EFFECTS OF MEDIA OF DIFFERENT IONIC COMPOSITION ON THE ACTIVATION POTENTIAL OF ANURAN EGG CELLS

Activation potentials of the eggs of two anuran species, Bufo vulgaris formosus and Rana rugosa, were measured in media of different ionic composition, and the mechanism of their generation is discussed. The same trends of ionic effects upon the activation potential were consistently obtained in both species. The membrane potential of the unactivated eggs was negative with respect to tap water, Ringer's solution, and the media described below except isotonic KCl and KNO3 , in which the potentials were nearly zero or sometimes slightly positive. Upon activation induced by pricking in tap water, isotonic Na2 SO4 , or 42 mM or 63 mM buffered sodium phosphate solution, depolarization followed by reversal of the membrane potential took place, associated with a decrease in the effective resistance. Then the potential gradually decreased and returned to the initial value. On the other hand, in Ringer's solution, or in isotonic NaBr, NaNO3 , KCl or KNO3 , the activation potential occurred in a hyperpolarizing direction, associated with a decrease in the effective resistance. These results suggest that the ooplasmic membrane of the anuran egg surface during activation is selectively permeable to NO3 - and Br- as well as Cl- , but not to SO4 - and phosphate ions, or alternatively, that the activation potential is due to this selective permeability and to the concentration gradients of these monovalent anions across the ooplasmic membrane.