Detrimental effects of beta-adrenergic stimulation on beta-adrenoceptors and microtubules in the heart

Effect of Digoxin on Ventricular Remodeling and Responsiveness of beta-Adrenoceptors in Chronic Volume Overload

BACKGROUND: Digoxin improves baroreflex function and reduces neurohumoral activation in severe heart failure, but it is uncertain how digoxin affects ventricular remodeling and progression to left ventricular dysfunction. In addition, the effect of digoxin in in vitro beta-adrenoceptor density and function, and contractile reserve in vivo is not well understood. METHODS AND RESULTS: To study this, we compared digoxin with placebo treatment in rats with chronic volume overload induced by aortocaval fistula and in sham-operated control animals. Left ventricular end-diastolic cavity dimensions (LVDd) and wall thickness were measured weekly by in vivo transthoracic echocardiography, and left ventricular mass (LVM) and percent fractional shortening (%FS) were calculated. Six weeks after fistula creation, simultaneous echocardiographic and invasive hemodynamic evaluation at rest and in response to incremental dobutamine (1-10 µg/kg/min intravenously) were measured. Myocardial plasma membrane beta-adrenoceptor density and maximal adenylate cyclase responses (V(max)) to isoproterenol, 5'-guanylylimi dodiphosphate, and forskolin were measured in vitro. Volume overload induced progressive increases in LVDd and LVM over the 6-week study period. Percent fractional shortening at rest, and the change in %FS in response to dobutamine stress were dramatically reduced 6 weeks after fistula creation. Although 6-week fistula animals had unchanged beta-adrenoceptor density (B(max)) and binding affinity (K(d)) as compared with controls, maximal adenylate cyclase responses to stimulation in vitro (V(max)) were markedly reduced. Digoxin treatment prevented this loss of responsiveness of adenylate cyclase but did not affect beta-adrenoceptor density or affinity in vitro. Digoxin had no effect on LVDd, LVM, %FS, or the response to dobutamine infusion in vivo. CONCLUSIONS: Although digoxin prevented beta-adrenoceptor desensitization and improved in vitro myocardial adenylate cyclase response, the cardiac response to adrenergic stimulation in vivo was not significantly improved. These results suggest that the role of beta-adrenoceptor desensitization in the progression from volume overload hypertrophy to left ventricular dysfunction and heart failure may be less important than previously thought. Furthermore, although digoxin treatment did produce modest hemodynamic benefits, it did not prevent progressive remodeling in this model.

Beta-adrenergic blockade in dilated cardiomyopathy, ischemic cardiomyopathy, and other secondary cardiomyopathies

Beta-blockers were initially given to patients with chronic heart failure due to ischemic heart disease and resting tachycardia. The prompt effect on severe backward heart failure was directly associated with an immediate fall in heart rate. This observation led to long-term administration to patients with idiopathic dilated cardiomyopathy and, later, to patients with ischemic cardiomyopathy and secondary cardiomyopathies as well. Due to marked down-regulation of beta receptors, patients with heart failure are extremely sensitive to beta blockade. A test dose of metoprolol 5 mg b.i.d. for 2 days is recommended to select patients for long-term beta-blockade, followed by careful titration with increment in dose over 6 weeks. One important effect of beta-blockade in the early phase of treatment is a reduction in the myocardial energy demand early after the onset of long-term treatment. After 1 month of treatment with beta-blockers, marked improvement of diastolic function is observed. This effect might be attributed to inhibition of calcium overload. After 3 months of treatment, an increase in ejection fraction can be observed, which might be attributed to upregulation of beta receptors. The withdrawal of long-term treatment was followed by a deterioration of heart function in 61% of patients and improvement was seen after reinstitution of beta-blockade. There was an increase in cardiac index and stroke work index at rest as well as during supine exercise. A marked fall in left ventricular filling pressure at rest and unchanged filling pressure during supine exercise was noted, while exercise capacity increased by 25%. A similar pattern was seen in patients with ischemic cardiomyopathies and other secondary cardiomyopathies. However, the increase in ejection fraction in the ischemic cardiomyopathy group was lower (0.06) compared to the groups with dilated cardiomyopathy and other secondary cardiomyopathies (0.18).