Regulation of the L-type Ca2+ channel current in rat pinealocytes: Role ofbasal phosphorylation

In the present study, the role of phosphoprotein phosphatase in the regulat ion of L-type Ca2+ channel currents in rat pinealocytes was investigated us ing the whole-cell version of the patch-clamp technique. The effects of thr ee phosphatase inhibitors, calyculin A, tautomycin, and okadaic acid, were compared. Although all three inhibitors were effective in inhibiting the L- type Ca2+ channel current, calyculin A was more potent than either tautomyc in or okadaic acid, suggesting the involvement of phosphoprotein phosphatas e-1. To determine the kinase involved in the regulation of these channels, cells were pretreated with H7 (a nonspecific kinase inhibitor), H89 (a spec ific inhibitor of cyclic AMP-dependent kinase), KT5823 (a specific inhibito r of cyclic GMP-dependent kinase), or calphostin C (a specific inhibitor of protein kinase C), Pretreatment with either H7 or calphostin C decreased t he inhibitory effect of calyculin A on the L-type Ca2+ channel current. In contrast, pretreatment with H89 or KT5823 had no effect on the inhibition c aused by calyculin A. Based on these observations, we conclude that basal p hosphatase activity, probably phosphoprotein phosphatase-l, plays an import ant role in the regulation of L-type Ca2+ channel currents in rat pinealocy tes by counteracting protein kinase C-mediated phosphorylation.