The two components in the shortening of unloaded ventricular myocytes: their voltage dependence.

In isolated myocytes from mammalian ventricles a fast and a slow component in the contractile response to depolarizing voltage clamp steps were identified. The potential dependence of the slow component was identical to the activation curve of iCa. The fast component, however, remained at its maximal amplitude at potentials positive to +10 mV (up to +100 mV), in which potential range iCa declined and eventually disappeared. The results suggest that the slow component may be activated by Ca++ entering through sarcolemmal Ca channels, whereas the fast component depends on Ca release from intracellular sites and may depend on both Cai and voltage.