The ionic basis of the membrane potential in the early Asterias embryo

The ionic basis of the resting potential in the fertilized Asterias embryo has been investigated. The response of the membrane potential (as determined by intracellular microelectrodes) to changes in external K concentration was stable and reversible up to 200 m M K. The time course and magnitude of the response observed upon changes in external K is essentially independent of the concentration of external Cl. There is a significant effect of Na ion on the membrane potential, especially at low external K. The response of the membrane potential to changes in external K could be predicted by a form of the constant field equation. Plots of e VF RT versus external K, in conjunction with the constant field equation, allow an estimate of the internal K concentration and the ratio of Na to K permeability ( P Na P K ). These plots indicate at this stage of development an internal K concentration of approximately 200 m M and a P Na P K of 0.03.