Correlations between Time-Dependent and Cytochalasin B Affected Sperm Entry in Voltage-Clamped Sea Urchin Eggs

Abstract

Sperm entry is only successful in approximately 15% of Lytechinus variegates eggs voltage-clamped at −70 mV, where as 100% of eggs voltage-clamped at −20 mV are penetrated by sperm (Lynn and Chambers 1984). Suppression of sperm penetration at −70 mV is reversible if the clamped egg membrane potential is stepped to −20 mV (a permissive potential for sperm entry). Sperm incorporation occurred in 50% of the eggs when the clamped potential was shifted at approximately 7.5s with an increasing percentage of penetrations as the time period from the sperm-initiated conductance increase to the time of the step was decreased. In the reciprocal experiments where the membrane potential was first clamped at −20 mV and then stepped to −70 mV at specific time points following a sperm-egg interaction, 50% of the eggs were penetrated at an average time of 10.4s with an increasing percentage of sperm incorporation occurring as the time to the step was increased. A similar failure of sperm penetration is induced by treating the sea urchin egg with cytochalasin B (cyto B) (Longo 1978; Schatten and Schatten 1980). Experiments with cyto B-pretreated eggs clamped at −20 mV (an otherwise permissive potential for sperm entry) revealed that not only is sperm entry blocked, but voltage-clamp current profiles associated with the failure of sperm entry at −70 mV were diagnostic of nonpermissive voltage-clamped potentials. In addition, localized FE elevation lifting the sperm from the surface of the egg was frequently observed (an event normally seen only at clamped potentials of −90 and −100 mV). One extended interpretation of these experiments is that microfilament polymerization is interrupted in both the cyto B experiments and in experiments where the egg is clamped at −70 mV and more negative potentials without cyto B treatments.