Effects of alpha-adrenoceptor stimulation on electrophysiological properties and mechanics in rat papillary muscle.

1. Electrophysiological and inotropic responses to stimulation of alpha-adrenoceptors were examined in isolated rat papillary muscles. 2. Stimulation of alpha-adrenoceptors caused two major electrophysiological changes, i.e. prolongation of action potential duration (APD) and hyperpolarization of resting membrane potential. 3. The time course of the inotropic responses to alpha-adrenoceptor stimulation was composed of an initial, short-lasting and small positive phase followed by a negative phase and then a second increasing phase. 4. Nifedipine abolished the alpha-adrenoceptor-mediated positive inotropic effect whereas unaffecting the negative inotropic effect, the APD prolongation and the hyperpolarization. 5. In quiescent muscles alpha-adrenoceptor stimulation also produced hyperpolarization, which was blocked by Ba2+.