Ionic mechanism of the fertilization potential of the marine worm, Urechis caupo (Echiura).

Abstract

Microelectrode and tracer flux studies of the U. caupo egg during fertilization were done. Insemination causes a fertilization potential with the membrane potential rising from an initial level of -33 .+-. 6 mV to a peak at +51 .+-. 6 mV (n = 16), falling to a plateau of about +30 mV and then returning to the original resting potential 9 .+-. 1 min (n = 10) later. The fertilization potential results from an increase in Na+ permeability, which is amplified during the first 15 s by a Ca2+ action potential. The maximum amplitude of the fertilization potential, excluding the first 15 s, changes by 51 mV for a 10-fold change in external [Na+]. In the 10 min period after insemination, both Na+ and Ca2+ influxes increase relative to unfertilized egg values by factors of 17 .+-. 17 (n = 6) and 34 .+-. 14 (n = 4), respectively; the absolute magnitude of the Na+ influx is 16 .+-. 6 times larger than that of Ca2+. In the absence of seprm these same electrical and ionic events are elicited by trypsin; thus, the ion channels responsible must preexist in the unfertilized egg membrane. Increased Na+ influx under conditions of experimentally induced polyspermy indicates that during normal monospermic fertilization, only a fraction of available Na+ channels are opened. These channels are apparently sperm-gated. Ca2+ influx at fertilization is primarily via the membrane potential-gated channel, because kinetics are appropriate, and influx depends on potential in solutions of varying [Na+], but is independent of number of sperm incorporations in normal sea water.