EFFECTS OF PROTEIN PHOSPHATASE AND KINASE INHIBITORS ON THE CARDIAC L-TYPE CA CURRENT SUGGEST 2 SITES ARE PHOSPHORYLATED BY PROTEIN-KINASE-A AND ANOTHER PROTEIN-KINASE

We previously showed (Frace, A. M. and H. C. Hartzell. 1993. Journal of Physiology. 472:305-326) that internal perfusion of frog atrial myocytes with the nonselective protein phosphatase inhibitors microcystin or okadaic acid produced an increase in the L-type Ca current (P-Ca) and a decrease in the delayed rectifier K current (I-K). We hypothesized that microcystin revealed the activity of a protein kinase (PKX) that was basally active in the cardiac myocyte that could phosphorylate the Ca and K channels or regulators of the channels. The present studies were aimed at determining the nature of PKX and its phosphorylation target. The effect of internal perfusion with microcystin on I-Ca or I-K was not attenuated by inhibitors of protein kinase A (PKA). However, the effect of microcystin on I-Ca was largely blocked by the nonselective protein kinase inhibitors staurosporine (10-30 nM), K252a (250 nM), and H-7 (10 mu M). Staurosporine and H-7 also decreased the stimulation of I-Ca by isoproterenol, but K252a was more selective and blocked the ability of microcystin to stimulate I-Ca, without significantly reducing isoproterenol-stimulated current. Internal perfusion with selective inhibitors of protein kinase C (PKC), including the autoinhibitory pseudosubstrate PKC peptide (PKC19-31) and a myristoylated derivative of this peptide had no effect. External application of several PKC inhibitors had negative side effects that prevented their use as selective PKC inhibitors. Nevertheless, we conclude that PKX is not PKC. PKA and PKX phosphorylate sites with different sensitivities to the phosphatase inhibitors calyculin A and microcystin. In contrast to the results with I-Ca, the effect of microcystin on I-K was not blocked by any of the kinase inhibitors tested, suggesting that the effect of microcystin on I-K may not be mediated by a protein kinase but may be due to a direct effect of microcystin on the I-K channel.