The role of intracellular calcium and pH during fertilization and egg activation in the hydrozoan Phialidium.

A calcium transient occurs at fertilization in the eggs of the hydrozoans Mitrocomella and Phialidium. The eggs of Phialidium have an intracellular pH (pHi) of 7.6-7.95. There is no increase in pHi following fertilization. Both calcium ionophore and ammonia treatments activate Phialidium eggs. Calcium ionophore causes a calcium transient without changing pHi. Ammonia concentrations of 10-20 mM at pH 8 cause a rise in pHi but no detectable calcium transient. Both activating agents can (1) block subsequent fertilization, (2) initiate cell cycle events such as DNA synthesis, nuclear envelope breakdown, chromosome condensation, and cleavage-related events, and (3) initiate voltage-dependent calcium channel function. While calcium ionophore treatment invariably elicits all of these manifestations of egg activation, ammonia sometimes induces only the induction of voltage-dependent calcium channel function. Oocytes were also treated with calcium ionophore or ammonia at different stages during their maturation. Ammonia treatment did not induce egg activation when applied at any stage during maturation; however, calcium ionophore initiated voltage-dependent calcium channel function when oocytes were treated after germinal vesicle breakdown. Ionophore treatment during maturation did not render these eggs unfertilizable or initiate cell cycle events. These experiments show that elevation of [Ca2+] with ionophore and increasing pHi with ammonia may activate these eggs through different pathways, that specific maturational events must occur before they can do so, and that the activation of voltage-dependent calcium channel function can be dissociated from other egg activation events.