Calcium-dependent events at fertilization of the frog egg: injection of a calcium buffer blocks ion channel opening, exocytosis, and formation of pronuclei.

Eggs of Xenopus laevis were injected with a calcium buffer before insemination, to examine the effect of preventing or suppressing the sperm-induced increase in intracellular calcium on the fertilization potential, exocytosis, and pronuclear formation. Microinjection of BAPTA [(1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid)] at concentrations between 0.2 and 0.7 mM usually suppressed the fertilization potential to a series of transient depolarizations. The fertilization potential was completely inhibited when the final concentration of BAPTA in the egg was greater than 0.7 mM. These observations support the hypothesis that activation of the chloride conductance responsible for the fertilization potential depends on an increase in intracellular calcium. Exocytosis of cortical granules and elevation of the fertilization envelope were prevented by injecting BAPTA at concentrations greater than 0.2 mM. Injection of BAPTA to suppress the rise in calcium did not inhibit sperm entry and BAPTA-injected eggs were highly polyspermic. Examination by light and electron microscopy revealed that sperm decondensation and pronuclear formation were prevented by injection of the calcium buffer before insemination.