Inositol tri-phosphate in human and ascidian spermatozoa

Using a specific protein binding assay we have shown that a spermatozoon of the ascidian Ciona intestinalis contains 1.58 ± 0.74 × 10^?19 moles of inositol 1,4,5-tri-phosphate (InsP₃), while a human spermatozoon contains 6.4 ± 0.14 × 10^?19 moles. Induction of the acrosome reaction (AR) in both species, by exposure to the calcium ionophore A23187, does not significantly alter levels of InsP₃, suggesting that phosphatidylinositol (PI) turnover is not necessary for the calcium ionophore induced AR. Furthermore, PI turnover in ascidian spermatozoa appears to be insensitive to lithium and phorbol ester. The high intracellular concentration of InsP₃ in spermatozoa, corresponding to 50–200 μM, suggests it may play a role in egg activation. © 1993 Wiley-Liss, Inc.