Contributions of extracellular and intracellular Ca2+ to regulation of sperm motility: Release of intracellular stores can hyperactivate CatSper1 and CatSper2 null sperm

In order to fertilize, mammalian sperm must hyperactivate. Hyperactivation is triggered by increased flagellar Ca2+, which switches flagellar beating from a symmetrical to an asymmetrical pattern by increasing bending to one side. Thimerosal, which releases Ca2+ from internal stores, induced hyperactivation in mouse sperm within seconds, even when extracellular Ca2+ was buffered with BAPTA to approximately 30 nM. In sperm from CatSper1 or CatSper2 null mice, which lack functional flagellar alkaline-activated calcium currents, 50 μM thimerosal raised the flagellar bend amplitudes from abnormally low levels to normal pre-hyperactivated levels and, in 20-40% of sperm, induced hyperactivation. Addition of 1 mM Ni2+ diminished the response. This suggests that intracellular Ca2+ is abnormally low in the null sperm flagella. When intracellular Ca2+ was reduced by BAPTA-AM in wild-type sperm, they exhibited flagellar beat patterns more closely resembling those of null sperm. Altogether, these results indicate that extracellular Ca2+ is required to supplement store-released Ca2+ to produce maximal and sustained hyperactivation and that CatSper1 and CatSper2 are key elements of the major Ca2+ entry pathways that support not only hyperactivated motility but possibly also normal pre-hyperactivated motility.