Fertilization Potential of the Medaka Egg: (fertilization potential/depolarization/hyperpolarization/Ca

Fertilization of the medaka egg in 10% Ringer's solution generates a depolarization of 4 mV just before the appearance of a characteristically longer hyperpolarization (25). The depolarization appears to be the result of a nonspecific leak triggered by sperm stimulation and the amplitude of the depolarization is thought to be independent of [Ca2+ ]o (25). We have investigated the ionic dependence of this depolarization. An initial small depolarization (3-4 mV; duration, 5-8 sec) is followed by a rising phase of a spike-like depolarization in the range of 10-60 mV. The amplitude of this spike-like depolarization is propodional to log [Ca2+ ], ranging from 0.33-18 mM. Calcium antagonists, e.g. 10 mM cobalt or 10 μg/ml verapamil in 10% Ringer do not block the depolarization in the presence of 1.1 mM CaCl2 .