Effects of Microinjected Cations on the Early Event of Fertilization in the Medaka Egg: (microinjection/divalent cations/fertilization/medaka egg/membrane potential).

Effects of microinjected cations on the early events of fertilization were examined using eggs of Oryzias latipes. Microinjection of either Ca , Ba or Sr into the thin cortical cytoplasm induced breakdown of cortical alveoli (vesicles) (CABD) under Ca-Mg-free conditions, but microinjection of Mg , Mn or Co prevented CABD at the injected region when the eggs were inseminated in regular saline. Under Ca-Mg-free conditions, CABD could also be induced by microinjection of various solutions (NaCl, choline chloride, sucrose, pH buffer) without any divalent cations or ionophore A23187. Ca microinjected into the cortical cytoplasm did not play a role in sperm penetration. Upon microinjection with either Ca , Mg or K , the resting membrane potential leakage was transiently observed. However, depolarization of the membrane followed by slow hyperpolarization was observed only upon microinjection of Ca . From these experiments, it was inferred that microinjected divalent cations such as Ca , Ba or Sr do not act directly upon the cortical alveolus membrane, but trigger the induction of CABD via depolarization of the membrne and increase in intracellular Ca .