Modulation of N-type Ca2+ channel current kinetics by PMA in rat sympathetic neurons

The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) has been used extensively in studies of G protein modulation of Ca2+ channels. PMA has been shown to be a powerful tool for inducing phosphorylation and interrupting G-protein-mediated signaling pathways. Here we re-examine the effects of PMA on whole-cell N-type Ca2+-channel currents in rat sympathetic neurons. We found that, along with an increase in the current amplitude previously reported by others, PMA pretreatment leads to alterations in current activation and inactivation kinetics. These alterations in current kinetics are voltage-dependent and are not reproduced by internal dialysis with the G protein inhibitor GDPbetaS. Alterations in current kinetics by PMA may therefore indicate the existence of a modulated state, presumably phosphorylated, of N-type Ca2+ channels. We propose that the increase in current amplitude is due primarily to alterations in current kinetics rather than to removal of tonic inhibition.