G-PROTEIN REGULATION OF CAPACITATIVE CALCIUM-ENTRY MAY BE MEDIATED BYPROTEIN-KINASE-A AND PROTEIN-KINASE-C IN XENOPUS OOCYTES

Inositol 2,4,5-trisphosphate irreversibly activated capacitative calci um entry in Xenopus oocytes, whereas guanosine thiotriphosphate (GTP[S ]) and AlF4- only activated capacitative calcium entry transiently. Bo th GTP[S] and AIF(4)(-) inhibited capacitative calcium entry activated by thapsigargin pretreatment, but guanosine thiodiphosphate (GDP[S]), inositol 2,4,5-trisphosphate and dibutyryl cyclic GMP did not affect capacitative calcium entry. This suggests the involvement of heterotri meric GTP-binding proteins in the regulation of capacitative calcium e ntry. Activation of protein kinase C or cyclic-AMP-dependent protein k inase had profound effects on capacitative calcium entry, which were c onsistent with the hypothesis that the effects of GTP[S] and AlF4- on capacitative calcium entry may be mediated via heterotrimeric GTP-bind ing protein stimulation of kinases. Further evidence for this hypothes is was derived from the result that the effects of GTP[S] on calcium e ntry could be inhibited by the application of the protein kinase inhib itor staurosporine.