Characterization of Ca2+ transients induced by intracellular photorelease of InsP3 in mouse ovarian oocytes.

Ca2+ transients (measured with Fluo-3) were induced in single mouse ovarian oocytes by photolytic liberation of InsP3. The time course of cytosolic Ca2+ changes induced in this way is composed of distinct phases: upstroke, fast decline, slow declining plateau and fast decline to rest level. All the phases reflect mainly intracellular redistributions of the ion and not influx, since they are not strongly dependent on external Ca2+ or on changes in transmembrane potential. Often sustained Ca2+ oscillations followed the first InsP3-induced Ca2+ transient. These persisted for several minutes in the absence of external Ca2+. The initial rate of Ca2+ rise and the delay between the InsP3 stimulus and Ca2+ upstroke are correlated with the amount of liberated InsP3. A second InsP3 stimulation, applied during the plateau, causes only small Ca2+ elevations, lacking the upstroke phase. A second, full sized, transient could be elicited only after a complete return to the basal level. Vanadate, applied intracellularly, appeared to inhibit the re-uptake phase into the stores, stabilizing the plateau level. The present observations suggest that in mouse oocytes the InsP3-sensitive stores provide only a small and graded Ca2+ release which may then act as a trigger for a more substantial Ca(2+)-induced Ca2+ release (CICR) process.