Mechanical and electrophysiological effects of thiopental on rat cardiac left ventricular papillary muscle.

Thiopental induces a negative inotropic effect on mammalian heart muscle, where it decreases Ca2+ current and Ca2+ release from the sarcoplasmic reticulum and reduces K+ currents. We analysed the effects of thiopental on the mechanical and electrical activities of rat myocardium, which differ markedly from those of other mammals. The effects of thiopental on mechanical parameters and on the transmembrane resting (RP) and action (AP) potentials of rat left ventricular papillary muscle were investigated. These effects were also studied in the presence of atenolol, a beta-blocking agent, and 4-aminopyridine (4-AP), a blocker of the transient outward K+ current. Thiopental (3.8 x 10(-6), 3.8 x 10(-5) and 1.1 x 10(-4) M) induced a dose-dependent positive inotropic effect. This positive inotropic effect persisted in the presence of atenolol (1 x 10(-6) M) but did not develop in the presence of 1 mM 4-AP; 4-AP had a positive inotropic effect but not in the presence of thiopental. Moreover, thiopental (3.8 x 10(-5) M) lengthened the plateau and the slow repolarizing phase of the AP, while 1 mM 4-AP only prolonged the plateau duration. In rat myocardium, the positive inotropic effect of thiopental in part mimics that of 4-AP, and in part may be explained by the lengthening of the slow repolarizing phase of the AP.