CYCLIC-GMP INHIBITS CYTOPLASMIC CA2-ATPASE ACTIVITY IN RAT MEGAKARYOCYTES( OSCILLATION BY INCREASING CA2+)

The regulatory effects of cyclic GMP on purinoceptor-operated cytoplas mic Ca2+ oscillation of rat megakaryocytes were investigated by using whole-cell patch-clamp technique. ATP-induced oscillatory K+ currents though Ca2+-activated K+ channels (I(KCa)s) were depressed by pretreat ment 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 nucleo tide-dependent protein kinase inhibitor, N-[2-(methylamino)]-5-isoquin olinesulfonamide . HCl (H-8) (10 mu M), but not by a selective cAMP-de pendent-protein kinase inhibitor, Rp-cAMPS (100 mu M). The oscillatory I-KCa directly evoked by intracellular D-myo-inositol-trisphosphate ( IP3) perfusion was also inhibited by the application of sodium nitropr usside. The inhibitory effect of sodium nitroprusside disappeared when the ATP-induced oscillatory I-KCa was changed to a monophasic sustain ed I-KCa current by inhibition of Ca2+-ATPase. These results suggested that cGMP depressed Ca2+ mobilization by improving Ca2+-ATPase activi ty by phosphorylation. (C) 1998 Elsevier Science B.V.