PMA counteracts G protein actions on CaV2.2 channels in rat sympathetic neurons

Protein kinase C (PKC)-induced phosphorylation and G protein-mediated inhibition of CaV2.2 N-type Ca2+ channels counteract exerting opposing modulatory responses at the channel level. At present, the most striking question remaining is whether prominent enhancement of the Ca2+ current (ICa) observed under PKC activation arises from relief of G-protein tonic inhibition. Here, by using patch-clamp methods in superior cervical ganglion (SCG) neurons of rat, we show the following: First, that PKC activation by phorbol-12-myristate-13-acetate (PMA) not only counteracts mutually with noradrenaline (NA) and GTPγS-induced ICa inhibition, but also reverses current inhibition by Gβγ subunits over-expression. Second, that PMA increases ICa beyond the enhancement expected by sole removal of the G protein-mediated tonic inhibition. Accordingly, PMA increases conductance through N-type Ca2+ channels, unlike the G protein inhibitor GDPβS. Together, our results support that PMA-induced phosphorylation produces changes in ICa that cannot be accounted for by prevention of G protein inhibition. They may have important implications in reinterpretation of existing data with PMA. Furthermore, counteracting modulation of ion channels and reversibility within a short time frame are better support for a dynamic system with short-term adaptive responses.