Mechanism of Ca2+ release in medaka eggs microinjected with inositol 1,4,5-trisphosphate and Ca2+

The reaction time of Ca2+ release from cytoplasmic stores induced by microinjection of inositol 1,4,5-trisphosphate (IP3), calcium ionophore A23187, Ca2+, Sr2+, Ba2+, and cyclic guanosine 5'-monophosphate (cGMP) in Oryzias latipes eggs in Ca2+-free medium was measured by the luminescence of aequorin injected into the egg. Microinjection of IP3 or calcium ionophore induced rapid Ca2+ release without a time lag, while microinjection of either Ca2+ or cGMP required a time lag of 5-30 sec for Ca2+ release. Following microinjection of both IP3 and Ca2+, Ca2+ release commenced in a cytoplasmic region close to the egg surface. These results suggest that in the medaka egg, cytoplasmic Ca2+ induces Ca2+ release from cytoplasmic stores indirectly, probably via a membrane factor such as IP3.