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 surface of the cell membrane. We studied the relationship between Ca efflux and intracellular Ca mobilization in Xenopus oocytes. Elevation of cytosolic Ca by a direct injection of 1 nmol 45CaCl2 resulted in a typical Ca-activated chloride current, but not in 45Ca efflux. This demonstrated that a Ca rise at the cytoplasmic surface of the membrane is not sufficient to produce an increased efflux. Co-injection of inositol 1,4,5-trisphosphate (InsP3), to prevent rapid Ca sequestration, also failed to cause Ca efflux. Smaller amounts of labelled Ca (0.05 nmol) equilibrated with Ca stores in a time-dependent pattern with an optimum at 2 h after injection. In contrast, Ca taken up from the medium was immediately available for agonist- or InsP3-induced efflux. Emptying the agonist-sensitive stores with thapsigargin (TG) did not affect chloride currents induced by Ca injection, indicating that these currents were due to direct elevation of Ca at the plasma membrane, rather than Ca-induced Ca release from InsP3-sensitive stores. Agonist-induced depletion of Ca stores enhanced uptake from the extracellular medium and the subsequent release of the label by an agonist. Similar protocol when the label was injected into the oocytes, failed to affect agonist induced efflux. We suggest that, under physiological conditions, agonist-dependent Ca extrusion or uptake in oocytes is executed exclusively via a functionally restricted compartment, which is closely associated with both agonist-sensitive Ca stores and the plasma membrane.