Cotical granule exocytosis in sea urchin eggs is inhibited by drugs that alter intracellular calcium stores

In sea urchin eggs fertilization is accompanied by cortical granule exocytosis, a secretory event thought to be initiated by release of intracellularly sequestered calcium. We have examined the effect of two drugs on this process: chlortetracycline (CTC), a known chelator of intracellular calcium, and 8‐(N,N‐diethylamino)octyl‐3,4,5‐trimethoxybenzoate (TMB‐8), an antagonist of intracellular calcium release in both skeletal and smooth muscle. Preincubation of eggs for 10 min with either CTC or TMB‐8 blocked sperm entry, inhibited the burst of ⁴⁵Ca²⁺ efflux normally seen postinsemination, and prevented fertilization envelope elevation. Half‐maximal inhibition occurred with 200 μM CTC and 60 μM TMB‐8. Electron microscopy confirmed that cortical granule exocytosis had been blocked, although inhibition was not due to a direct effect on exocytosis. CTC and TMB‐8 had no effect on Ca²⁺‐stimulated granule fusion in isolated egg cortices. Rather, these drugs block the early events in egg activation: sperm incorporation and triggering of exocytosis. These two effects appear to be independent since addition of either drugs just before insemination permits sperm entry but inhibits calcium release and cortical granule exocytosis.