Involvement of a Phorbol Ester-Insensitive Protein Kinase C in the α₂-Adrenergic Inhibition of Voltage-Gated Calcium Current in Chick Sympathetic Neurons

alpha2-Adrenoceptors regulate the efficacy at the sympathoeffector junction by means of a feedback inhibition of transmitter release. In chick sympathetic neurons, the mechanism involves an inhibition of N-type calcium channels, and we now present evidence that this effect involves an atypical, phorbol ester-insensitive protein kinase C (PKC). The inhibition of voltage-gated Ca2+ currents by the specific alpha2-adrenergic agonist UK 14,304 was significantly attenuated when the PKC inhibitors PKC(19-36), staurosporine, or calphostin C were included in the internal solution used to fill the patch pipettes, or if staurosporine 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, ever though PKCalpha and -epsilon isozymes were translocated to plasma membranes by PDBu. The atypical isozyme PKCzeta was translocated by 100 micrometer AA and this effect was attenuated when PKC(19-36) was added to the patch pipette solution. Our observations indicate that classical, new, and atypical PKC isozymes are present in chick sympathetic neurons and that an atypical, phorbol ester-insensitive PKC is involved in the inhibition of voltage-activated calcium currents by alpha2-adrenoceptor activation.