Calcium and thiopental-induced spontaneous activity in rabbit papillary muscle.

Isolated rabbit papillary muscles exposed to 20 to 30 mg/l of thiopental developed spontaneous contractile activity. The spontaneous activity often appeared following the cessation of high frequency stimulation. The maximum rate of spontaneous beating decreased in parallel with the force of contraction when the extracellular Ca2+ concentration was lowered or when ryanodine, an inhibitor of sarcoplasmic reticular function, was added. Stretching of the muscles increased the incidence of thiopental-induced spontaneous activity. These results suggest that intracellular Ca2+ influences thiopental-induced spontaneous activity, analogous to arrhythmias associated with afterdepolarization.