Transregulation of adenylyl-cyclase-coupled inhibitory receptors in heart failure enhances anti-adrenergic effects on adult rat cardiomyocytes.

Abstract

In congestive heart failure (CHF), a desensitisation of stimulatory beta-receptors and of adenylyl cyclase in the heart is associated with an increase in inhibitory Gi proteins. To investigate whether the regulation of the Gi-mediated inhibitory side of the adenylyl cyclase system may be of functional importance in the failing myocardium, the contractile response of isolated adult cardiomyocytes to stimulation of inhibitory muscarinic M2 and A1 adenosine receptors was analysed. CHF was induced in rats by banding of the ascending aorta and was verified by doubling of lung wet weight. After four weeks, contraction amplitude (delta L) and the velocity (dL/dtmax) of isolated ventricular cardiomyocytes during electrical field stimulation in the presence of 1 mM Ca2+ were measured using video micrometry. Contractile responses of failing cardiomyocytes to 5 mM Ca2+ were unchanged. The response to increasing concentrations of the beta-adrenergic agonist, isoproterenol (0.1-30 nM), and to forskolin (0.1 nM-1 microM) were significantly blunted. When A1 receptors were activated with N6-(R-phenyl-isopropyl)-adenosine (PIA; 0.01-1 microM) in the presence of 3 nM isoproterenol, contractility was unchanged in cells compared with those from sham-operated rats, but delta L was reduced by up to 23% and dL/dtmax by 35% in failing cardiomyocytes (P < 0.01), demonstrating an enhanced inhibitory effect of A1 receptors. The response to the M2 receptor agonist, carbachol (0.01-3 microM), was augmented to a comparable extent (delta L, -22%, dL/dtmax, -39%; P < 0.01). In CHF, the inotropic responses to beta-receptor-stimulation and to direct stimulation of adenylyl cyclase, but not to Ca2+, are diminished due to desensitisation of the stimulatory side of the adenylyl cyclase signal transduction system. In parallel, the responses to inhibitory receptors are augmented, leading to a pronounced Gi-mediated negative inotropic effect on failing heart muscle cells. Those anti-adrenergic effects could contribute to the contractile dysfunction of the failing heart. Reversal of the sensitisation to inhibitory stimuli might be one of the desirable mechanisms of medical therapy in CHF.