An inositol 1,4,5-trisphosphate receptor-gated intracellular Ca2+ store is involved in regulating sperm hyperactivated motility

Hyperactivated motility, a swimming pattern displayed by mammalian sperm in the oviduct around the time of ovulation, is essential to fertilization. Ca2+ has been shown to be crucial for the initiation and maintenance of hyperactivated motility. Nevertheless, how Ca2+ reaches the axoneme in the core of the flagellum to switch on hyperactivation is unknown. Ca2+-releasing agents were used to determine whether an intracellular store provides Ca2+ 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 Ca2+ 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 Ca2+ 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 (IP3R) 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 IP3R-gated Ca2+ store in the neck region of sperm that regulates hyperactivated motility.