Differential isoform-specific regulation of calcium-independent protein kinase C in rat cerebral cortex

Regulation of the Ca(2+)-independent protein kinase C (PKC) activity and isoforms by phorbol esters was investigated in rat cerebral cortex. Loss of soluble PKC eta immunoreactivity from the soluble fraction was dramatic with only a small increase in the membrane fraction. The kinetics of PKC epsilon and -delta translocation were slower than that for PKC eta, while phorbol esters had no effect on PKC zeta translocation. Despite the translocation of PKC delta, -epsilon and -eta from the soluble to the membrane fraction, both fractions showed a loss of PKC activity. These data indicate that the rates of translocation, inactivation and/or downregulation appear to be different not only among these Ca(2+)-independent isozymes, but also from that reported for the Ca(2+)-dependent PKCs. In addition, these results emphasize the importance of measuring both Ca(2+)-independent PKC activity and immunoreactivity in evaluating activation of these isoforms.