Elevating intracellular calcium levels in human sperm using an internal calcium ATPase inhibitor, 2,5-di(tert-butyl) hydroquinone (TBQ), initiates capacitation and the acrosome reaction but only in the presence of extracellular calcium

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The aim of this study was to investigate the effect of an internal calcium ATPase inhibitor, TBQ, on human sperm capacitation and the acrosome reaction during incubation in a calcium-depleted media. Sperm were isolated into and incubated for up to 6 hr in media depleted of Ca2+ and two Ca(2+)-containing media controls. At set time intervals, sperm in each media group were treated with 100 microM TBQ for 5 min. Afterwards, sperm were induced to acrosome react using the divalent cation ionophore, A23187, as a measure of sperm fertilizing potential. It was established, using the Chlortetracycline assay, that incubation of sperm in a Ca(2+)-depleted media inhibited or delayed sperm capacitation resulting in fewer spontaneous or A23187-induced acrosome reacted sperm. However, incubation of sperm in a Ca(2+)-depleted media did not appear to inhibit sperm motility. The treatment of sperm with TBQ during their incubation in Ca(2+)-depleted media was found to have very little effect resulting in low numbers of capacitated and acrosome reacted sperm. The results from this study suggest that human sperm have an obligatory requirement for extracellular calcium during capacitation and the acrosome reaction, but may require either very little extracellular Ca2+ to maintain motility or possess internal Ca2+ stores sufficient for their requirements. In addition, TBQ did not increase the number of capacitated and acrosome reacted sperm during incubation in a Ca(2+)-depleted media suggesting that the TBQ-effect of accelerating sperm capacitation is dependent on presence of extracellular Ca2+.