Cyclic GMP inhibits cytoplasmic Ca 2+ oscillation by increasing Ca 2+-ATPase activity in rat megakaryocytes

Abstract

The regulatory effects of cyclic GMP on purinoceptor-operated cytoplasmic Ca 2+ oscillation of rat megakaryocytes were investigated by using whole-cell patch-clamp technique. ATP-induced oscillatory K + currents though Ca 2+-activated K + channels ( I KCas) were depressed by pretreatment with the guanylate cyclase activator, sodium nitroprusside, and a stable membrane-permeable cGMP analogue, 8-bromo-cGMP. The inhibition by sodium nitroprusside was blocked by treatment with a cyclic nucleotide-dependent protein kinase inhibitor, N-[2-(methylamino)]-5-isoquinolinesulfonamide·HCl (H-8) (10 μM), but not by a selective cAMP-dependent-protein kinase inhibitor, Rp-cAMPS (100 μM). The oscillatory I KCa directly evoked by intracellular d-myo-inositol-trisphosphate (IP 3) perfusion was also inhibited by the application of sodium nitroprusside. The inhibitory effect of sodium nitroprusside disappeared when the ATP-induced oscillatory I KCa was changed to a monophasic sustained I KCa current by inhibition of Ca 2+-ATPase. These results suggested that cGMP depressed Ca 2+ mobilization by improving Ca 2+-ATPase activity by phosphorylation.