Development of inositol trisphosphate-induced calcium release mechanism during maturation of hamster oocytes

Mature hamster eggs exhibit repetitive increases in the intracellular Ca2+ concentration ([Ca2+]i) at fertilization, caused by inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release (IICR) from stores. Oscillating Ca2+ rises also occurred in inseminated immature oocytes at the germinal vesicle (GV) stage but the peak [Ca2+]i of each response was about half of that in mature eggs. Like the responses to sperm in the mature egg, those in the immature oocyte were also blocked by injection of a monoclonal antibody against the InsP3 receptor (18A10). Compared with mature eggs, immature oocytes were more sensitive to externally applied serotonin which caused repetitive IICR, but each response was smaller. The development of the IICR mechanism during oocyte maturation was investigated using iontophoretic injection of InsP3 and Ca2+ imaging with fura 2. In immature oocytes the rise in [Ca2+]i with an InsP3 pulse of 1.2 nA for 1 sec was 35% of that in mature eggs. The response increased with increasing doses of InsP3. The nearly maximum response, obtained with a 20-nA 2-sec pulse, was 80% of that in mature eggs, suggesting that immature oocytes are less sensitive to InsP3 but that most of Ca(2+)-releasable stores are already present in the immature oocyte. IICR developed in two phases during in vivo maturation. The sensitivity to InsP3 increased gradually between the GV stage and prometaphase of the first meiosis. Then a nonlinear dose-response relation developed during the second meiosis to metaphase II, resulting in regenerative, propagating Ca2+ release induced by a threshold pulse of 1 nA for 1 sec in mature eggs. Similar changes occurred during in vitro maturation of oocytes isolated from follicles at GV stage and cultured for up to 16 hr. The development of IICR is thought to be a prerequisite factor for the acquisition of the ability of an egg to undergo normal fertilization.