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-NNN′,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.
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