Abstract
The fertilization reaction of echinoderm eggs (Lytechinus pictus, a sea urchin, and Dendraster excentricus, a sand dollar) was followed with intracellular electrodes. Membrane potential and K+ activity were recorded.
The unfertilized egg of Lytechinus has a membrane potential of -8 mV, inside negative. Within 5 sec after the addition of sperm, a fertilization action potential develops, going to +10 mV, inside positive. The time from the initial depolarization to a return to the original -8 mV is 120-150 sec. The repolarization continues until a potential of -10 to -14 mV is reached, at which point it pauses for 3-4 min. At 6-8 min after fertilization, a further and relatively rapid hyperpolarization begins, going to -60 to -65 mV by 15-25 min after fertilization and remaining constant at these values.
The membrane potential of the unfertilized egg appears to depend on a general permeability to anions. The fertilization action potential seems to reflect a prolonged influx of sodium. The final depolarization to -60 mV is attributable to the development of potassium conductance.
Simultaneous measurements with a K+ ion-selective electrode gives constant readings of about 240 mM K+ in the unfertilized eggs throughout the fertilization process.
Similar results were obtained with Dendraster eggs. The resting potential of the unfertilized eggs was -7 mV; the action potential on activation attained +18 mV; the repolarization paused at -16 to -24 mV and the final potential attained was -70 mV. The electrical changes after fertilization with spermatozoa or activation with Pronase were identical.
Keywords: fertilization, sea-urchin egg, membrane potential, sodium permeability, intracellular potassium, potassium permeability
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