An inositol 1,4,5-trisphosphate receptor-gated intracellular Ca(2+) store is involved in regulating sperm...

Hyperactivated motility, a swimming pattern displayed by mammalian sperm in the oviduct around the time of ovulation, is essential to fertilization. Ca(2+) has been shown to be crucial for the initiation and maintenance of hyperactivated motility. Nevertheless, how Ca(2+) reaches the axoneme in the core of the flagellum to switch on hyperactivation is unknown. Ca(2+)-releasing agents were used to determine whether an intracellular store provides Ca(2+) to the axoneme. Hyperactivation was induced immediately in bull sperm by thapsigargin, caffeine, and thimerosal. The responses were dose-dependent and were induced in both capacitated and uncapacitated sperm. When external Ca(2+) was buffered below 50 nM with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, the response to caffeine was significantly reduced; however, the responses to thapsigargin and thimerosal were not affected. This indicates caffeine-induced hyperactivation depends on external Ca(2+) influx, whereas hyperactivation by thapsigargin and thimerosal do not. Acrosome reactions were not induced by these treatments; therefore, an acrosomal store was probably not involved. Indirect immunofluorescence labeling showed type I inositol 1,4,5-trisphosphate receptors (IP(3)R) in the acrosome and neck region, but no ryanodine receptors (RyR) were found using anti-RyR antibodies or BODIPY FL-X ryanodine. These data indicate that there is an IP(3)R-gated Ca(2+) store in the neck region of sperm that regulates hyperactivated motility.