INVOLVEMENT OF A PHORBOL ESTER-INSENSITIVE PROTEIN-KINASE-C IN THE ALPHA(2)-ADRENERGIC INHIBITION OF VOLTAGE-GATED CALCIUM CURRENT IN CHICKSYMPATHETIC NEURONS

alpha(2)-Adrenoceptors regulate the efficacy at the sympatho-effector junction by means of a feedback inhibition of transmitter release. In chick sympathetic neurons, the mechanism involves an inhibition of N-t ype calcium channels, and we now present evidence that this effect inv olves an atypical, phorbol ester-insensitive protein kinase C (PKC). T he inhibition of voltage-gated Ca2+ currents by the specific alpha(2)- adrenergic agonist UK 14,304 was significantly attenuated when the PKC inhibitors PKCI(19-36), staurosporine, or calphostin C were included in the internal solution used to fill the patch pipettes, or if stauro sporine or calphostin C were applied extracellularly; however, phorbol esters as classical activators of PKC or oleoylacetylglycerol did not mimic the effect of UK 14,304, and chronic exposure to 4-beta-phorbol dibutyrate (PDBu) did not attenuate it, even though PKC alpha and -ep silon isozymes were translocated to plasma membranes by PDBu. The atyp ical isozyme PKC zeta was translocated by 100 mu M arachidonic acid (A A), but not by PDBu; 100 mu M AA and linoleic acid inhibited voltage-a ctivated Ca2+ currents, and this effect was attenuated when PKCI(19-36 ) was added to the patch pipette solution. Our observations indicate t hat classical, new, and atypical PKC isozymes are present in chick sym pathetic neurons and that an atypical, phorbol ester-insensitive PKC i s involved in the inhibition of voltage-activated calcium currents by alpha(2)-drenoceptor activation.