High concentration of inositol 1,4,5-trisphosphate in sea urchin sperm

We measured inositol 1,4,5-trisphosphate (InsP 3) content of sea urchin gametes by using a specific protein binding assay, and found that a spermatozoon contains 4×10 −19 to 1×10 −18 moles of InsP 3 before the acrosome reaction. Since the acrosome reaction has previously been shown to increase the InsP 3 content of sperm severalfold, our measurement indicates that a spermatozoon contains at least 2×10 −18 moles of InsP 3 at fertilization, corresponding to a concentration in the spermatozoon of about 1 mM. The threshold for activation of eggs by injection of InsP 3 dissolved in a much larger volume of solution has been found to be about 3×10 −18 moles, corresponding to a concentration in the injectate of 1 μM. This suggests that sea urchin sperm may contain enough InsP 3 to activate eggs. With an electroporation method, we also showed that sperm extract acts on eggs only from inside, consistent with a primary messenger role for InsP 3.