Positive inotropic effect of somatostatin in guinea-pig ventricular muscles

We investigated the effect of somatostatin (SS) in guinea-pig ventricular muscles using a force transducer and a conventional microelectrode method. Instead of a negative inotropic effect in atrial muscles, SS (10(-11) to 10(-7) M) elicited a positive inotropic effect in ventricular muscles in a concentration-dependent fashion, without changing the time course of contraction. The positive inotropic effect of SS was accompanied by a significant enhancement of the slow action potentials and was suppressed by diltiazem and phentolamine. An increase of extracellular Ca2+ concentration or stimulation frequency enhanced the positive inotropic effect of SS. The positive inotropic effect of SS was not suppressed in the presence of propranolol, metoclopramide, cimetidine or indomethacin, and it appeared even under cold conditions. These results suggest that SS has a positive inotropic effect in guinea-pig ventricular muscle, which is at least partly due to an increase in the slow inward Ca2+ current.