Regulation of Ca influx by a protein kinase C activator in chromaffin cells: differential role of PrQ- and L-type Ca channels

Phorbol esters reduce depolarization-evoked Ca influx in adrenal chromaffin cells, suggesting that voltage-sensitive Ca channels Ž . 2q VSCCs are inhibited by protein kinase C-mediated phosphorylation. We now address the possibility that Land PrQ-type Ca channel Ž . subtypes might be differentially involved in phorbol ester action. In bovine chromaffin cells, short-term 10 min incubations with phorbol Ž . q 2q Žw 2qx . 12-myristate 13-acetate PMA inhibited early high K -evoked rises in cytosolic free Ca concentration Ca and the early i 2q Ž component of the depolarization-evoked Mn quenching of fura-2 fluorescence in a dose-dependent manner IC : 18 and 7 nM; 50 . Ž . maximal inhibitions: 45 and 48%, respectively . The protein kinase C inhibitor staurosporine 100 nM reverted the inhibitory action of Ž . Ž . w 2qx PMA. PMA 0.1–1 mM inhibited the early and late phases of the ionomycin 2 mM -evoked Ca transients by 14–23%. v-Agatoxin i 2q q w 2qx Ž . IVA, a blocker of PrQ-type Ca channels, inhibited high K -evoked Ca rises in a dose-dependent fashion IC s50 nM . In i 50 Ž . 2q contrast, 0.1 mM v-conotoxin GVIA, a blocker of N-type channels, was without effect. A sizeable -45% component of early Ca Ž . Ž . influx persisted in the combined presence of v-agatoxin IVA 100 nM and nitrendipine 1 mM . Simultaneous exposure to v-agatoxin w 2qx 2q IVA and PMA inhibited both the early Ca transients and Mn quenching to a much greater extent than each drug separately. i w 2qx Inhibition of the Ca transients by nitrendipine and PMA did not significantly exceed that produced by PMA alone. It is concluded i that phorbol ester-mediated activation of protein kinase C inhibits preferentially L-type VSCCs over PrQ type channels in adrenal chromaffin cells. However, the possibility cannot be ruled out that dihydropyridine-resistant, non-PrQ type channels might also be negatively regulated by protein kinase C. This may represent an important pathway for the specific control of VSCCs by protein kinase C-linked receptors, not only in paraneurones but presumably also in neurones and other excitable cells. q 1999 Elsevier Science B.V. All rights reserved.