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Cardiovascular Research 2000 47(2):350-358; doi:10.1016/S0008-6363(00)00099-7
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Copyright © 2000, European Society of Cardiology

Experimental heart failure in rats: effects on cardiovascular circadian rhythms and on myocardial β-adrenergic signaling

Klaus Wittea,*, Kai Hub, Johanna Swiateka, Claudia Müssiga, Georg Ertlb and Björn Lemmera

aInstitute of Pharmacology and Toxicology, Faculty of Clinical Medicine Mannheim, University of Heidelberg, Maybachstraße 14–16, D-68169 Mannheim, Germany
bDepartment of Internal Medicine, University of Würzburg, Josef-Schneider-Strasse 2, 97080 Würzburg, Germany

* Corresponding author. Tel.: +49-621-330-030; fax: +49-621-3300-333 klaus.witte{at}urz.uni-heidelberg.de

Received 15 October 1999; accepted 10 April 2000


   Abstract
Top
 Abstract
1 Introduction
 2 Methods
 3 Results
 4 Discussion
 References
 
Objectives: Patients with chronic heart failure frequently show blunted circadian blood pressure profiles. The mechanisms involved in the loss of physiological day–night variation are still unclear, but a continuously active sympathetic nervous system could play a role. The present study evaluated long-term consequences of rat heart failure on cardiovascular circadian patterns in vivo, and on density and function of cardiac β-adrenoceptor subtypes in vitro, as a marker of cardiac adrenergic drive. Methods: Heart failure in rats was induced by coronary artery ligation leading to infarct sizes of >30% of left ventricular circumference. Blood pressure and heart rate were monitored for 10 weeks after infarction using radiotelemetry. Density and function of cardiac β1 and β2-adrenoceptors were measured by radioligand binding and adenylyl cyclase stimulation. Results: During the activity period at night blood pressure and heart rate were lower in rats with heart failure than in sham controls, leading to reduced night–day variation in the heart failure group. Depression of circadian rhythmicity in blood pressure was found over the whole study period, while that in heart rate occurred with a lag-time of several weeks. In failing left ventricles β-adrenoceptors showed reduced high affinity agonist binding, a shift in the β1:β2 ratio towards the β2-subtype, and decreased β1-adrenergic stimulation of adenylyl cyclase. In right ventricles no differences were found between failing and control rats. The blunted nocturnal increase in blood pressure and heart rate as well as β1-adrenergic desensitization were correlated with the severity of left ventricular dysfunction. Conclusions: Heart failure in rats leads to disturbed circadian patterns in blood pressure and heart rate, and to desensitization of cardiac β1-adrenoceptors, indicating chronic sympathetic overactivity.

KEYWORDS Adrenergic (ant)agonists; Blood pressure; Heart failure; Receptors


   1 Introduction
Top
 Abstract
 1 Introduction
2 Methods
 3 Results
 4 Discussion
 References
 
In human congestive heart failure, neurohumoral counterregulatory mechanisms are activated, such as the renin–angiotensin and sympathetic nervous system. Sustained activation of these systems, which support blood pressure, is now known to contribute to the progressive deterioration of cardiac function. Thus, it has been shown that increased plasma concentration [1] and cardiac spill-over of noradrenaline [2] are associated with increased mortality of the patients. Consequently, current treatment guidelines of heart failure include inhibitors of the renin–angiotensin and the sympathetic nervous system [3,4].

It has been shown that the circadian blood pressure profile is altered in patients with heart failure [5–7]. The mechanisms involved in the disturbed variation in blood pressure and heart rate of these patients are still unclear, but a chronically activated sympathetic nervous system is likely to play a role. The reduced day–night variation in heart failure was significantly correlated with reduced left ventricular ejection fraction [5], and with increased plasma concentrations of norepinephrine and atrial natriuretic peptide [7]. Thus, a decreased circadian amplitude in blood pressure and heart rate of patients with congestive heart failure may represent a marker of these neurohumoral adaptations. It is well known that, in human heart failure, adrenergic overactivity is accompanied by downregulation of β1-adrenergic receptors, increased expression of GRK2 (β-adrenoceptor kinase) and of the Gi{alpha}2-protein. Only recently it has been shown that β2-adrenoceptors, which are preserved in the failing heart, are able to activate Gi- as well as Gs-proteins (reviews in Refs. [8,9]). The β2-subtype may, therefore, play two opposing roles in the failing heart, in which β1-adrenoceptors are reduced: its stimulation could maintain cardiac contractility via activation of Gs and a subsequent increase in cAMP formation, or contribute to the depressed β-adrenergic signaling via enhanced coupling to Gi and a further decrease in adenylyl cyclase activity. These findings clearly indicate that changes in the density of β-adrenoceptor subtypes in cardiac diseases may have unexpected consequences with regard to the post-receptor pathways.

In the present study, we used radiotelemetry for long-term monitoring of circadian blood pressure and heart rate rhythms in an established animal model of congestive heart failure, i.e. rats with myocardial infarction after ligation of the left coronary artery. At the end of the monitoring period we measured the density and functional efficacy of β1- and β2-adrenoceptors in ventricular myocardium of failing and control hearts in vitro, in order to evaluate the contribution of both subtypes to the depressed β-adrenergic signaling. While some of these issues have been addressed in previous investigations, the present study is the first to combine long-term cardiovascular monitoring in vivo with detailed data on β-adrenergic function in vitro. Moreover, no information was available so far about changes in the functional contribution of β-adrenoceptor subtypes in the rat model of chronic myocardial infarction.


   2 Methods
Top
 Abstract
 1 Introduction
 2 Methods
3 Results
 4 Discussion
 References
 
2.1 Study protocol
Female Wistar rats (strain Crl:(WI) BR, Charles River, Sulzfeld, Germany) were obtained at an age of 8 weeks, kept under constant environmental conditions with free access to food and tap water and a light–dark cycle of 12:12 h with lights on from 07.00–19.00 h. At the age of 10 weeks rats underwent sham surgery or coronary artery ligation. After 1 week of recovery, left ventricular hemodynamics were measured invasively, and telemetry transmitters were implanted for monitoring of blood pressure and heart rate patterns. Animal experiments were approved by German federal regulations, which meet fully the requirements of the Guide for the Care and Use of Laboratory Animals by the US National Institutes of Health.

2.2 Experimental myocardial infarction
Coronary artery ligation was performed as described in detail in [10]. Briefly, rats were intubated and anesthetized by ether. After a left thoracotomy, the left coronary artery was ligated by a 5-0 Prolene suture. Sham surgery was performed identically without occlusion of the artery.

2.3 Left ventricular hemodynamics
In anesthetized rats the carotid artery was cannulated, and a fluid-filled catheter was placed in the left ventricle. LVSP, LVEDP and LV dP/dtmax were measured by using a Millar micromanometer as described [11].

2.4 Telemetric blood pressure monitoring
Telemetric blood pressure transmitters (TA11PA-C40, Data Sciences, St. Paul, MN, USA) for continuous monitoring of aortic blood pressure were implanted as described in detail [12]. Blood pressure measurements of 4 s duration were taken every 5 min throughout the whole study period and corrected for changes in ambient pressure. Prior to further analyses 15-min averages were calculated from three consecutive readings. Data were then analysed in weekly segments using our previously published software DQ-Fit [13]. For reasons of clarity, only mean values are shown for the day (rest) and night (activity) period. The difference between night and daytime means was calculated as a measure of diurnal variation.

2.5 Preparation of cardiac tissue
At the end of the study rats were sacrificed by decapitation, hearts removed and dissected quickly (within 1 min after decapitation). A transverse section was obtained of the left ventricle including the interventricular septum and fixed in buffered formaldehyde for determination of infarct size. Left and right ventricular tissue was frozen in liquid nitrogen and stored at –60°C until the biochemical assays were performed. Only rats with infarct sizes greater than 30% of the left ventricular circumference were used for the infarct group. Lung weights were taken as a marker of pulmonary congestion.

2.6 Biochemical experiments
Single left ventricles were weighed and immediately homogenized in ice-cold assay buffer (50 mmol/l Tris, 10 mmol/l MgCl2, pH 7.4 at 37°C) using an Ultra-Turrax-homogenizer (IKA, Staufen, Germany) at 20 000 rpm. The resulting suspension was divided into three subsets for determination of β-adrenoceptor density (suspension A), adenylyl cyclase activity (suspension B), and 5-nucleotidase activity (suspension C). Suspension A was homogenized a second time using a Potter S glass-homogenizer (Braun, Melsungen, Germany). After 10 min centrifugation at 25 000 g the supernatants of suspensions A, B and C were discarded. The pellet of suspension A was resuspended in 15 µl assay buffer per mg of tissue and pellet B in 150 µl/mg. An aliquot of resuspended pellet B was stored at –20°C for measurement of G-protein content. Pellet C was homogenized a second time in assay buffer (150 µl/mg) with the addition of 0.3% Triton-X 100 for solubilization of sarcolemmal 5-nucleotidase. After 10 min centrifugation at 25 000 g the supernatant was used for photometric measurement of 5-nucleotidase activity.

Right ventricles were processed in the same manner for determination of adenylyl cyclase activity.

2.6.1 Radioligand binding studies
Cardiac β-adrenoceptor density (Bmax) and affinity (KD) were determined in saturation experiments using the non-selective β-adrenoceptor antagonist [3H]CGP-12177 (0.125–4 nmol/l). For determination of β-adrenoceptor subtypes displacement experiments were performed using increasing concentrations of the β1-adrenoceptor antagonist CGP-20712A in the presence of [3H]CGP-12177 (2 nmol/l). High and low affinity agonist binding and its sensitivity to guanine-nucleotides was studied by competition of [3H]CGP-12177 (2 nmol/l) with isoprenaline in the absence or presence of guanylyl-imidodiphosphate (GppNHp, 100 µM).

2.6.2 Adenylyl cyclase assay
The formation of cAMP was determined in the presence of 3-isobutyl-1-methylxanthine (IBMX) and an ATP-regenerating system as described [14]. Briefly, 0.3 ml of membrane suspension was added to 1.2 ml of prewarmed assay buffer (37°C, pH 7.4) containing 1 mmol/l IBMX, 0.5 mmol/l ATP, 10 mmol/l phosphocreatine and 0.1 mg/ml creatine phosphokinase. The reaction was stopped after 8 min by heating the tubes at 120°C, cAMP formed was measured in the supernatant by radioassay (TRK 432, Amersham Buchler, Braunschweig, Germany).

β-Adrenergic stimulation was determined in the presence of GTP 10 µmol/l by two different approaches [14]. For subtype-selective stimulation, concentration–response curves were constructed using the non-selective β-adrenoceptor agonist isoprenaline and the rather β1-selective β-adrenoceptor agonist noradrenaline in the presence of ICI-118.551 (1 µmol/l), a highly selective β2-adrenoceptor antagonist. In the second approach, cAMP formation was stimulated by a fixed concentration of isoprenaline in the absence or presence of either ICI-118.551 (10–100 nmol/l) or the β1-selective β-adrenoceptor antagonist CGP-20712A (1–10 µmol/l).

2.6.3 5-Nucleotidase activity
Activity of solubilized 5-nucleotidase was measured photometrically at 37°C by using a commercially available reagent kit (Sigma, Deisenhofen, Germany).

2.6.4 G-protein content
The amounts of Gs{alpha} (45- and 52-kDa splice variants) and Gi{alpha} in left ventricular membranes were measured by immunoblotting using a chemoluminescence detection system (RPN 2106, Amersham Buchler) and quantitative densitometry as described in detail [14]. Antibodies used were RM/1 and AS/7 (NEN, Bad Homburg, Germany) for Gs{alpha} and Gi{alpha}, respectively. Specific bands were quantified using defined G-protein standards (recombinant Gs{alpha}, No. 371766, Calbiochem, Bad Soden, Germany; recombinant Gi{alpha}, NEI816, NEN).

2.6.5 Protein measurement
The protein content of the membrane fractions A and B was determined by the method of Lowry et al. [15] with minor modifications. Protein concentration in supernatant of solubilized pellet C was measured with a commercially available Coommassie reagent (Pierce, oud-Beijerland, The Netherlands). Bovine serum albumin was used as standard and dissolved in assay buffer.

2.7 Statistical analyses
Spectral analysis was done in weekly data segments (=672 data points) from individual rats by using the Lomb algorithm [16]. Saturation curves for determination of Bmax and KD, competition curves for calculation of β-adrenoceptor subtypes and agonist binding patterns, and concentration–response curves for analysis of subtype-selective stimulation were fitted to the experimental data using PHARMFIT [17]. Differences between groups were tested by Mann–Whitney U-test, with P<0.05 as level of significance. The statistics software BIAS [18] was used. Data are expressed as mean values±S.D. unless otherwise indicated.

2.8 Chemicals
Isoprenaline, noradrenaline and IBMX were obtained from Sigma. GTP, GppNHp, and the components of the ATP-regenerating system were purchased from Boehringer Mannheim (Mannheim, Germany). [3H]CGP-12177 (4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]-benzimidazol-2-one) was obtained from Amersham, ICI-118.551 (erythro[±]-1-[7-methylindan-4-yloxyl]-3-isopropylamino-butan-2-ol) was from Tocris Cookson (Bristol, UK). CGP-20712A (1-[2-(3-carbamoyl-4-hydroxy-phenoxy)-ethylamino]-3-[4-(1-methyl-4-trifluormethyl-2-imidazolyl)-phenoxy]-propanol-methansulfonate) was donated by Novartis (Basel, Switzerland).


   3 Results
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
4 Discussion
 References
 
3.1 Cardiovascular parameters
One week after experimental myocardial infarction, LVEDP was significantly higher in rats of the infarct group than in sham controls (15.4±5.9 vs. 7.4±4.0 mmHg, U-test P<0.01). LV dP/dtmax (11486±1404 vs. 16840±1560 mmHg/s in sham rats, U-test P<0.001) and LVSP (113.9±8.8 vs. 134.0±7.7 mmHg in sham rats, U-test P<0.001) were significantly lower in rats after myocardial infarction.

Throughout the monitoring period sham operated control rats had clear circadian rhythms in blood pressure and heart rate, with peaks occurring in the activity period at night. Representative circadian profiles during 1 week of monitoring are shown in Fig. 1.


Figure 1
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  		Fig. 1 Circadian variation in locomotor activity (bars), systolic (open circles), diastolic blood pressure (closed circles), and heart rate (closed diamonds) 3 weeks after sham surgery (left) or experimental myocardial infarction (right). Data were sampled by radiotelemetry in unrestrained, freely moving animals using implantable blood pressure transmitters. In rats with heart failure, blood pressure and its circadian rhythmicity was markedly reduced, while heart rate was not affected. Mean values±SEM, n=9 (sham) and n=7 (heart failure).

 
In the heart failure group blood pressure was lower than in sham controls (Fig. 2). The reduction was more prominent during the night, and statistically significant differences were found from week 2 after infarction until week 6 (systolic) and 9 (diastolic), respectively. As a consequence night–day differences in blood pressure, representing a simple estimate of circadian variability, were markedly and significantly reduced in the heart failure group (Table 1). Spectral analysis confirmed the reduced circadian variation in blood pressure early after myocardial infarction (Fig. 3).


Figure 2
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  		Fig. 2 Day and night averages in diastolic (circles), systolic blood pressure (squares) and heart rate (diamonds) in rats after experimental myocardial infarction (open symbols) and sham controls (closed symbols). In the first weeks after infarction, blood pressure was lower in the heart failure group, mainly due to a reduction at night, indicating a loss of the physiological blood pressure rise in the rat's activity period. Heart rate did not differ significantly between heart failure and control rats until week 8. Thereafter the nocturnal rise in heart rate was blunted in the heart failure group. Data from the first week of monitoring are shown separately, because the animals of both groups were still affected by the surgical implantation of the transmitters. Mean values±SEM, n=9 (sham) and n=7 (heart failure), *, P<0.05 (Mann–Whitney U-test, sham versus heart failure).

 

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  		Table 1 Nocturnal increases in blood pressure and heart rate (night minus day mean) in Wistar rats after sham surgery (Sham) or experimental myocardial infarction (Heart Failure)a

 

Figure 3
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  		Fig. 3 Spectral analysis of systolic blood pressure (SBP, left) and heart rate (HR, right) in rats after experimental myocardial infarction (dashed line) and sham controls (solid line). Data were taken from the third week after surgery and correspond to those in Fig. 2. Spectral analysis was done in individual data. The curves represent group means of n=9 (sham) and n=7 (heart failure) individual power spectra, open (heart failure) and closed circles (sham) indicate peak spectral power values±SEM. In rats with heart failure the circadian rhythmic component was clearly reduced in systolic blood pressure and preserved in heart rate early after myocardial infarction.

 
Until 5 weeks after infarction, heart rate (Fig. 2) and its circadian rhythmicity (Fig. 3) did not differ between the groups. Thereafter, nocturnal heart rate values were lower in rats with heart failure, and statistically significant differences between the groups were found in week 9 and 10. Consequently, the difference between night and daytime means was smaller in the heart failure group than in sham controls, and statistical significance was achieved from weeks 6–10 (Table 1).

3.2 Myocardial parameters and lung weight
The average size of myocardial infarction in the heart failure group was 45.0±7.9% of the left ventricular circumference. Absolute and relative (divided by body weight) left as well as right ventricular weight were slightly but not significantly higher in rats of the heart failure group than in control animals. The specific activity of left ventricular 5-nucleotidase was markedly and significantly elevated in failing compared to control tissue (Table 2). Lung weight was significantly higher in the heart failure group, suggesting the presence of pulmonary congestion.


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  		Table 2 Myocardial parameters and lung weight in Wistar rats 10 weeks after sham surgery (Sham) or experimental myocardial infarction (Heart Failure)a

 
3.3 β-Adrenoceptors
The density of left ventricular β-adrenoceptors per mg of protein and their affinity towards the radioligand did not differ between groups. However, after correction for the increased activity of 5-nucleotidase, a marker of sarcolemmal membrane content, overall β-adrenergic receptor density was reduced in preparations from failing left ventricles.

Competition experiments with the β1-selective antagonist CGP-20712A demonstrated a shift in the β1:β2-ratio towards the β1-subtype (Table 3), while the affinities of both subtypes towards CGP-20712A were unchanged. Competition experiments with isoprenaline in the absence or presence of GppNHp showed a reduced proportion of guanine-nucleotide sensitive, high affinity agonist binding sites in the heart failure group (Table 3), indicating desensitized β-adrenoceptors.


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  		Table 3 Left ventricular β-adrenoceptors in Wistar rats 10 weeks after sham surgery (Sham) or experimental myocardial infarction (Heart Failure)a

 
3.4 Adenylyl cyclase
In rats of the heart failure group, stimulation of left ventricular adenylyl cyclase by isoprenaline resulted in a smaller increase in cAMP formation than in tissue of the control animals. Stimulation of the enzyme by noradrenaline in the presence of the β2-adrenoceptor antagonist ICI-118.551 was also less effective in left ventricles of the heart failure rats (Fig. 4), indicating a disturbed function of the β1-adrenoceptor. The predominant reduction in β1-adrenergic efficacy was confirmed by the second experimental approach (Table 4), in which cAMP formation was activated by isoprenaline with or without addition of β1- and β2-selective antagonists, respectively. In the absence of antagonists, the increase in cAMP formation by isoprenaline was reduced by 40% in the heart failure group. A comparable reduction of 40% was also observed after stimulation by isoprenaline when β2-adrenoceptors were blocked. In contrast, the increase in cAMP formation via β2-adrenoceptors by isoprenaline in the presence of the β1-selective antagonist CGP-20712A was slightly but not significantly lower in heart failure than in control rats.


Figure 4
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  		Fig. 4 Increase in left ventricular cAMP formation after stimulation of β-adrenoceptors by isoprenaline (left) and after stimulation of the β1-subtype by noradrenaline (right) in the presence of the β2-adrenoceptor antagonist ICI-118.551 (1 µmol/l). Total β-adrenergic and β1-adrenergic stimulation were significantly less effective in rats with experimental heart failure (open symbols) than in sham controls (closed symbols). The potencies (EC50) of both agonists were not different between the groups of animals. Mean values±SEM, n=10 (sham) and n=7 (heart failure).

 

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  		Table 4 Increase in left and right ventricular adenylyl cyclase activity by β-adrenergic stimulation in Wistar rats 10 weeks after sham surgery (Sham) or experimental myocardial infarction (Heart Failure)a

 
In right ventricular tissue, stimulation of total β-, β1- and β2-adrenoceptors did not differ between heart failure and control animals.

3.5 G-proteins
The left ventricular content of short (45 kDa) and long (52 kDa) splice variants of Gs{alpha} amounted to 1.5±0.4 and 0.8±0.2 pmol/mg in sham controls, versus 1.1±0.3 and 0.8±0.3 pmol/mg in rats of the heart failure group. Myocardial density of Gi{alpha} was 2.4±0.8 pmol/mg in control tissue versus 2.8±0.9 pmol/mg in failing left ventricles. There were no statistically significant differences between the groups with regard to Gs{alpha} and Gi{alpha} proteins.

3.6 Correlation between hemodynamic and biochemical parameters
A significant positive correlation (r=0.75, P<0.001) was found between left ventricular dP/dtmax at study entry and left ventricular β1-adrenergic efficacy, determined at the end of the study. There was no correlation between dP/dtmax and left ventricular β2-adrenergic function, nor between dP/dtmax and right ventricular β-adrenoceptors. In contrast, LVEDP was negatively correlated with the functional efficacy of left ventricular β1 (r=–0.80, P<0.001), β2 (r=–0.73, P<0.001) and right ventricular β1-adrenoceptors (r=–0.59, P<0.05).

Circadian blood pressure and heart rate patterns showed weaker associations with left ventricular hemodynamics at study entry: significant correlations were found for LVEDP and the nocturnal increase in heart rate (r=–0.66, P<0.01), and for LV dP/dtmax and nocturnal increases in systolic (r=0.53, P<0.05), diastolic blood pressure (r=0.60, P<0.05) and in heart rate (r=0.63, P<0.01).


   4 Discussion
Top
 Abstract
 1 Introduction
 2 Methods
 3 Results
 4 Discussion
References
 
In rats with experimental heart failure we observed a reduction in blood pressure at night, i.e. in the activity period of rats, leading to a blunted circadian profile. Initially, heart rate was less affected but, starting at 6 weeks after infarction, its nocturnal increase was also depressed. Heart failure was accompanied by β1-adrenergic desensitization in left ventricular tissue, whereas right ventricular β1-adrenoceptors and the β2-subtype in both ventricles were not affected.

4.1 Cardiac β-adrenergic signaling
Changes in the cardiac adrenergic signaling system after experimental myocardial infarction have been addressed in earlier studies. Discrepant results have been reported with regard to β-adrenoceptor density, which was either unchanged [19,20] or reduced [21–24] in left ventricular tissue of rats with heart failure. However, in studies in which β-adrenoceptor subtypes were differentiated, including the present one, a reduced proportion of β1-adrenoceptors has been observed [22–24]. It is likely that agonist-induced down-regulation plays an important role in the subtype-shift, because the concentration and cardiac turnover of the rather β1-selective endogenous agonist noradrenaline is known to be increased 8 weeks after myocardial infarction in rats [25]. The present study is the first to show that left ventricular β1-adrenergic efficacy is selectively altered in rats with heart failure. A reduced response of cardiac adenylyl cyclase to β-adrenergic stimulation has been observed in earlier studies [21,23], but no information was available regarding subtype-selective stimulation of cAMP formation. In the present study, β1-adrenergic efficacy was markedly reduced in the failing left ventricles but unchanged in right ventricles of the same animals, suggesting a local rather than systemic mechanism of desensitization. This hypothesis is supported by the previous finding that, in patients with isolated right ventricular heart failure, changes in β-adrenergic signaling were restricted to the right ventricle [26]. The density of left ventricular G-proteins was unchanged in rats with heart failure, which is in agreement with previous observations [19]. Therefore, phosphorylation of the β1-adrenoceptor itself by G-protein coupled receptor kinases and/or proteinkinase A is likely to be involved in the functional uncoupling of the β1-subtype. This hypothesis is supported by our observation of reduced high-affinity agonist binding of left ventricular β-adrenoceptors. Finally, it is of interest that functional uncoupling of left ventricular β1-adrenoceptors was positively correlated with the severity of heart failure.

4.2 Cardiovascular circadian rhythms
The sympathetic nervous system in rats shows pronounced circadian rhythmicity, with higher rates of cardiac noradrenaline turnover at night [27]. Moreover, the sympathetic nervous system is known to contribute to circadian variation in blood pressure and heart rate in rats [28]. In the rat model of chronic myocardial infarction, a reduced circadian variation in blood pressure and heart rate was first described by Teerlink and Clozel [29]. The present study extends their observation by showing that after myocardial infarction: (i) the blunted rhythm in blood pressure is an early and persistent finding, (ii) the reduction in diurnal heart rate variation occurs later, and (iii) the severity of left ventricular dysfunction is correlated with the reduction in cardiovascular 24 h variation and accompanied by left ventricular β1-adrenergic dysfunction.

An involvement of the cardiac β-adrenergic signaling pathway in heart rate variability has already been shown in studies with transgenic overexpression of atrial β1-adrenoceptors [30] and cardiac Gs{alpha}-subunits [31]. Both transgenic models are characterized by increased adrenergic drive to the heart and show blunted heart rate variability suggesting sympathovagal imbalance. Most interestingly, in mice overexpressing cardiac Gs{alpha} circadian rhythmicity in heart rate was almost completely lost, and these animals develop a cardiomyopathic phenotype [31]. Taken together, the present findings and the data obtained in transgenic mice suggest that blunted circadian (and short-term) heart rate variability could represent a marker of enhanced adrenergic drive which, in the long term, is deleterious to the heart.

The different time courses of changes in blood pressure and heart rate during the development of heart failure points to an involvement of different underlying mechanisms. In human heart failure, a blunted circadian rhythm in blood pressure has been observed [5–7] which is thought to be due, at least partly, to chronically elevated concentrations of atrial natriuretic peptide [6,7], and to a loss of its circadian rhythmicity [6]. An increase in atrial natriuretic peptide has also been observed in rats after myocardial infarction [32] but, unfortunately, no information is available about its circadian rhythmicity. In the present study, night–day variation in heart rate was significantly correlated with LV dP/dt as well as LVEDP, whereas that in blood pressure showed a weaker correlation with LV dP/dt only. Because LVEDP is regarded as a sensitive marker of congestive heart failure one may, indeed, conclude that the reduced circadian variation in blood pressure in rats after myocardial infarction is not directly linked to cardiac failure. On the other hand, the increase in atrial natriuretic peptide observed in rats with heart failure could indicate a state of sodium retention which, in salt-sensitive subjects, is associated with blunted 24-h blood pressure profiles, and with inappropriately high sympathetic nervous activity at night [33].

In conclusion, the present study demonstrates for the first time that left ventricular dysfunction induced by myocardial infarction leads to subtype-selective functional desensitization of cardiac β1-adrenoceptors, and is accompanied by blunted circadian profiles in blood pressure and heart rate. While changes in heart rate variation are most likely due to chronic activation of the sympathetic nervous system, it remains to be shown whether the blunted circadian blood pressure profile involves additional neurohumoral mechanisms.

Time for primary review 24 days.


   Acknowledgements
 
This study was supported by the Forschungsfonds der Fakultät für Klinische Medizin Mannheim, no. 34/96.


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

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