AGONIST-EVOKED CALCIUM EFFLUX FROM A FUNCTIONALLY DISCRETE COMPARTMENT IN XENOPUS OOCYTES

Agonist-induced calcium (Ca) mobilization is accompanied by Ca efflux, presumably reflecting the rise in Ca concentration at the cytosolic s urface of the cell membrane. We studied the relationship between Ca ef flux and intracellular Ca mobilization in Xenopus oocytes. Elevation o f cytosolic Ca by a direct injection of 1 nmol (CaCl2)-Ca-45 resulted in a typical Ca-activated chloride current, but not in Ca-45 efflux. T his demonstrated that a Ca rise at the cytoplasmic surface of the memb rane is not sufficient to produce an increased efflux. Go-injection of inositol 1,4,5-trisphosphate (InsP(3)), to prevent rapid Ca sequestra tion, also failed to cause Ca efflux. Smaller amounts of labelled Ca ( 0.05 nmol) equilibrated with Ca stores in a time-dependent pattern wit h an optimum at 2 h after injection. In contrast, Ca taken up from the medium was immediately available for agonist- or InsP(3)-induced effl ux. Emptying the agonist-sensitive stores with thapsigargin (TG) did n ot affect chloride currents induced by Ca injection, indicating that t hese currents were due to direct elevation of Ca at the plasma membran e, rather than Ca-induced Ca release from InsP(3)-sensitive stores. Ag onist-induced depletion of Ca stores enhanced uptake from the extracel lular medium and the subsequent release of the label by an agonist. Si milar protocol when the label was injected into the oocytes, failed to affect agonist induced efflux. We suggest that, under physiological c onditions, agonist-dependent Ca extrusion or uptake in oocytes is exec uted exclusively via a functionally restricted compartment, which is c losely associated with both agonist-sensitive Ca stores and the plasma membrane.