Voltage clamp studies of fertilization in sea urchin eggs: II. Current patterns in relation to sperm entry, nonentry, and activation

Following attachment of a sperm to the surface of a sea urchin egg clamped at a membrane potential ( V m) more positive than +17 mV, no changes in membrane conductance can be detected, the sperm does not enter the egg, and no morphological changes can be detected. At V m from +17 to −100 mV three characteristically different types of current profiles are observed: Type I are activation currents in eggs penetrated by a sperm. These have three phases, which occur in all eggs clamped at V m from +17 to −20 mV and in decreasing percentages at clamped V m more negative than −20 mV (to −75 mV). Complete fertilization envelopes are elevated, relatively large mound-shaped fertilization cones form, and the eggs develop to normal embryos. Type II are sperm transient currents in eggs not penetrated by a sperm, the eggs otherwise remaining in the unfertilized state. These transients are simpler and shorter than type I currents, and are observed only at clamped V m more negative than −20 mV. Type III are modified activation currents in eggs not penetrated by a sperm. These have three phases, are observed only at clamped V m more negative than −20 mV, and are the only type of activation current seen at clamped V m more negative than −75 mV. Complete fertilization envelopes are elevated, the fertilization cones are small and filament-like, and the eggs fail to cleave. We conclude that (a) the sperm transient currents (type II) and phase 1 of the activation currents (types I and III) are similar events generated by a sperm-initiated localized conductance increase, (b) the abrupt decrease of current which terminates the sperm transients and phase 1 of type III currents results from a turnoff of the sperm-induced conductance increase and signals that the sperm will not enter the egg, and (c) the occurrence of phase 2 during an electrophysiological response induced by a sperm indicates that the egg is activating.