IP3R1-mediated Ca2+ release is enhanced independently of [Ca2+]ER during oocyte maturation. (A) Oocytes were cultured in the presence of CPA (50µM) for 12hr but for variable times in IBMX (GV, GVBD and MI oocytes were cultured in IBMX for 12, 8 and 4 hr, respectively). [Ca2+]ER content was estimated following addition of 2 µM ionomycin in Ca2+ free medium and shown to be comparable in all stages of maturation (bar graph on right). Error bar, SEM (n=9–10). (B) IP3-induced Ca2+ release was caused by release of cIP3 in CPA-treated oocytes. UV flashes of 0.001 sec (open arrow) were applied to oocytes in all stages of maturation. A representative trace is shown for each stage, and the number in parentheses represents oocytes responding to cIP3 in each stage.