Differential modulation of pharmacologically distinct components of Ca2+ currents by protein kinase C activators and phosphatase inhibitors in nerve-growth-factor-differentiated rat pheochromocytoma (PC12) cells.

We have studied the effects of protein kinase C (PKC) activators 4 beta-phorbol 12-myristate 13-acetate (4 beta-PMA) and 1-oleoyl-2-acetylglycerol (OAG) and of phosphatase inhibitors (okadaic acid and calyculin A) on voltage-activated Ca2+ and K+ channels in nerve-growth-factor-(NGF)-differentiated pheochromocytoma (PC12) cells. Whole-cell Ba2+ and K+ currents were recorded at room temperature with the patch-clamp technique. By using omega-conotoxin (CgTX) and isradipine, two specific Ca2+ channel blockers, we found three types of Ba2+ currents (IBa): (1) a omega-CgTX-sensitive IBa; (2) an isradipine-sensitive IBa; and (3) a omega-CgTX plus isradipine-resistant IBa. The external application of 4 beta-PMA or OAG down-modulated the isradipine-sensitive IBa whereas the two other IBa were not affected. 4 beta-PMA-induced inhibition of IBa was prevented by staurosporine (a protein kinase inhibitor) and PKC (19-31) (a specific PKC inhibitor). The delayed rectifier K+ current (IK) was unaffected by PKC activators. Both okadaic acid and calyculin A affected the components of the IBa in different manners. The presence of okadaic acid decreased the isradipine-sensitive IBa more than the omega-CgTX-sensitive IBa. The omega-CgTX plus isradipine-resistant IBa was not affected. Calyculin A down-modulated all three components of IBa to a similar degree. Our results suggest a differential modulation of voltage-activated Ca2+ and K+ channels by the PKC signalling pathway in NGF-differentiated PC12 cells.