Are there inositol 1,4,5-triphosphate (IP3) receptors in human sperm?

Calcium signaling plays important roles in mammalian fertilization, such as the calcium wave in egg and the acrosome reaction (AR) in sperm. The calcium wave is accompanied by a transient increase of cytosolic calcium, which is mediated by inositol 1,4,5-triphosphate (IP3) induced calcium release (IICR). During AR, it is commonly recognized that influx of extracellular calcium causes a dramatic increase of cytosolic calcium. Participation of IICR in the phenomena, however, remains unclear. To investigate whether IICR also participates in calcium mobilization during AR, the present study examined the existence of the IP3 receptor family (IP3R type 1, 2 and 3) in human sperm and their changes during the reaction. Immunoblot analyses showed the existence of IP3R types 1 and 3 in human sperm, but IP3R type 2 was undetectable. The expression of IP3R type 1 was diminished after AR and was also detected in the vesiculated membrane fragment which was released by the fusion of the plasma membrane and the outer acrosomal membrane during AR. In contrast, the expression of IP3R type 3 on the blot was similar before and after the reaction. Immunohistochemical observations suggested that IP3R type 1 localized in the anterior portion of the sperm head and the expression was decreased after AR. IP3R type 3 was observed in the posterior portion of the sperm head, midpiece, and tail, and little change was found even after AR. The [3H]-IP3 binding assay suggested stoichiometric interactions between IP3 and IP3Rs of non-acrosome reacted and acrosome reacted human sperm.