Inhibition in L-type Ca2+ channel by stimulation of protein kinase C in isolated guinea pig ventricular cardiomyocytes.

1. Electrophysiological effects of phorbol esters on the L-type Ca2+ current (ICa(L)) in isolated single ventricular cells from guinea pig hearts were investigated. 2. In whole-cell voltage-clamped myocytes, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) at 10(-7) M inhibited ICa(L). An antagonist of protein kinase C (PK-C), H-7, at 10(-5) M did not modify the TPA-induced inhibition. The time-course of inactivation process for ICa(L) was greatly slowed. 3. In cell-attached patch-clamp experiments, TPA (10(-7) M) also markedly decreased the opening of L-type Ca2+ channels. The conductance was unaffected. 4. Even H-7 (10(-5) M) alone inhibited the opening of the channels. Addition of TPA (10(-7)-10(-8) M) caused further decrease in the opening. 5. On the other hand, 4-alpha-phorbol-12,13-didecanoate (not a PK-C activator) had no effect on the Ca2+ channels. 6. These results indicate that the PK-C activation induced by TPA greatly depresses the opening of L-type Ca2+ channels in ventricular cell membranes.