Characterization of postjunctional adrenoceptor subtypes in isolated rat myocardial cells in culture

The effects of agonists and antagonists, specific for the different adrenoceptor subtypes on the automaticity of cultured ventricular cells from postnatal rat, were studied. Chronotropic responses were assessed by recording monophasic action potentials using intracellular microelectrodes. Contraction was assessed by an electro-optical procedure. (-)-Isoproterenol, salbutamol, (-)-phenylephrine, and methoxamine increased the spontaneous rate in a dose-dependent manner, but the stimulatory potencies of alpha-adrenoceptor agonists were weaker than those of the beta-alternates. The frequency response to (-)-isoproterenol was inhibited by atenolol but not by butoxamine. Atenolol was also more effective than butoxamine in antagonizing the rate acceleration by salbutamol. The chronotropic effects of phenylephrine and methoxamine were inhibited by prazosin. In contrast, neither clonidine nor yohimbine displayed any chronotropic action. These findings suggest that the postjunctional adrenoceptors present in the sarcolemma of the isolated cardiac muscle cells, which mediate automaticity responses to catecholamines, are of beta 1, beta 2, and alpha 1 types, the physiological contribution of the beta 1-adrenoceptors being predominant. Applicability of these conclusions to the in situ myocardiocytes is discussed with respect to the level of functional differentiation achieved by the rat myocardial cells in culture.