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Cardiovascular Research 2005 66(2):374-383; doi:10.1016/j.cardiores.2004.11.005
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Copyright © 2004, European Society of Cardiology

Aging decreases vasoconstrictor responses of coronary resistance arterioles through endothelium-dependent mechanisms

Robert D. Shipley and Judy M. Muller-Delp*

Department of Health and Kinesiology, Texas A & M University, College Station, Texas 77843, USA

* Corresponding author. Tel.: +1 979 458 3502; fax: +1 979 847 8987. Email address: jmd{at}hlkn.tamu.edu

Received 23 April 2004; revised 15 October 2004; accepted 3 November 2004


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: Aging decreases coronary blood flow and maximal reserve capacity. Impaired blood flow capacity may be related to an increased vasoconstrictor capacity of coronary resistance vessels. This study tested the hypothesis that aging increases the vasoconstrictor responsiveness of coronary arterioles isolated from myocardium of young (4 months) and old (24 months) Fischer 344 rats.

Methods: Isolated coronary arterioles were cannulated and pressurized (60 cm H2O) via hydrostatic pressure reservoirs.

Results: Contrary to our hypothesis, aging decreased responsiveness of coronary arterioles to endothelin (ET, 1 x 10–11–3 x 10–8 M), potassium chloride (KCl, 10–100 mM), and pressure-induced myogenic responses (0–140 cm H2O). Removal of the endothelium from coronary arterioles increased vasoconstriction to all agonists; however, age-related KCl vasoconstrictor response differences remained, suggesting that K+ channel activity and/or the relative contribution of specific K+ channels to maintenance of vascular smooth muscle membrane potential may change with age. Removal of the endothelium, in addition to increasing responsiveness, eliminated aging-induced differences in ET- and pressure-induced vasoconstriction. L-NAME (10–5) incubation resulted in a greater enhancement of spontaneous tone in arterioles from old rats compared to those of young rats. ETB (BQ-788, 3 x 10–8) receptor blockade eliminated the age-associated differences.

Conclusion: Collectively, these data suggest an age-associated increase in endothelial modulation of coronary resistance vessel constriction. This enhanced endothelial attenuation of coronary arteriolar constriction appears to result from increased basal release of nitric oxide. These alterations of coronary vascular reactivity may contribute to age-induced redistribution of coronary blood flow and diminished cardiac function.

KEYWORDS Aging; Coronary circulation; Microcirculation; Endothelins; Nitric oxide; Endothelial function


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
Cardiovascular function declines with advancing age. At least part of the age-related decline in the cardiovascular system is attributable to reduced cardiac function. Cardiac output, peak heart rate, and peak stroke volume all decrease with age [1]. Because cardiac function is closely linked to coronary blood flow, the age-associated impairment of cardiac function may be related to reduced coronary blood flow capacity and/or altered distribution of coronary blood flow [2]. Indeed, impairments of maximal coronary vascular reserve capacity have been reported in aged animals [1,3] and humans [4], and age significantly reduces endocardial-to-epicardial blood flow ratio during maximal vasodilation in rats [3].

Studies performed in both humans and animals indicate that alterations of coronary blood flow distribution and coronary blood flow capacity that occur with advancing age may be associated with changes in vascular reactivity of both large epicardial arteries and resistance vessels. In human subjects, the coronary blood flow response to acetylcholine (i.e., vasodilation) declines with age [5], and contractile responses of epicardial arteries to norepinephrine increase significantly with age [6]. In aged rats, endothelin (ET)-induced reductions in coronary blood flow are increased [7], and constrictor responses of large epicardial arteries to ET, serotonin, and KCl are augmented [8]. These studies suggest that age results in greater responsiveness of the coronary conduit vasculature to vasconstrictor stimuli; however, the effects of age on the vasoconstrictor responses of the coronary resistance vasculature remain unknown. Specifically, a major portion of coronary vascular resistance resides in arterial vessels less than 150 µm in diameter [9]; thus, the purpose of this study was to test the hypothesis that age increases vasoconstrictor responsiveness of coronary resistance arterioles.

Vasoconstriction of coronary arterial vessels depends upon intrinsic contractile responses of vascular smooth muscle and their modulation by the endothelium. To investigate the effects of age on vasoconstrictor responses of the coronary resistance vasculature, isolated arterioles were studied in the absence of age-related changes in neural and humoral influences. To determine the modulatory effects of the endothelium, vasoconstrictor responses were studied in both endothelium-intact arterioles and in arterioles denuded of endothelium.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1. Animals
All procedures performed in this study were approved by the Texas A&M Laboratory Animal Care Committee. All methods conformed to the National Institutes of Health Guide for the Care and Use of Laboratory Animals (National Research Council, Washington, DC, Revised 1996).

Fifty-five young (4 months) and 60 old (24 months) male Fischer-344 rats were obtained from Harlan (Indianapolis, IN). The animals were housed in a temperature-controlled (23 ± 2 °C) room with a 12:12-h light–dark cycle. Water and rat chow were provided ad libitum.

2.2. Microvessel preparation
Rats were anesthetized with pentobarbital sodium (60 mg/kg i.p.). Hearts were dissected free and placed in cold (4 °C) physiological saline solution (PSS) containing 145.0 mM NaCl, 4.7 mM KCl, 2.0 mM CaCl2, 1.17 mM MgSO4, 1.2 mM NaH2PO4, 5.0 mM glucose, 2.0 mM pyruvate, 0.02 mM EDTA, 3.0 mM MOPS buffer, and 1 g/100 ml BSA, pH 7.4. Resistance vessels (<150 µm) branching from the left anterior descending artery were dissected free under a dissection microscope (Olympus SVH10) and removed from the surrounding cardiac tissue. The arterioles were transferred to a Lucite chamber containing PSS equilibrated with room air. The ends of the arteriole were cannulated with a micropipette and secured with nylon suture (10-0 opthalmic, Alcon). The chamber containing the cannulated arteriole was then place on an inverted microscope (Olympus IX70) equipped with a video camera and micrometer (Panasonic BP310; Texas A&M Cardiovascular Research Institute) to measure intraluminal diameter. The coronary arterioles were then pressurized at 60 cm H2O with two hydrostatic columns. Arterioles unable to hold pressure due to leaks or branches were discarded. Those without leaks were warmed to 37 °C and allowed to develop spontaneous tone.

2.3. Active pressure responses
After equilibration and development of spontaneous tone at 60 cm H2O, pressure was decreased to 0 cm H2O. Pressure was increased by increments of 10 to 140 cm H2O, and then decreased in 10 cm H2O increments back down to 0 cm H2O. After each step change in pressure, diameter was recorded for 3 min.

2.4. Passive pressure responses
To determine maximal diameter and passive responses to pressure, the solution in the bath and pressure lines was replaced with calcium-free PSS containing 2.0 mM EDTA. Arterioles were washed every 15 min and allowed to completely relax at 60 cm H2O for 45 min. Maximal diameter at 60 cm H2O was recorded. Passive responses to increasing intraluminal pressure were determined by lowering the pressure reservoirs to 0 cm H2O and recording diameters as pressure was increased incrementally by 10 to 140 cm H2O.

2.5. Responses to endothelin
To determine whether aging alters receptor-dependent vasoconstriction, a concentration–response curve to ET was generated. Changes in diameter were measured in response to cumulative additions of ET (1 x 10–11–3 x 10–8 M) to the vessel bath. To determine whether ET-induced constriction mediated through the ETA receptor on vascular smooth muscle was reduced in the absence of ET-stimulated NO release, the ET concentration–response was evaluated in the presence of the ETB receptor antagonist BQ-788 (3 x 10–8 M) [10].

2.6. Responses to KCl
To evaluate receptor-independent vasoconstriction, a KCl concentration–response curve was generated using isotonic PSS solutions with increasing concentrations of KCl (10–100 mM) substituted for Na+.

2.7. Removal of the endothelium
To determine the contribution of the endothelium to age-induced differences in vasoconstrictor responses, concentration response curves for ET and KCl, and the active pressure curve were repeated in a separate set of arterioles in which the endothelium was removed. Endothelial removal was accomplished by passing approximately 5 cc of air through the vessel lumen and confirmed by the absence of relaxation to 3 x 10–4 M ACh.

2.8. Blockade of nitric oxide synthase
To determine whether constrictor responses to endothelin and pressure changes were modulated by nitric oxide production, responses were reevaluated during nonspecific blockade of nitric oxide synthase (NOS) with NG-nitro-L-arginine methyl ester (L-NAME; 1 x 10–5 M) or during blockade of inducible nitric oxide synthase (iNOS) with aminoguanidine (1 x 10–4 M) [11].

2.9. Evaluation of eNOS expression
Arterioles were snap frozen and stored at –80 °C. Arterioles were pulverized in lysate buffer, and RNA was extracted with the RNAqueous filter system (Ambion). Real-time PCR was performed with TaqMan® probes designed with the use of Primer Express® from the published sequence for rat ecNOS (forward primer: GAA CCT ACA GAG CAG CAA ATC CA; reverse primer: CAG TCC CTC CTG GCT TCC TCC A) and a TaqMan® oligonucleotide probe (probe: CGA GCC ACA ATC CTG GTC CGT CTT) labeled with a fluorescent reporter dye and a quencher dye (Applied Biosystems) as described previously. Levels of the target sequence and levels of coamplified 18S ribosomal RNA were quantified relative to the cycle number (cycle threshold, CT), at which the target and 18S reach a fixed threshold as described previously [12].

2.10. Data analysis
Data are expressed as means ± standard error. Spontaneous tone was calculated as a percent constriction in relation to maximal diameter as determined by the following equation


Formula

where IDm is the maximal diameter recorded at 60 cm H2O, and IDbase is the steady-state baseline diameter recorded at the same pressure. Active and passive responses to pressure changes were normalized to the maximal diameter according to the formula


Formula

where IDs is the steady-state diameter recorded after each pressure change. The vasoconstriction responses to KCl and ET are expressed as percent constriction as calculated by the formula


Formula

where IDb is the baseline diameter immediately prior to addition of the first dose of vasoconstrictor agonist, and IDs is the steady-state diameter measured after addition of each dose.

Two-way repeated measures analysis of variance was used to determine overall group differences. Student's t-tests were performed to identify differences between the maximal responses to KCl and ET, to identify differences in the amount of tone with respect to age and intervention, and to determine differences in sensitivity (EC50) to KCl and ET. A P-value less than 0.05 was considered to be statistically significant.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1. Body and heart weights
Table 1 displays data from the rats included in this study. Old rats had a significantly higher body weight and heart weight than young rats. The ratio of heart weight to body weight was also significantly higher in old rats.


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  		Table 1 Body weight, heart weight, and heart weight/body weight of young and old rats

 
3.2. Vessel characteristics
The amount of spontaneous tone present before each intervention is shown in Table 2. Tone achieved prior to any intervention was not different between arterioles from young and old animals. Treatment with L-NAME significantly increased tone in arterioles from both young and old rats; however, the increase in spontaneous tone elicited by L-NAME was significantly greater in the arterioles from old rats compared to that of young rats.


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  		Table 2 Characteristics of coronary resistance arterioles

 
3.3. Myogenic response
Fig. 1 illustrates active and passive responses of coronary arterioles as pressure was increased from 0 to 140 cm H2O. Arterioles from both young and old rats displayed myogenic constriction in response to increases in pressure; however, myogenic constriction was reduced in arterioles from old rats. Age did not alter passive responses to increasing pressure. In arterioles from old rats, removal of the endothelium increased myogenic tone, whereas endothelial removal did not significantly alter myogenic responses in arterioles from young rats (Fig. 2A). This differential affect of endothelial removal resulted in an elimination of the age-associated difference in the myogenic response. Because age has been reported to increase iNOS expression and function in vascular smooth muscle, we also sought to determine whether age enhanced production of NO by iNOS during myogenic stimulation. Specific blockade of iNOS with aminoguanidine treatment did not alter myogenic responses in denuded arterioles from either young or old rats (Fig. 2B).


Figure 1
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  		Fig. 1 Active and passive pressure responses of coronary arterioles of young and old rats. No age-associated difference was observed in the passive pressure response. The active pressure response was significantly reduced in arterioles from aged rats. Values are means ± S.E. *P<0.05 young versus old.

 

Figure 2
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  		Fig. 2 (A) Myogenic responses in intact and denuded coronary arterioles from young and old rats. Removal of the endothelium increased vasoconstriction in old rats, and eliminated differences between young and old rats. (B) Myogenic responses in denuded arterioles before and after treatment with aminoguanidine. Aminoguanidine had no significant effect on myogenic responses in denuded arterioles from young (P=0.19) and old (P=0.18) rats.

 
3.4. Responses to KCl
Age reduced vasoconstriction to KCl (Fig. 3). Removal of the endothelium significantly increased the constriction to KCl in arterioles from both young and old rats; however, in the absence of the endothelium, the age-associated decrease in responsiveness to KCl remained. Both sensitivity (EC50) and maximal response were reduced in denuded arterioles from aged rats as compared to denuded arterioles from young rats.


Figure 3
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  		Fig. 3 Vasoconstrictor responses to KCl in intact and denuded coronary arterioles from young and old rats. Sensitivity in denuded arterioles from old (EC50=46.8 mM KCl) rats was reduced (P=0.02) compared to denuded arterioles from young (EC50=38.9 mM KCl) rats. Values are means ± S.E. *P<0.05 old versus young; {ddagger}P<0.05 old denuded versus young denuded (maximal response).

 
3.5. Responses to ET
Age decreased vasoconstrictor responsiveness to ET (Fig. 4A). Blockade of the ETB receptor increased ET-induced constriction in arterioles from aged rats and eliminated the differences between young and aged groups (Fig. 4B). Removal of the endothelium augmented constriction to ET in arterioles from both young and old rats (Fig. 5A); however, the enhancement of ET-induced constriction was greater in arterioles from old rats. Following endothelial denudation, the age-related difference in ET responsiveness was eliminated.


Figure 4
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  		Fig. 4 (A) ET concentration–response curve in coronary resistance arterioles from young and old rats. Values are means ± S.E. *P<0.05 old versus young. (B) Effect of BQ-788 (ETB) receptor blockade on ET concentration–response curves in coronary arterioles from young and old rats. Values are means ± S.E.

 

Figure 5
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  		Fig. 5 ET concentration–response curve in coronary arterioles following (A) removal of the endothelium, (B) L-NAME treatment, and (C) endothelial removal+aminoguanidine treatment. In coronary arterioles from young rats, L-NAME treatment (EC50=5.7 x 10–10 M ET) increased (P=0.01) sensitivity to ET compared to control conditions (EC50=1.3 x 10–10 M ET). Values are means ± S.E. *P<0.05 old versus young; +P<0.05 control versus treatment.

 
To evaluate the role of NO in endothelial modulation of ET-induced constriction, ET concentration–response curves were generated after blockade of nitric oxide synthase (NOS) with L-NAME. L-NAME treatment increased baseline constriction in arterioles from both age groups; however, the increase in tone produced by L-NAME was significantly greater in arterioles from aged rats. Similarly, L-NAME treatment significantly increased ET-induced constriction in arterioles from old rats and increased sensitivity (EC50) to ET in young rats (Fig. 5B); however, the inhibitory effect of L-NAME was greater in arterioles from aged rats compared those from young rats. Thus, following L-NAME treatment, contractile responses to ET were greater in arterioles from old rats compared to those of young rats.

To determine whether generation of NO by iNOS in the vascular smooth muscle contributed to the reduction of ET-induced constriction in arterioles from aged rats, ET concentration–response curves were constructed in denuded arterioles in the presence of aminoguanidine. Aminoguanidine treatment did not alter responses to ET in denuded vessels from either young or aged rats (Fig. 5C). To determine whether age-related differences in the effectiveness of NOS blockade was related to a difference in eNOS expression, eNOS mRNA was quantitatively evaluated in coronary arterioles from young and old rats. eNOS mRNA was greater in coronary arterioles from aged rats as compared to those from young rats (Fig. 6).


Figure 6
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  		Fig. 6 eNOS mRNA expression in coronary arterioles from young (n=19) and old (n=22) rats. Values are means ± S.E. *P=0.057 old versus young.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
The purpose of this study was to test the hypothesis that age augments vasoconstrictor responses of coronary resistance arterioles. Contrary to our hypothesis, the present findings indicate that age decreases the reactivity of coronary resistance arterioles to ET, KCl, and changes in transmural pressure. Our hypothesis was based largely on results from previous studies indicating that alterations of coronary vasomotor control at the level of the whole organ and/or epicardial arteries contribute to an age-related increase in coronary vascular resistance [1,3,7,8,13]. Reports in the literature indicate that age increases constriction to ET and sarafotoxin in large coronary arteries, [7,8], and ET-induced reductions in coronary blood flow are greater in aged rats [7]. Similarly, total coronary blood flow responses to pharmacological dilators decrease with age in both rats [1,3] and humans [5]. In contrast, our results demonstrate that age decreases constrictor responses in coronary resistance arterioles, indicating that age dichotomously alters reactivity to vasoconstrictor agents in coronary conduit arteries and coronary resistance arterioles. The current results obtained in resistance arterioles are consistent with reports of age-induced reductions in response to ET and pressure (the myogenic response) in resistance arterioles from mesentery [14] and skeletal muscle [15]. Although our results do not indicate that enhanced vasoconstrictor responses of coronary arterioles contribute to the age-related increase in total coronary vascular resistance, the reduced reactivity to vasoconstrictor stimuli may be a factor in the altered distribution of coronary blood flow previously reported in aged rats [1,3].

Our results indicate that age decreases constrictor responses of coronary arterioles primarily through augmented release of endothelial vasodilator factors. Indeed, enhanced constriction upon removal of the endothelium was consistently greater in coronary arterioles from aged rats. Treatment with L-NAME also increased spontaneous tone (Table 2) and constriction to ET to a greater degree in arterioles from old rats, suggesting the age-induced increase in endothelial modulation of vasoconstrictor responses is due to increased production or bioavailability of nitric oxide. Consistent with this observation, expression of eNOS mRNA was greater in coronary arterioles from aged rats, suggesting an up-regulation of NO-mediated modulation of coronary vasoconstrictor tone. Interestingly, the current results are also consistent with reports where conditions, such as age and hypertension, increase cardiac work and stimulate mechanisms to increase coronary blood flow. In aged humans, cardiac work was increased at rest, and this increase was accompanied by significantly augmented basal coronary blood flow [4]. In rats, age increases central arterial stiffness and pulse pressure [16–18], thereby increasing ventricular afterload, and presumably contributing to the cardiac hypertrophy that is a common feature in several strains of senescent rats [1,17–20]. An up-regulation of eNOS signaling may contribute to the increases in resting coronary blood flow that occur in conjunction with cardiac hypertrophy in aged rats [1,20]. A similar phenomenon occurs in spontaneously hypertensive rats, where basal release of NO increases in the coronary circulation, potentially compensating for increased cardiac work and elevated coronary vascular resistance [21]. An increased endothelial modulation of myogenic vasoconstriction has also been observed in coronary arteries of spontaneously hypertensive rats [13]. In contrast to our findings indicating that NO release increases in coronary resistance vessels, Amrani et al. [22] reported that basal and stimulated release of coronary nitric oxide is reduced in aged rats; however, their measurements reflect nitric oxide production from the entire coronary vasculature. Vasomotor responses are not uniform throughout the coronary vascular tree; therefore, regional adaptations in vascular reactivity may be masked by compensatory changes at other levels of the vascular tree [23,24]. For example, increased resistance in larger arteries of aged rats could stimulate compensatory metabolic dilation or increased production of NO in smaller resistance arterioles. Moreover, the time course of age-related adaptations may differ between epicardial arteries and coronary arterioles; compensatory endothelial mechanisms present in coronary arterioles of senescent rats may no longer be functional in epicardial arteries. Our results are specific to coronary arterioles, again indicating that the effects of age on vascular control mechanisms are heterogeneous within the coronary vascular tree.

Endothelin plays a crucial role in the regulation of coronary vascular tone [25–27]. The concentration of ET is increased [28] and has been implicated in the pathophysiology of a number of coronary disorders, including myocardial infarction [29], coronary artery spasm [30], and atherosclerosis [31]. The vasoconstrictor responses to ET have been shown to increase with age in Langendorf hearts, as well as in isolated rings of the left anterior descending coronary artery [7,8]. In contrast, the present results indicate that vasoconstriction to ET decreases in coronary arterioles from aged rats. ET stimulates both dilation and constriction of arterioles through direct smooth muscle contraction and release of NO from the vascular endothelium [32]. The ETB receptor located on the endothelium signals the release of NO and modulates the vasoconstrictor effects of ET bound to smooth muscle ETA and ETB receptors [33]. Basal or stimulated nitric oxide production also attenuates ET-mediated constriction by displacing ET from its receptor [34]. In this study, blockade of the ETB receptor eliminated the age-related difference in responsiveness to ET, suggesting that responsiveness of ETA receptors on the vascular smooth muscle does not change with age. Moreover, the effect of ETB receptor blockade (i.e., normalized constriction; Fig. 4B) substantiates our observation that the reduced responsiveness to ET in coronary arterioles from aged rats occurs through alterations in the production of endothelial NO. Our current results indicate that (1) the age-related difference in ET-induced constriction occurs through an endothelial pathway, (2) NO production by iNOS does not affect ET-induced constriction in denuded arterioles from either young or old rats, and (3) NO production in intact arterioles attenuates ET-mediated constriction of coronary arterioles of aged rats to a greater degree than those of young rats. Furthermore, incubation with L-NAME, under basal conditions, resulted in a greater enhancement of spontaneous tone in arterioles from aged rats compared to those of young rats (Table 2), and eNOS mRNA expression is increased in coronary arterioles from aged rats. Together, these data, along with our findings that removal of the endothelium eliminates age-related differences in myogenic constriction of coronary arterioles, suggest that age enhances basal endothelial NO production.

The effect of age on NOS function and expression varies between vascular beds [12,35–39]. eNOS protein content has been shown to increase in the aorta of old rats relative to that in young and middle aged rats [36,38]. In tail arteries of aged rats, norepinephrine-induced constriction is reduced due to enhanced iNOS expression and activity [37]. Similarly, iNOS expression increases in coronary resistance arteries from middle-aged rats [39]. In contrast, numerous investigations indicate that NO-mediated vasodilation decreases with age [12,35,39–42]; however, this decrease in NO-mediated function may be related to decreased bioavailability of NO. For example, in coronary resistance arteries of middle-aged rats, flow-induced, NO-mediated dilation is reduced with age, but the impaired response to flow is improved significantly by treatment with scavengers of superoxide [4]. It is possible that reduced availability of NO acts as a stimulus to increase NOS expression and function. van der Loo et al. [36] reported decreased nitric oxide levels in aged rat aorta, accompanied by sevenfold higher expression and activity of eNOS. Although several studies have also indicated that iNOS expression and function in vascular smooth muscle increase with age [37–39,43], we found that iNOS inhibition did not significantly alter vasocnstrictor responses to either pressure or ET in coronary arterioles from old rats. In contrast, eNOS mRNA expression was increased in arterioles from old rats as compared to young rats. These results suggest that the attenuation of constrictor responses in coronary arterioles from aged rats is likely due to increased production of NO by eNOS. Further experiments are necessary to determine whether relative changes in activity of NOS isoforms and/or NO bioavailability underlie enhanced NO-mediated modulation of vasoconstrictor responses in coronary arterioles of aged rats.

In this study, KCl-induced constriction of coronary arterioles was increased with age (Fig. 3). Although removal of the endothelium augmented KCl-induced constriction, age-related differences in the responses to KCl were not eliminated in the absence of the endothelium, as was the case with constrictor responses to ET. In large coronary arteries, Marijic et al. [44] reported that age reduced expression of both voltage and Ca++-activated K+ channels. These findings suggest that K+ channel activity and/or the contribution of K+ channels to coronary vasomotor tone changes with age. Thus, alterations in the responsiveness of both the vascular smooth muscle and the endothelium may occur in coronary arterioles with age; however, the age-related reduction of ET-induced constriction does not appear to result from adaptations of the vascular smooth muscle.

In conclusion, aging reduces the vasoconstriction mediated by changes in pressure, ET, and KCl in rat coronary arterioles. Removal of the endothelium eliminated aging-induced differences in myogenic and ET-induced constriction, indicating an age-associated increase in endothelial modulation of coronary resistance vessel constriction. This enhanced endothelial attenuation of coronary arteriolar constriction appears to result from increased basal release of nitric oxide. These alterations of coronary vascular reactivity may contribute to a compensatory increase in basal coronary blood flow and altered coronary flow distribution in aged animals.


   Notes
 
Time for primary review 23 days


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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