The ionic basis of membrane potential changes from before fertilization through the first cleavage in the egg of the frog Rana cameranoi

Abstract

Experiments were performed to identify the ionic basis of membrane potential changes in the Rana cameranoi egg, from prior to fertilization through the first cleavage. The membrane potential was monitored continuously during this period. Ten per cent Ringer was used as the recording solution in the control group. The effects of Na+ or Ca2+ conductances were observed by altering external concentrations of these ions. K+ and Cl- conductances were tested by adding channel blockers of these ions (TEA and SITS, respectively) to the extracellular medium. The resting potential of the unfertilized egg is mainly affected by K+ conductance. Chloride conductance is responsible for the depolarization phase of the fertilization potential evoked by sperm entry, and K+ conductance is responsible for the repolarization phase of this potential. We suggest that Na+ permeability does not directly contribute to the fertilization potential; however fertilization potential peak is significantly reduced upon a reduction of extracellular sodium. The fertilization potential is not significantly influenced by extracellular Ca2+, and eggs fertilized in calcium-free solutions maintain their normal development; these results suggest that extracellular Ca2+ does not significantly contribute to the electrical and mechanical blocks that prevent polyspermy. The membrane potential of the fertilized egg does not alter significantly until the first cleavage. Potassium conductance contributes to hyperpolarization generated upon the first cleavage, whereas sodium is the basic ion responsible for the phase which follows peak hyperpolarization, and which plays a role in the return of the post-cleavage membrane potential to a steady level. Cl- conductance, which is important as the ionic basis of the fertilization potential, does not significantly influence any parameter of the cleavage cycle.