MODULATION OF CA2-CHANNEL CURRENTS BY PROTEIN-KINASE-C IN ADULT-RAT SYMPATHETIC NEURONS()

1. Modulation of Ca2+-channel currents by phorboi-12-myristate-13-acet ate (PMA) was investigated in acutely dissociated adult rat superior c ervical ganglion neurons using the whole cell variant of the patch-cla mp technique. 2. PMA (500 nM) increased the current amplitudes, accele rated the inactivation of step currents, retarded the deactivation of tail currents, and shifted the tail current activation to more negativ e potentials. 3. The effects of PMA were concentration and voltage dep endent and mediated through activation of protein kinase C (PKC). PMA also increased Ca2+ currents recorded with the perforated patch techni que. 4. PMA affected the N-type Ca2+ channels and an omega-conotoxin G VIA-resistant current component. Ca2+ currents affected by PMA were no t sensitive to omega-agatoxin IVA or nimodipine. 5. PMA not only atten uated Ca2+-channel inhibition induced by alpha(2)-adrenoceptor agonist , which modulates Ca2+ channels via a pertussis toxin (PTX)-sensitive pathway, but also attenuated current inhibition by vasoactive intestin al polypeptide, which modulates Ca2+ channels via a PTX-insensitive bu t cholera toxin-sensitive pathway. 6. PMA reversed Ca2+-channel inhibi tion induced by tonic activation of G-protein in the absence of neurot ransmitter (even in neurons pretreated with PTX) or induced by activat ion of G-proteins with guanosine 5'-O-(3-thiotriphosphate) (GTP)-gamma -S. 7. Inhibition of phosphatase by okadaic acid or substitution of Ba 2+ for Ca2+ in the external solutions accelerated the PMA effect. 8. O ur results suggest that activation of PKC antagonizes G-protein mediat ed inhibition of Ca2+ channels by shifting Ca2+ channels from the ''re luctant'' state to the ''willing'' state. The G-proteins and, more lik ely, the N-type Ca2+ channels may be the target of PKC phosphorylation . Protein phosphatases may be involved in counteracting the PKC phosph orylation in rat sympathetic neurons.