Ca-mediated activation of a K current at fertilization of golden hamster eggs

Golden hamster eggs respond to fertilization with recurring hyperpolarizations [Miyazaki, S. & Igusa, Y. (1981) Nature (London) 290, 703-705]. We analyzed the ionic mechanism of the fertilization potential and examined whether the fertilization potential plays a role in polyspermy block. Each hyperpolarizing response (HR) during fertilization is found to be caused by an increase in the K conductance activated by an increase in the intracellular Ca2+ concentration. This conclusion is based on the following: (i) The reversal potential of the HR shifted with the Nernstian slope for K ions when the external K concentration was changed, whereas it was unaltered by the removal of Cl ions. (ii) The HR was blocked by the intracellular injection of EGTA. (iii) Injection of Ca2+ into an egg induced a hyperpolarization of the membrane similar to the HR. The Ca-activated K conductance shows an apparent outward rectification, which could be explained by an asymmetric distribution of K ions across the membrane. The HR associated with sperm entry into the egg occurred at any membrane potential between -160 and +50 mV. Therefore, a potential-dependent block of sperm entry does not occur in the hamster egg.