ removal and increases cardiac muscle relaxation in vivo.76,77

At room and body temperatures, the rate-limiting step of isometric relaxation of intact rat trabeculae appears to be determined by the myofilaments.78,79 For instance, stretching cardiac muscle increases the number of available strongly bound cross-bridges and slows relaxation without changing the Ca2+ transient (for review78). This effect of slowed relaxation was also observed in ventricular isovolumic contractions and in ventricles of conscious dogs (for review78). One interpretation of these results is that a feedback mechanism of strongly bound cross-bridges increases the subsequent strong binding of other cross-bridges, causing relaxation to be prolonged. In support of this hypothesis, relaxation was drastically impaired when the strongly bound cross-bridge analogue, NEM-S1, was introduced into rat’s skinned ventricular myofibrils.40

More direct effects of cross-bridge kinetics on relaxation can be determined in skinned cardiac preparations induced to relax by rapid Ca2+ sequestration by the photo-labile Ca2+ chelator diazo-2.80 For instance, increasing the concentration of ADP decreases cross-bridge detachment and the rate of relaxation, whereas increasing the concentration of Pi increases cross-bridge detachment and the rate of relaxation.81,82 Similarly, relaxation is accelerated by Pi in small bundles of cardiac myofibrils induced to relax by removal of Ca2+ by rapid solution exchange.83 Thus, intracellular metabolites known to alter the kinetics of cross-bridge detachment modify the rate of cardiac muscle relaxation independent of the fall in Ca2+. In addition, guinea pig and human adults primarily express the slower β-myosin isozyme, whereas adult mice and rats primarily express the faster {alpha}-myosin isozyme in their ventricles.38,58,59 However, hypothyroidism in rats shifts myosin expression to nearly 100% β-myosin without any changes in Tn.38 Skinned cardiac muscles form the same or different species expressing {alpha}-myosin relax two to five times faster than those expressing β-myosin.38,58–60 Thus, changing myosin heavy chain expression also modulates the rate of cardiac muscle relaxation. The conclusion from some of these studies is that myosin kinetics alone are rate-limiting for relaxation, based on the assumption that Ca2+ dissociation from TnC is extremely rapid (for review63), which is true for isolated TnC, but may not be the case with the more physiologically relevant biochemical model systems (Figure 3).24 In any regard, there is no doubt that the intrinsic characteristics of myosin play a fundamental role in regulating the rate of cardiac muscle relaxation.

The evidence suggesting that the Ca2+ exchange kinetics with TnC may also influence the rate of cardiac muscle relaxation comes from studies of TnI modification. For instance, ssTnI increases the Ca2+ affinity and decreases the rate of Ca2+ dissociation from the Tn complex in isolation, on the thin filament, and in the presence of myosin S1,24 consistent with delayed myocyte relaxation and in vivo diastolic dysfunction in transgenic mice expressing ssTnI in the heart.47 Furthermore, it has long been known that β-adrenergic stimulation of the heart accelerates the rate of cardiac muscle relaxation and is associated with increasing the rate of Ca2+ dissociation from TnC through PKA phosphorylation of TnI.26,84 In addition, truncation of TnI associated with cardiac ischaemia, or a familial hypertrophic cardiomyopathy-related TnI mutation, increased the Ca2+ sensitivity of force development and delayed the rate of relaxation of isolated cardiac myofibrils.85,86 However, the Ca2+-sensitizing compound bepridil had no effect on the rate of relaxation of isolated cardiac myofibrils, but did sensitize the force developed by the myofibrils to Ca2+.60 It is currently unknown how bepridil or any of the disease-associated Tn modifications affect TnC kinetics in the more sophisticated biochemical systems, or for that matter in muscle, although it is typically assumed only the Ca2+ dissociation rate is altered. However, it has been proposed that ssTnI may affect the Ca2+ association rate to a greater extent than the rate of Ca2+ dissociation from TnC on the thin filament in the presence of myosin S1.24 Since the rate of Ca2+ dissociation from TnC has not been measured in muscle or force-bearing myofibrils, it is difficult to know with certainty whether the rate of Ca2+ dissociation from TnC is too rapid or slow enough to influence relaxation. However, there is circumstantial evidence that TnC is still bound with Ca2+ during relaxation on the basis of ‘extra’ Ca2+ being released from the myofilaments late in relaxation and after rapid release of force during relaxation in intact trabeculae (see Jiang et al.87 and references within). This extra Ca2+ is thought to be released from TnC as cross-bridges are rapidly detached, which in turn decreases the Ca2+ sensitivity of Tn by accelerating the rate of Ca2+ dissociation from TnC.24 Thus, it may be that Ca2+ dissociation from TnC and cross-bridge kinetics work together to modulate the rate of cardiac muscle relaxation at the level of the myofilaments. The effects of TnC on relaxation could be determined by utilizing TnC mutants that alter the rate of Ca2+ dissociation from TnC in the complex biochemical systems.


   5. Conclusions
Top
 Abstract
 1. Introduction
 2. Troponin C structure...
 3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
 5. Conclusions
Funding
 References
 
In summary, controversy abounds in the cardiac muscle literature over the rate-limiting steps of cardiac muscle contraction and relaxation. The idea of cardiac muscle contraction/relaxation being controlled by a single biochemical step may be oversimplified. For instance, as argued earlier, cardiac muscle contraction/relaxation can be modulated by Ca2+ levels/kinetics, Ca2+ association/dissociation from the regulatory binding site of TnC, and cross-bridge kinetics. Thus, Ca2+ exchange with TnC may be one of a number of mechanisms in the arsenal of cardiac muscle that can be modulated to alter cardiac muscle performance in health and disease.


   Funding
Top
 Abstract
 1. Introduction
 2. Troponin C structure...
 3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
 5. Conclusions
 Funding
References
 
We acknowledge support from the National Institutes of Health (K99 HL087462 to S.B.T.) and an award from the American Heart Association (SDG to J.P.D.).


   Acknowledgements
 
We would like to thank Drs Jack Rall and Mark Ziolo for critical reading of the manuscript.

Conflict of interest: none declared.


   References
Top
 Abstract
 1. Introduction
 2. Troponin C structure...
 3. Measurements of Ca2+...
 4. Rate-limiting step(s) of...
 5. Conclusions
 Funding
 References
 

  1. Ebashi S, Endo M. Calcium ion and muscle contraction. Prog Biophys Mol Biol (1968) 18:123–183.[CrossRef][Medline]
  2. Farah CS, Reinach FC. The troponin complex and regulation of muscle contraction. FASEB J (1995) 9:755–767.[Abstract]
  3. Gordon AM, Homsher E, Regnier M. Regulation of contraction in striated muscle. Physiol Rev (2000) 80:853–924.[Abstract/Free Full Text]
  4. Gordon AM, Regnier M, Homsher E. Skeletal and cardiac muscle contractile activation: tropomyosin ‘rocks and rolls. News Physiol Sci (2001) 16:49–55.[Abstract/Free Full Text]
  5. Chandra M, Tschirgi ML, Ford SJ, Slinker BK, Campbell KB. Interaction between myosin heavy chain and troponin isoforms modulate cardiac myofiber contractile dynamics. Am J Physiol Regul Integr Comp Physiol (2007) 293:R1595–R1607.[Abstract/Free Full Text]
  6. Kass DA. Mechanisms and efficacy of LV pre-excitation for patients with heart failure and supra-normal systolic function. Heart Fail Rev (2000) 5:357–365.[CrossRef][Medline]
  7. Borlaug BA, Kass DA. Mechanisms of diastolic dysfunction in heart failure. Trends Cardiovasc Med (2006) 16:273–279.[CrossRef][Web of Science][Medline]
  8. Gomes AV, Potter JD. Molecular and cellular aspects of troponin cardiomyopathies. Ann NY Acad Sci (2004) 1015:214–224.[CrossRef][Web of Science][Medline]
  9. Kass DA, Solaro RJ. Mechanisms and use of calcium-sensitizing agents in the failing heart. Circulation (2006) 113:305–315.[Free Full Text]
  10. Tobacman LS. Thin filament-mediated regulation of cardiac contraction. Annu Rev Physiol (1996) 58:447–481.[CrossRef][Web of Science][Medline]
  11. van Eerd JP, Takahashi K. The amino acid sequence of bovine cardiac tamponin-C. Comparison with rabbit skeletal troponin-C. Biochem Biophys Res Commun (1975) 64:122–127.[CrossRef][Web of Science][Medline]
  12. Biesiadecki BJ, Kobayashi T, Walker JS, John Solaro R, de Tombe PP. The troponin C G159D mutation blunts myofilament desensitization induced by troponin I Ser23/24 phosphorylation. Circ Res (2007) 100:1486–1493.[Abstract/Free Full Text]
  13. Kleerekoper Q, Howarth JW, Guo X, Solaro RJ, Rosevear PR. Cardiac troponin I induced conformational changes in cardiac troponin C as monitored by NMR using site-directed spin and isotope labeling. Biochemistry (1995) 34:13343–13352.[CrossRef][Web of Science][Medline]
  14. Takeda S, Yamashita A, Maeda K, Maeda Y. Structure of the core domain of human cardiac troponin in the Ca2+-saturated form. Nature (2003) 424:35–41.[CrossRef][Medline]
  15. Li MX, Wang X, Sykes BD. Structural based insights into the role of troponin in cardiac muscle pathophysiology. J Muscle Res Cell Motil (2004) 25:559–579.[CrossRef][Web of Science][Medline]
  16. Kobayashi T, Solaro RJ. Calcium, thin filaments, and the integrative biology of cardiac contractility. Annu Rev Physiol (2005) 67:39–67.[CrossRef][Web of Science][Medline]
  17. Sia SK, Li MX, Spyracopoulos L, Gagne SM, Liu W, Putkey JA, et al. Structure of cardiac muscle troponin C unexpectedly reveals a closed regulatory domain. J Biol Chem (1997) 272:18216–18221.[Abstract/Free Full Text]
  18. Spyracopoulos L, Li MX, Sia SK, Gagne SM, Chandra M, Solaro RJ, et al. Calcium-induced structural transition in the regulatory domain of human cardiac troponin C. Biochemistry (1997) 36:12138–12146.[CrossRef][Web of Science][Medline]
  19. MacLachlan LK, Reid DG, Mitchell RC, Salter CJ, Smith SJ. Binding of a calcium sensitizer, bepridil, to cardiac troponin C. A fluorescence stopped-flow kinetic, circular dichroism, and proton nuclear magnetic resonance study. J Biol Chem (1990) 265:9764–9770.[Abstract/Free Full Text]
  20. Moyes CD, Borgford T, LeBlanc L, Tibbits GF. Cloning and expression of salmon cardiac troponin C: titration of the low-affinity Ca2+-binding site using a tryptophan mutant. Biochemistry (1996) 35:11756–11762.[CrossRef][Web of Science][Medline]
  21. Tikunova SB, Davis JP. Designing calcium-sensitizing mutations in the regulatory domain of cardiac troponin C. J Biol Chem (2004) 279:35341–35352.[Abstract/Free Full Text]
  22. Robinson P, Griffiths PJ, Watkins H, Redwood CS. Dilated and hypertrophic cardiomyopathy mutations in troponin and {alpha}-tropomyosin have opposing effects on the calcium affinity of cardiac thin filaments. Circ Res (2007) 101:1266–1273.[Abstract/Free Full Text]
  23. Kobayashi T, Solaro RJ. Increased Ca2+ affinity of cardiac thin filaments reconstituted with cardiomyopathy-related mutant cardiac troponin I. J Biol Chem (2006) 281:13471–13477.[Abstract/Free Full Text]
  24. Davis JP, Norman C, Kobayashi T, Solaro RJ, Swartz DR, Tikunova SB. Effects of thin and thick filament proteins on calcium binding and exchange with cardiac troponin C. Biophys J (2007) 92:3195–3206.[CrossRef][Web of Science][Medline]
  25. Dong WJ, An J, Xing J, Cheung HC. Structural transition of the inhibitory region of troponin I within the regulated cardiac thin filament. Arch Biochem Biophys (2006) 456:135–142.[CrossRef][Web of Science][Medline]
  26. Robertson SP, Johnson JD, Holroyde MJ, Kranias EG, Potter JD, Solaro RJ. The effect of troponin I phosphorylation on the Ca2+-binding properties of the Ca2+-regulatory site of bovine cardiac troponin. J Biol Chem (1982) 257:260–263.[Free Full Text]
  27. Metzger JM, Westfall MV. Covalent and noncovalent modification of thin filament action: the essential role of troponin in cardiac muscle regulation. Circ Res (2004) 94:146–158.[Abstract/Free Full Text]
  28. Solaro RJ, Moir AJ, Perry SV. Phosphorylation of troponin I and the inotropic effect of adrenaline in the perfused rabbit heart. Nature (1976) 262:615–617.[CrossRef][Medline]
  29. van der Velden J, Narolska NA, Lamberts RR, Boontje NM, Borbely A, Zaremba R, et al. Functional effects of protein kinase C-mediated myofilament phosphorylation in human myocardium. Cardiovasc Res (2006) 69:876–887.[Abstract/Free Full Text]
  30. Wang H, Grant JE, Doede CM, Sadayappan S, Robbins J, Walker JW. PKC-betaII sensitizes cardiac myofilaments to Ca2+ by phosphorylating troponin I on threonine-144. J Mol Cell Cardiol (2006) 41:823–833.[CrossRef][Web of Science][Medline]
  31. Dong W, Rosenfeld SS, Wang CK, Gordon AM, Cheung HC. Kinetic studies of calcium binding to the regulatory site of troponin C from cardiac muscle. J Biol Chem (1996) 271:688–694.[Abstract/Free Full Text]
  32. Hazard AL, Kohout SC, Stricker NL, Putkey JA, Falke JJ. The kinetic cycle of cardiac troponin C: calcium binding and dissociation at site II trigger slow conformational rearrangements. Protein Sci (1998) 7:2451–2459.[Web of Science][Medline]
  33. Li MX, Saude EJ, Wang X, Pearlstone JR, Smillie LB, Sykes BD. Kinetic studies of calcium and cardiac troponin I peptide binding to human cardiac troponin C using NMR spectroscopy. Eur Biophys J (2002) 31:245–256.[CrossRef][Web of Science][Medline]
  34. Johnson JD, Collins JH, Robertson SP, Potter JD. A fluorescent probe study of Ca2+ binding to the Ca2+-specific sites of cardiac troponin and troponin C. J Biol Chem (1980) 255:9635–9640.[Abstract/Free Full Text]
  35. Niederer SA, Hunter PJ, Smith NP. A quantitative analysis of cardiac myocyte relaxation: a simulation study. Biophys J (2006) 90:1697–1722.[CrossRef][Web of Science][Medline]
  36. Pan BS, Solaro RJ. Calcium-binding properties of troponin C in detergent-skinned heart muscle fibers. J Biol Chem (1987) 262:7839–7849.[Abstract/Free Full Text]
  37. Wang YP, Fuchs F. Length force Ca(2+)-troponin C affinity in cardiac slow skeletal muscle. Am J Physiol (1994) 266:C1077–C1082.[Web of Science][Medline]
  38. Fitzsimons DP, Patel JR, Moss RL. Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium. J Physiol (1998) 513:171–183.[Abstract/Free Full Text]
  39. Saeki Y, Takigiku K, Iwamoto H, Yasuda S, Yamashita H, Sugiura S, et al. Protein kinase A increases the rate of relaxation but not the rate of tension development in skinned rat cardiac muscle. Jpn J Physiol (2001) 51:427–433.[CrossRef][Web of Science][Medline]
  40. Fitzsimons DP, Patel JR, Moss RL. Cross-bridge interaction kinetics in rat myocardium are accelerated by strong binding of myosin to the thin filament. J Physiol (2001) 530:263–272.[Abstract/Free Full Text]
  41. Stehle R, Solzin J, Iorga B, Gomez D, Blaudeck N, Pfitzer G. Mechanical properties of sarcomeres during cardiac myofibrillar relaxation: stretch-induced cross-bridge detachment contributes to early diastolic filling. J Muscle Res Cell Motil (2006) 27:423–434.[CrossRef][Web of Science][Medline]
  42. Cassell M, Tobacman LS. Opposite effects of myosin subfragment 1 on binding of cardiac troponin and tropomyosin to the thin filament. J Biol Chem (1996) 271:12867–12872.[Abstract/Free Full Text]
  43. Gillis TE, Moyes CD, Tibbits GF. Sequence mutations in teleost cardiac troponin C that are permissive of high Ca2+ affinity of site II. Am J Physiol Cell Physiol (2003) 284:C1176–C1184.[Abstract/Free Full Text]
  44. Smith SJ, England PJ. The effects of reported Ca2+ sensitisers on the rates of Ca2+ release from cardiac troponin C and the troponin–tropomyosin complex. Br J Pharmacol (1990) 100:779–785.[Web of Science][Medline]
  45. Solaro RJ, Bousquet P, Johnson JD. Stimulation of cardiac myofilament force, ATPase activity and troponin C Ca2+ binding by bepridil. J Pharmacol Exp Ther (1986) 238:502–507.[Abstract/Free Full Text]
  46. Thijssen VL, Ausma J, Gorza L, van der Velden HM, Allessie MA, Van Gelder IC, et al. Troponin I isoform expression in human and experimental atrial fibrillation. Circulation (2004) 110:770–775.[Abstract/Free Full Text]
  47. Fentzke RC, Buck SH, Patel JR, Lin H, Wolska BM, Stojanovic MO, et al. Impaired cardiomyocyte relaxation and diastolic function in transgenic mice expressing slow skeletal troponin I in the heart. J Physiol (1999) 517:143–157.[Abstract/Free Full Text]
  48. Gomes AV, Venkatraman G, Davis JP, Tikunova SB, Engel P, Solaro RJ, et al. Cardiac troponin T isoforms affect the Ca2+ sensitivity of force development in the presence of slow skeletal troponin I: insights into the role of troponin T isoforms in the fetal heart. J Biol Chem (2004) 279:49579–49587.[Abstract/Free Full Text]
  49. Ward DG, Cornes MP, Trayer IP. Structural consequences of cardiac troponin I phosphorylation. J Biol Chem (2002) 277:41795–41801.[Abstract/Free Full Text]
  50. Pena JR, Wolska BM. Troponin I phosphorylation plays an important role in the relaxant effect of beta-adrenergic stimulation in mouse hearts. Cardiovasc Res (2004) 61:756–763.[Abstract/Free Full Text]
  51. van der Velden J, Papp Z, Zaremba R, Boontje NM, de Jong JW, Owen VJ, et al. Increased Ca2+-sensitivity of the contractile apparatus in end-stage human heart failure results from altered phosphorylation of contractile proteins. Cardiovasc Res (2003) 57:37–47.[Abstract/Free Full Text]
  52. White MY, Cordwell SJ, McCarron HC, Prasan AM, Craft G, Hambly BD, et al. Proteomics of ischemia/reperfusion injury in rabbit myocardium reveals alterations to proteins of essential functional systems. Proteomics (2005) 5:1395–1410.[CrossRef][Web of Science][Medline]
  53. Du CK, Morimoto S, Nishii K, Minakami R, Ohta M, Tadano N, et al. Knock-in mouse model of dilated cardiomyopathy caused by troponin mutation. Circ Res (2007) 101:185–194.[Abstract/Free Full Text]
  54. Gillis TE, Martyn DA, Rivera AJ, Regnier M. Investigation of thin filament near-neighbour regulatory unit interactions during force development in skinned cardiac and skeletal muscle. J Physiol (2007) 580:561–576.[Abstract/Free Full Text]
  55. Szczesna D, Zhang R, Zhao J, Jones M, Guzman G, Potter JD. Altered regulation of cardiac muscle contraction by troponin T mutations that cause familial hypertrophic cardiomyopathy. J Biol Chem (2000) 275:624–630.[Abstract/Free Full Text]
  56. Poggesi C, Tesi C, Stehle R. Sarcomeric determinants of striated muscle relaxation kinetics. Pflugers Arch (2005) 449:505–517.[CrossRef][Web of Science][Medline]
  57. Brenner B. Effect of Ca2+ on cross-bridge turnover kinetics in skinned single rabbit psoas fibers: implications for regulation of muscle contraction. Proc Natl Acad Sci USA (1988) 85:3265–3269.[Abstract/Free Full Text]
  58. Stehle R, Kruger M, Scherer P, Brixius K, Schwinger RH, Pfitzer G. Isometric force kinetics upon rapid activation and relaxation of mouse, guinea pig and human heart muscle studied on the subcellular myofibrillar level. Basic Res Cardiol (2002) 97(Suppl. 1):I127–I135.[Medline]
  59. Palmer S, Kentish JC. Roles of Ca2+ and crossbridge kinetics in determining the maximum rates of Ca2+ activation and relaxation in rat and guinea pig skinned trabeculae. Circ Res (1998) 83:179–186.[Abstract/Free Full Text]
  60. Piroddi N, Belus A, Scellini B, Tesi C, Giunti G, Cerbai E, et al. Tension generation and relaxation in single myofibrils from human atrial and ventricular myocardium. Pflugers Arch (2007) 454:63–73.[CrossRef][Web of Science][Medline]
  61. Regnier M, Martin H, Barsotti RJ, Rivera AJ, Martyn DA, Clemmens E. Cross-bridge versus thin filament contributions to the level and rate of force development in cardiac muscle. Biophys J (2004) 87:1815–1824.[CrossRef][Web of Science][Medline]
  62. Bell MG, Lankford EB, Gonye GE, Ellis-Davies GC, Martyn DA, Regnier M, et al. Kinetics of cardiac thin-filament activation probed by fluorescence polarization of rhodamine-labeled troponin C in skinned guinea pig trabeculae. Biophys J (2006) 90:531–543.[CrossRef][Web of Science][Medline]
  63. Stehle R, Iorga B, Pfitzer G. Calcium regulation of troponin and its role in the dynamics of contraction and relaxation. Am J Physiol Regul Integr Comp Physiol (2007) 292:R1125–R1128.[Free Full Text]
  64. Moss RL, Razumova M, Fitzsimons DP. Myosin crossbridge activation of cardiac thin filaments: implications for myocardial function in health and disease. Circ Res (2004) 94:1290–1300.[Abstract/Free Full Text]
  65. Campbell K. Rate constant of muscle force redevelopment reflects cooperative activation as well as cross-bridge kinetics. Biophys J (1997) 72:254–262.[Web of Science][Medline]
  66. Clark WA Jr, Chizzonite RA, Everett AW, Rabinowitz M, Zak R. Species correlations between cardiac isomyosins. A comparison of electrophoretic and immunological properties. J Biol Chem (1982) 257:5449–5454.[Abstract/Free Full Text]
  67. Rundell VL, Manaves V, Martin AF, de Tombe PP. Impact of beta-myosin heavy chain isoform expression on cross-bridge cycling kinetics. Am J Physiol Heart Circ Physiol (2005) 288:H896–H903.[Abstract/Free Full Text]
  68. Norman C, Rall JA, Tikunova SB, Davis JP. Modulation of the rate of cardiac muscle contraction by troponin C constructs with various calcium binding affinities. Am J Physiol Heart Circ Physiol (2007) 293:H2580–H2587.[Abstract/Free Full Text]
  69. Fitzsimons DP, Moss RL. Cooperativity in the regulation of force and the kinetics of force development in heart and skeletal muscles: cross-bridge activation of force. Adv Exp Med Biol (2007) 592:177–189.[Web of Science][Medline]
  70. de Tombe PP, Belus A, Piroddi N, Scellini B, Walker JS, Martin AF, et al. Myofilament calcium sensitivity does not affect cross-bridge activation-relaxation kinetics. Am J Physiol Regul Integr Comp Physiol (2007) 292:R1129–R1136.[Abstract/Free Full Text]
  71. Fukuta H, Little WC. Diagnosis of diastolic heart failure. Curr Cardiol Rep (2007) 9:224–228.[Medline]
  72. Bers DM. Calcium fluxes involved in control of cardiac myocyte contraction. Circ Res (2000) 87:275–281.[Free Full Text]
  73. Dillmann WH. Calcium regulatory proteins and their alteration by transgenic approaches. Am J Cardiol (1999) 83:89H–91H.[Web of Science][Medline]
  74. Bassani JW, Bassani RA, Bers DM. Relaxation in rabbit and rat cardiac cells: species-dependent differences in cellular mechanisms. J Physiol (1994) 476:279–293.[Abstract/Free Full Text]
  75. Sollott SJ, Lakatta EG. Novel method to alter length and load in isolated mammalian cardiac myocytes. Am J Physiol (1994) 267:H1619–H1629.[Web of Science][Medline]
  76. Szatkowski ML, Westfall MV, Gomez CA, Wahr PA, Michele DE, DelloRusso C, et al. In vivo acceleration of heart relaxation performance by parvalbumin gene delivery. J Clin Invest (2001) 107:191–198.[Web of Science][Medline]
  77. He H, Giordano FJ, Hilal-Dandan R, Choi DJ, Rockman HA, McDonough PM, et al. Overexpression of the rat sarcoplasmic reticulum Ca2+ ATPase gene in the heart of transgenic mice accelerates calcium transients and cardiac relaxation. J Clin Invest (1997) 100:380–389.[Web of Science][Medline]
  78. Hunter WC. Role of myofilaments and calcium handling in left ventricular relaxation. Cardiol Clin (2000) 18:443–457.[CrossRef][Medline]
  79. Janssen PM, Stull LB, Marban E. Myofilament properties comprise the rate-limiting step for cardiac relaxation at body temperature in the rat. Am J Physiol Heart Circ Physiol (2002) 282:H499–H507.[Abstract/Free Full Text]
  80. Adams SR, Kao JPY, Tsien RY. Biologically useful chelators that take up Ca2+ upon illumination. J Am Chem Soc (1989) 111:7957–7968.[CrossRef][Web of Science]
  81. Simnett SJ, Johns EC, Lipscomb S, Mulligan IP, Ashley CC. Effect of pH, phosphate, and ADP on relaxation of myocardium after photolysis of diazo 2. Am J Physiol (1998) 275:H951–H960.[Web of Science][Medline]
  82. Simnett SJ, Lipscomb S, Ashley CC, Mulligan IP. The effect of EMD 57033, a novel cardiotonic agent, on the relaxation of skinned cardiac and skeletal muscle produced by photolysis of diazo-2, a caged calcium chelator. Pflugers Arch (1993) 425:175–177.[CrossRef][Web of Science][Medline]
  83. Stehle R, Kruger M, Pfitzer G. Force kinetics and individual sarcomere dynamics in cardiac myofibrils after rapid ca(2+) changes. Biophys J (2002) 83:2152–2161.[Web of Science][Medline]
  84. Zhang R, Zhao J, Mandveno A, Potter JD. Cardiac troponin I phosphorylation increases the rate of cardiac muscle relaxation. Circ Res (1995) 76:1028–1035.[Abstract/Free Full Text]
  85. Narolska NA, Piroddi N, Belus A, Boontje NM, Scellini B, Deppermann S, et al. Impaired diastolic function after exchange of endogenous troponin I with C-terminal truncated troponin I in human cardiac muscle. Circ Res (2006) 99:1012–1020.[Abstract/Free Full Text]
  86. Kruger M, Zittrich S, Redwood C, Blaudeck N, James J, Robbins J, et al. Effects of the mutation R145G in human cardiac troponin I on the kinetics of the contraction-relaxation cycle in isolated cardiac myofibrils. J Physiol (2005) 564:347–357.[Abstract/Free Full Text]
  87. Jiang Y, Patterson MF, Morgan DL, Julian FJ. Basis for late rise in fura 2 R signal reporting [Ca2+]i during relaxation in intact rat ventricular trabeculae. Am J Physiol (1998) 274:C1273–C1282.[Web of Science][Medline]
  88. Sayle RA, Milner-White EJ. RASMOL: biomolecular graphics for all. Trends Biochem Sci (1995) 20:374.[CrossRef][Web of Science][Medline]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 Cardiovasc ResHome page
R. J. Solaro and P. P. de Tombe
Review focus series: sarcomeric proteins as key elements in integrated control of cardiac function
Cardiovasc Res, March 1, 2008; 77(4): 616 - 618.
[Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
77/4/619    most recent
cvm098v2
cvm098v1
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Davis, J. P.
Right arrow Articles by Tikunova, S. B.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Davis, J. P.
Right arrow Articles by Tikunova, S. B.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?