Reductions in muscle sympathetic nerve activity after long-term metoprolol for dilated cardiomyopathy: preliminary observations

Heart failure and its treatment from the perspective of sympathetic nerve activity

Heart failure is the end-stage phenotype of several cardiac diseases. The number of heart failure patients is increasing in accordance with an increase in the number of elderly people. The prognosis of heart failure is poor and its 5-year death rate is comparable to that of stage III cancer. It is important to understand the essential mechanism of the worsening prognosis of heart failure and to practice effective treatment from the perspective of improving the prognosis of heart failure based on its essential mechanism. Plasma noradrenaline level is a good predictor of the survival rate of heart failure patients, and sympathetic nerve activity is augmented in patients with heart failure as evidenced by a higher noradrenaline release rate (spillover) from the sympathetic nerve endings especially in the heart and kidney. Noradrenaline release is regulated by presynaptic receptors at the sympathetic nerve endings, and the kidney affects the sympathetic nerve activity. Although the short-term reflex augmentation of sympathetic nerve activity caused by reduced cardiac function may help to improve cardiac function, long-term augmentation of sympathetic nerve activity damages the heart and deteriorates the prognosis of heart failure. Currently, drugs such as angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, β-blockers, mineralocorticoid antagonists, ivabradine, angiotensin receptor-neprilysin inhibitor, and sodium-glucose transport protein 2 inhibitors, are used for the treatment of heart failure, and had a good prognosis in large randomized, controlled clinical trials. Interestingly, the same characteristics in common of these drugs is the ability to optimize excessively augmented sympathetic nerve activity. This review discusses insights into essential mechanism of heart failure that determines the prognosis of heart failure, focusing on the interaction between sympathetic nerve activity and anti-heart failure drugs currently recommended by the 2021 guidelines of the Japanese Circulation Society and the Japanese Heart Failure Society for heart failure treatment.

Muscle sympathetic nervous activity in depressed patients before and after treatment with sertraline

BACKGROUND

Sympathetic hyperactivity is one of the mechanisms involved in the increased cardiovascular risk associated with depression, and there is evidence that antidepressants decrease sympathetic activity.

OBJECTIVES

We tested the following two hypotheses: patients with major depressive disorder with high scores of depressive symptoms (HMDD) have augmented muscle sympathetic nervous system activity (MSNA) at rest and during mental stress compared with patients with major depressive disorder with low scores of depressive symptoms (LMDD) and controls; sertraline decreases MSNA in depressed patients.

METHODS

Ten HMDD, nine LMDD and 11 body weight-matched controls were studied. MSNA was directly measured from the peroneal nerve using microneurography for 3 min at rest and 4 min during the Stroop color word test. For the LMDD and HMDD groups, the tests were repeated after treatment with sertraline (103.3 +/- 40 mg).

RESULTS

Resting MSNA was significantly higher in the HMDD [29.1 bursts/min (SE 2.9)] compared with LMDD [19.9 (1.6)] and controls [22.2 (2.0)] groups (P = 0.026 and 0.046, respectively). There was a significant positive correlation between resting MSNA and severity of depression. MSNA increased significantly and similarly during stress in all the studied groups. Sertraline significantly decreased resting MSNA in the LMDD group and MSNA during mental stress in LMDD and HMDD groups. Sertraline significantly decreased resting heart rate and heart rate response to mental stress in the HMDD group.

CONCLUSION

Moderate-to-severe depression is associated with increased MSNA. Sertraline treatment reduces MSNA at rest and during mental challenge in depressed patients, which may have prognostic implications in this group.

Is kidney ischemia the central mechanism in parallel activation of the renin and sympathetic system?

In chronic kidney disease simultaneous activation of the renin-angiotensin and sympathetic systems occurs. Kidney ischemia seems to play a key role in the pathogenesis. This review firstly summarizes experimental and clinical evidence in chronic kidney disease supporting this idea and addresses the possibility that this mechanism is also relevant in some other disease conditions.

The effects of medications on circulating levels of cardiac natriuretic peptides

Circulating cardiac natriuretic peptide levels are being used increasingly in a range of clinical circumstances. Since it is evident that drugs used in the treatment of cardiovascular disorders can modulate natriuretic peptide levels, we here review the literature documenting these effects. Diuretics, blockers of the renin-angiotensin system, vasodilator agents, dopamine-like agonists, amiodarone, and perhaps allopurinol and statins suppress natriuretic peptide levels, most obviously in heart failure. Beta-blockers stimulate natriuretic peptide concentrations in hypertensive subjects, whereas in heart failure they have little effect or are stimulatory in the short term and inhibitory with sustained therapy. Digitalis compounds and aspirin tend to increase natriuretic peptide levels, and calcium channel blocking agents have varying effects depending on the individual drug and duration of administration. The effects of other drugs are less clear. Additional information is needed regarding the effects of medications along with dissection of the role of altered cardiac secretion versus changes in plasma clearance as explanation for drug-induced perturbations in natriuretic peptide concentrations. In the meantime, clinicians need to consider the known effects of medications when interpreting plasma levels of the cardiac natriuretic peptides.

Impact of 6 months of therapy with carvedilol on muscle sympathetic nerve activity in heart failure patients

BACKGROUND

The long-term effects of carvedilol on muscle sympathetic nerve activity (MSNA) and muscle blood flow at rest and exercise in patients with chronic heart failure (CHF) remain unknown.

METHODS AND RESULTS

Twenty-six patients (New York Heart Association class II-III) were randomized to carvedilol or placebo. Blood pressure, heart rate, MSNA, and forearm vascular resistance (FVR) at rest and during isometric forearm exercise (10% and 30% maximal voluntary contraction) were assessed before and after 6 months. Seven patients did not complete the study. Paired data were obtained in 19 (carvedilol 12, placebo 7). Carvedilol significantly decreased MSNA levels and heart rate at rest (-13 +/- 2 versus 3 +/- 8 bursts/min, P = .0001 and -16 +/- 3 vs -4 +/- 6 bpm, P = .05, respectively) and peak exercise (30% = -20 +/- 5 versus -3 +/- 7 bursts/min, P = 0.05 and -19 +/- 4 versus -4 +/- 6 bpm, P = 0.03, respectively) when compared with placebo. Carvedilol did not change a magnitude of response of MSNA and heart rate during exercise (-10 +/- 3 versus -7 +/- 2 bursts/min, P = 0.7 and 11 +/- 3 versus 6 +/- 1, P = .6, respectively). FVR was unchanged by carvedilol. When MSNA was quantified by burst incidence, the strength of reduction in MSNA was attenuated but still greater than placebo.

CONCLUSIONS

Carvedilol reduces MSNA in patients with CHF. Carvedilol does not reduce FVR at rest or during isometric exercise.

Effect of erythropoietin on cardiovascular prognosis parameters in hemodialysis patients

BACKGROUND

Renal anemia is an important determinant for left ventricular hypertrophy in dialysis patients and an independent prognosis parameter for the cardiovascular survival in dialysis patients. In addition, an autonomic dysfunction is associated with the uremic state and influences the cardiovascular risk in patients with end-stage renal disease (ESRD).

METHODS

We investigated in this prospective longitudinal study the effect of hemoglobin normalization by a chronic treatment with recombinant human erythropoietin (rhEPO) on cardiovascular prognosis parameters in 23 patients on chronic hemodialysis with renal anemia (hemoglobin concentration < or =10.5 g/dL) and echocardiographically proven left ventricular hypertrophy. We studied muscle sympathetic nerve activity measured by microneurography; cardiopulmonary baroreflex activity by lower-body negative pressure (LBNP-) testing; left ventricular structure and mass index (LVMI) by echocardiography; blood pressure by 24-hour readings; peripheral blood flow and vascular resistance by plethysmography before (U1) and after 7 months of chronic rhEPO treatment (U2).

RESULTS

In the anemic state, mean (+/- SD) muscle sympathetic nerve activity in ESRD was elevated (U1 rest, 34 +/- 13 bursts per minute) and cardiopulmonary baroreflex response during LBNP markedly lacking (U1 -15 mm Hg, 34 +/- 13 bursts per minute) reflecting a severely impaired autonomic function. Normalization of the hemoglobin concentration by chronic rhEPO treatment (U1, 10.5 +/- 0.9 g/dL versus U2, 13.4 +/- 3.1 g/dL, P <0.001) did not influence sympathetic nerve activity (U2, 34 +/- 15 bursts per minute, NS) and cardiopulmonary baroreflex sensitivity did not change (U2 -15 mm Hg, 37 +/- 16 bursts per minute, NS). LVMI decreased significantly after chronic treatment with rhEPO (U1, 134 +/- 26 g/m2 versus U2, 97 +/- 25 g/m2, P < 0.001) and left ventricular geometry developed from an asymmetric to a symmetric configuration (U1, relative wall thickness 0.58 versus U2, 0.43, P < 0.001). Under treatment with rhEPO, 24-hour systolic and diastolic blood pressure did not increase (systolic U1, 132 +/- 4 mm Hg versus U2, 128 +/- 3 mm Hg, NS, and diastolic U1, 76 +/- 2 mm Hg versus U2, 73 +/- 2 mm Hg, NS). Peripheral blood flow (U1, 6.1 +/- 3.3 mL/100 mL/min versus U2, 6.2 +/- 0.6 mL/100 mL/min, NS) as well as forearm vascular resistance (U1, 15.7 +/- 3.3 mm Hg/mL/100 mL versus U2, 14.9 +/- 3.1 mm Hg/mL/100 mL, NS) did not change by chronic rhEPO treatment.

CONCLUSION

Normalization of hemoglobin by chronic rhEPO treatment in dialysis patients has beneficial cardiovascular effects with regression of left ventricular hypertrophy and improvement of left ventricular geometry. However, a reduction of sympathetic overactivity or a resetting of baroreceptor sensitivity by a rhEPO treatment in dialysis patients in the medium-term could not be demonstrated. The reason for this may be the complex and multifactorial pathomechanism of autonomic dysfunction and cardiovascular disease in ESRD.

Effects of carvedilol therapy on QT-interval dispersion in congestive heart failure: is there a difference in the elderly?

In cases of chronic congestive heart failure, QT-interval dispersion is a strong predictor of death. Carvedilol therapy appears to decrease QT-interval dispersion. We investigated whether carvedilol reduces QT-interval dispersion in congestive heart failure and whether this pharmaceutical agent has additional effects on elderly patients. Seventy-seven ambulatory patients who had chronic congestive heart failure were evaluated for hypertension, diabetes mellitus, smoking, alcohol abuse, concomitant medications, and QT-interval dispersion. Carvedilol therapy was then initiated. Six months later, we re-evaluated the same variables, as well as morbidity and mortality rates, and number of hospitalizations. The patients were divided into 2 groups: Group I, aged < 65 years (n = 42); and Group II, aged > or = 65 years (n = 35). Statistics were analyzed with the Student's t-test chi2 test, and Cox regression model. At 6 months, both groups showed significantly decreased QT-interval dispersion values compared with baseline values (76.9 +/- 29.3 vs 104.3 +/- 41.5 ms, respectively; P < 0.0001). An elevated QT-interval dispersion value at baseline increased morbidity (P = 0.041) but not hospitalization (P > 0.05). Group II had a smaller reduction in QT-interval dispersion than did Group I (24.41 +/- 29.36 and 30.98 +/- 32.70 ms, respectively), but this difference was not significant. We conclude that in ambulatory patients with chronic congestive heart failure, long-term carvedilol therapy significantly decreases QT-interval dispersion, and this effect is uniformly distributed between patients aged < 65 years and those aged 265 years.

Lack of evidence for peripheral alpha(1)- adrenoceptor blockade during long-term treatment of heart failure with carvedilol

OBJECTIVES

The purpose of this study was to determine whether carvedilol's alpha(1)-adrenoceptor antagonism persists during long-term therapy of patients with congestive heart failure (CHF).

BACKGROUND

Carvedilol and metoprolol differ in that carvedilol also antagonizes beta(2)- and alpha(1)-adrenoceptors. We hypothesized that in contrast to metoprolol, carvedilol would increase calf vascular conductance (CVC), blunt neurally mediated vasoconstriction and attenuate neuroeffector transfer function gain.

METHODS

We randomized 36 patients with CHF (age 55 +/- 1 years, ejection fraction 19 +/- 1%, means +/- SE) to either drug. Blood pressure (BP), heart rate, muscle sympathetic nerve activity (MSNA) and CVC were assessed before and after four months of treatment. The variability of BP and MSNA was determined using fast Fourier transformation.

RESULTS

Paired data were obtained in 23 (carvedilol, 13; metoprolol, 10) subjects. Both beta-blockers decreased heart rate, but neither affected mean BP or CVC (carvedilol: 0.016 +/- 0.002 to 0.018 +/- 0.003 U; metoprolol: 0.020 +/- 0.002 to 0.020 +/- 0.004 U). Isometric handgrip exercise (30% of maximum) increased heart rate, mean BP and MSNA. The calf vasoconstrictor response to handgrip exercise was not affected by carvedilol (from 16 +/- 6 resistance U to 25 +/- 10 resistance U, NS). The gain of the transfer of oscillations in MSNA into BP under resting conditions was not attenuated by carvedilol.

CONCLUSIONS

Carvedilol did not increase CVC, blunt the calf vasoconstrictor response to handgrip or attenuate the gain of the neuroeffector transfer function, indicating the absence of functionally important peripheral alpha(1)-adrenoceptor antagonism during long-term treatment of CHF.

Nonselective versus selective beta-adrenergic receptor blockade in congestive heart failure: differential effects on sympathetic activity

BACKGROUND

Activation of the sympathetic nervous system has important prognostic implications in chronic heart failure. Nonselective versus selective beta-adrenergic receptor antagonists may have differential effects on norepinephrine release from nerve terminals mediated by prejunctional beta(2)-adrenergic receptors.

METHODS AND RESULTS

Thirty-six patients with chronic heart failure were randomized to the nonselective beta-blocker carvedilol or the selective beta-blocker metoprolol (double-blind). Measurements of hemodynamics and cardiac and systemic norepinephrine spillover as well as microneurographic recordings of muscle sympathetic nerve traffic were made before and after 4 months of therapy. In the carvedilol group (n=17), there were significant reductions in both total body (-1.7+/-0.5 nmol/min, P<0.01) and cardiac norepinephrine spillover (-87+/-29 pmol/min, P<0.01). By contrast, in the metoprolol group (n=14), there were no significant changes in total body or cardiac norepinephrine spillover. Responses in the carvedilol group were significantly different from those observed in the metoprolol group (P<0.05). Both agents caused a reduction in heart rate and increases in pulse pressure, although mean arterial pressure did not change. Importantly, microneurographic measures of sympathetic nerve traffic to skeletal muscle did not change in either group.

CONCLUSIONS

Therapy with carvedilol caused significant decreases in systemic and cardiac norepinephrine spillover, an indirect measure of norepinephrine release. Such changes were not observed in patients treated with metoprolol. There was no effect of either agent on sympathetic efferent neuronal discharge to skeletal muscle. These findings suggest that carvedilol, a nonselective beta-blocker, caused its sympathoinhibitory effect by blocking peripheral, prejunctional beta-adrenergic receptors.

Effects of beta-blockers on neurohormonal activation in patients with congestive heart failure

The effect of beta-adrenoceptor antagonists (beta-blockers) on neurohormonal activation in patients with congestive heart failure has been the subject of study in numerous small clinical trials. Short term therapy with beta-blockers is associated with a variable acute neurohormonal response which may be determined by the pharmacology of the agent under study and the baseline characteristics of the patient population. Long term therapy with beta-blockers devoid of intrinsic sympathomimetic activity (partial agonist activity) is associated with evidence of decreased plasma markers of activation of the sympathetic nervous system, the renin-angiotensin system, and endothelin-1. Beta1-selective and nonselective beta-blockers appear to be associated with evidence of decreased neurohormonal activation, with differential effects on beta-adrenoceptor density. Agents with partial agonist activity appear to differ from pure antagonists, with some studies reporting evidence of increased neurohormonal activation. The mechanisms by which beta-blockers reduce neurohormonal activation and the clinical relevance of changes in adrenergic function to their use in the treatment of heart failure require further investigation.

Echocardiographic parameters in reversible idiopathic dilated cardiomyopathy

BACKGROUND

Reversible idiopathic dilated cardiomyopathy (IDCM) is a rare entity. It has been hypothesized that the degree of left ventricular end diastolic dilation is an important independent predictor of prognosis. We undertook a study to identify cases of reversible IDCM and to evaluate the echocardiographic findings in these patients.

METHODS

We identified 5 patients with IDCM who showed normalization of left ventricular function over a follow up period of 5 months. The findings were compared with those of 10 patients with IDCM who did not show improvement of left ventricular function. The mean (+/-SE) left atrial and left ventricular (LV) dimensions and ejection fraction at baseline and follow-up were compared in both groups.

RESULTS

There were no statistical differences between the mean (+/-SE) left atrium sizes in the cases and control group at baseline [4.52 (+/-0.24) cm v4.6 (+/-0.13) cm; P = 0.758]. Also, no differences were observed between mean (+/-SE) LV dimensions in diastole and systole in both groups at baseline [LV diastole, 6.72 (+/-0.35) cm versus 6.56 (+/-0.22) cm; P = 0.711; LV systole, 5.6 (+/-0.27) cm versus 5.59 (+/-0.29) cm; P = 0.712] as well as in mean (+/-SE) ejection fraction [24% (+/-3.96) versus 21.7% (+/-3.30); P = 0.623].

CONCLUSION

Based on initial echocardiographic parameters, chamber dimensions and baseline LV ejection fraction are not predictors of reversibility.

Effect of beta-blocker on left ventricular function and natriuretic peptides in patients with chronic heart failure treated with angiotensin-converting enzyme inhibitor

To evaluate whether or not beta-blockers can improve the condition of patients with heart failure treated with a combination of diuretics, digitalis and angiotensin-converting enzyme inhibitor (ACEI), 52 patients with chronic heart failure who have been treated with ACEI for more than 6 months were enrolled. They were divided into 2 groups: 26 patients continued the same therapy another 6 months or more (group A), and 26 patients were given oral metoprolol for 6 months or more, in addition to the ACEI (group B). Echocardiographic parameters and atrial and brain natriuretic peptides (ANP, BNP) were measured. The left ventricular dimensions at end-diastole and end-systole were significantly decreased and fractional shortening was significantly increased in group B after 6 months' treatment with the beta-blocker, but these parameters remained unchanged in group A. Plasma levels of both ANP and BNP were significantly decreased in group B, but remained unchanged in group A. These results indicate that concomitant beta-blocker therapy can improve left ventricular function and attenuate plasma ANP and BNP levels in patients with chronic heart failure treated with ACEI.

Effect of beta-blocker therapy on severe ventricular arrhythmias in patients with idiopathic dilated cardiomyopathy

Beta-blocker therapy has been shown to improve cardiac function and prognosis in patients with idiopathic dilated cardiomyopathy (DCM). However, whether beta-blockers reduce severe ventricular arrhythmias and sudden cardiac death has not been clarified. The present study was designed to investigate the effects of beta-blockers on non-sustained ventricular tachycardia (VT) and sudden cardiac death in patients with DCM. Sixty-five patients with DCM treated with diuretics, digitalis and angiotensin-converting enzyme inhibitors were assigned to receive beta-blockers (n = 33) or not (n = 32). Mean follow-up was 53+/-30 months. The echocardiographic indices of cardiac function, the incidence of non-sustained VT on Holter monitoring electrocardiograms, and sudden cardiac death rate were compared between the 2 groups. Comparable improvement in cardiac function on echocardiograms was found in the 2 treatment groups. The patient group treated with beta-blockers showed a significant reduction in the prevalence of VT (from 43 to 15%, p<0.05) and the development of new episodes of VT (5 vs. 16%) compared to the group without beta-blockers. The sudden cardiac death rate did not differ between the 2 groups. The results of the present study suggest that beta-blockers are effective in reducing severe ventricular arrhythmias in patients with DCM.

Neurohumoral activation and progression of heart failure: hypothetical and clinical considerations

The model for heart failure has changed radically over the past 20 years. No longer a simple hemodynamic paradigm of pump dysfunction, heart failure is now characterized as a complex clinical syndrome with release of many neurohormones and cytokines, which are believed to be most responsible for progression of disease. This change in our understanding of the pathophysiology of heart failure has important therapeutic implications. Drugs designed to influence the myocardial contractile state have been found over the past few decades to have either a neutral or an adverse effect on long-term survival, whereas agents designed to block the renin-angiotensin-aldosterone and other neurohormonal systems have proved to be remarkably effective treatment. Recently, drugs designed to block excessive sympathetic nervous system activity have been demonstrated in well-controlled studies to be safe and effective forms of therapy for heart failure. Carvedilol, a nonselective beta-adrenergic blocker with alpha1-blocking and antioxidant properties, is associated with prevention of progression of heart failure as manifested by improvement in left ventricular (LV) function, reduction in heart size, and improved survival in patients with New York Heart Association functional Class II and III symptoms. This improvement is observed equally in patients with ischemic and non-ischemic heart failure. It is tempting to speculate that beta-adrenergic blockers prevent the progression of heart failure by reducing LV mass and LV chamber size. In essence, carvedilol, and perhaps other beta-adrenergic blockers, appear to abrogate relentless LV remodeling which is typically associated with progression of heart failure. The combination of angiotensin-converting enzyme inhibitors and beta-adrenergic blockers may be particularly effective in this regard, although more data on beta-adrenergic blockers in patients with advanced heart failure are needed. Data from experimental heart failure animal models also suggest that endothelin (ET) subtype A (ET(A)) receptor blockers have the potential to lessen the pace of progressive LV remodeling. As our understanding of the neuroendocrine response to diminished cardiac performance improves, novel and even more imaginative neurohormonal and cytokine antagonists are likely to emerge as important new treatments for both hypertension and heart failure.

Restoring function in failing hearts: the effects of beta blockers

Until recently, clinical management of congestive heart failure was purely palliative. The drugs used in patients with failing hearts--digoxin, vasodilators, and positive inotropic agents--improved contractility, reversed hemodynamic abnormalities, and enhanced functional status, but they failed to confer a survival benefit. Indeed, the use of inotropic agents often resulted in excess mortality--a paradox explained in part by the pharmacological properties of these agents, which increase production of cAMP, the intracellular messenger for the beta-adrenergic system. The short-term pharmacological benefits of these drugs may be offset by deleterious long-term biological effects on the heart muscle itself. The use of beta-blockers in heart failure is counterintuitive, given that their initial pharmacological effect is to reduce heart rate and contractility in a faltering heart, thus producing an effect diametrically opposed to that of inotropic agents. However, it is becoming more clear that beta-blocker therapy in patients with heart failure not only improves left ventricular function, but may actually reverse pathological remodeling in the heart. Accumulating clinical evidence indicates that these beneficial changes are the result of secondary biological changes in the myocardium rather than a response to the pharmacological effects of the drugs themselves. Mounting evidence suggest that these agents may prolong survival in patients with heart failure, and ongoing clinical trials may soon confirm these preliminary findings.

Acute effects of beta 1-selective and nonselective beta-adrenergic receptor blockade on cardiac sympathetic activity in congestive heart failure

BACKGROUND

beta-Blockers may reduce cardiac sympathetic activity in patients with heart failure by antagonizing beta-adrenergic receptors that facilitate sympathetic outflow to the heart. To explore this possible effect of beta-blockade, we measured cardiac norepinephrine spillover responses in patients with heart failure after the acute administration of either propranolol, a nonselective beta-blocker, or metoprolol, a beta 1-selective agent.

METHODS AND RESULTS

Eighteen patients were studied. Repeated intravenous doses of propranolol (0.5 mg; nine patients; left ventricular ejection fraction, 14 +/- 2%) or metoprolol (1.0 mg; nine patients; left ventricular ejection fraction, 18 +/- 2%) were administered until one of the following end points was reached: a 15% decrease in heart rate, left ventricular +dP/dt, or mean arterial blood pressure or a 5 mm Hg increase in left ventricular end-diastolic pressure. Propranolol (mean dose, 2.0 mg) and metoprolol (mean dose, 3.6 mg) caused similar reductions in heart rate, +dP/dt, and coronary sinus plasma flow. Cardiac norepinephrine spillover was reduced after propranolol (277 +/- 55 to 262 +/- 53 pmol/min, P < .05) but was increased after metoprolol (233 +/- 57 to 296 +/- 82 pmol/min, P < .05). In a comparison of the two groups, the decrease in spillover after propranolol was significantly different than the increase seen after metoprolol (P < .01).

CONCLUSIONS

The administration of a beta 1-selective antagonist was associated with increased cardiac norepinephrine spillover. In contrast, the administration of a nonselective beta-blocker until similar hemodynamic end points were reached caused a reduction in norepinephrine spillover. This suggests that in patients with heart failure, nonselective beta-blockade may have favorable inhibitory effects on cardiac sympathetic activity.