Electrophysiological and mechanical studies of frog heart adrenoceptor stimulation by epinine.

Electrophysiological and mechanical effects of epinine, an alpha- and beta-agonist, were investigated in frog atrial fibres. From dose-response curves it was demonstrated that epinine produced an increase in amplitude and duration of the plateau, and in duration of the action potential. Corresponding increases in peak tension, rate of tension development, time to peak tension and rate of relaxation were noted. The positive inotropic effect of epinine was mediated only by myocardial beta-adrenoceptor stimulation; over the whole range of concentrations, the inotropic action of epinine was not influenced by phentolamine (3.10(-6) mol . litre-1), but was inhibited by propranolol (3.10(-8) mol . litre-1). The mechanism of epinine action involved an important increase in slow calcium current Isi responsible for the changes in the action potential, while the positive inotropic effect of the drug was attributed to the resulting increase in intracellular calcium concentration. The action of epinine seemed independent of cAMP concentration, because the effects of epinine on contraction were not significantly increased by papaverine (10(-5) mol . litre-1). In frog heart, a beta-stimulation induced specific effects on action potential and tension. The positive inotropic effect of beta-catecholamines resulted directly from electrical changes induced by an increase in slow calcium current.