Time and voltage windows for reversing the electrical block to fertilization.

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Abstract
The electrical block to fertilization of sea urchin eggs can be overcome by very brief periods of inside-negative egg membrane potential. Lytechinus pictus eggs whose membrane potentials have been clamped at +15 mV cannot be fertilized. If the membrane potential is repolarized to inside-negative voltages for a brief interval, the egg can be successfully fertilized. By varying the duration and voltage of these brief periods of inside negativity, 3 general properties of the electrically sensitive step in fertilization were uncovered. A membrane-potential step that becomes rate limiting at inside-positive voltages can be initiated within a few milliseconds of inside negativity (30-60 ms at -60 mV). At the time that the electrically sensitive step is being completed, there are other potential-independent steps with probably slower time constants because the duration of negativity was more effective applied as paired pulses rather than as a single long pulse. The permissive state is more quickly established by inside negativity than the nonpermissive state is established by inside positivity because the interval between paired pulses could be a few times longer than the effective single pulse in duration. In these voltage-clamped eggs the intervals from the successful completion of the electrically sensitive step to the next identifiable signs of activation were on the order of several seconds and highly variable.