A rapid sodium-dependent block to polyspermy in sea urchin eggs

The rapid electrical depolarization of the egg's plasma membrane which protects sea urchin ova against polyspermy in the interval between stimulation by the fertilizing spermatozoon and completion of the cortical reaction is believed to be mediated by the influx of sodium (Na⁺) ions. This hypothesis was tested in Arbacia punctulata and Strongylocentrotus purpuratus by inhibiting the rapid block to polyspermy with low-Na⁺ (choline-substituted) seawater, and the cortical granule secretion-mediated block with soybean trypsin inhibitor (SBTI). Eggs inseminated in low-Na⁺ seawater or SBTI became heavily polyspermic. Polyspermy elicited by low Na⁺ or SBTI was increased in dejellied Strongylocentrotus eggs. However, the severity of polyspermy was not enhanced in low Na⁺ plus SBTI because the fertilizing capacity of sperm and gamete binding were reduced in low-Na⁺ media. Since SBTI completely suppresses the cortical granule secretion-mediated block to polyspermy in Arbacia for about 3 min postinsemination, the rate at which SBTI-treated eggs became polyspermic was used to measure the duration and efficacy of the rapid block. The half-time for SBTI-treated Arbacia eggs to become polyspermic in natural (425 mM Na⁺) seawater was 89.9 ± 4.7 sec (N = 4). The plot of incidence of polyspermy vs time was essentially an inverse mirror image of electrophysiologic data on repolarization of the oolemma during fertilization. The rapid block is also Na⁺ dependent, since SBTI-treated eggs became polyspermic more rapidly in 26 mM Na⁺ seawater (half-time, 15.8 ± 1.6 sec; N = 3, P < 0.01).