Intracellular Mg(2+) concentrations are differentially regulated in the sperm head and mid-piece in acrosome reaction inducing conditions.

The sperm ability to fertilize involves the regulation of ATP levels. Because inside cells, ATP is complexed with Mg(2+) ions, changes in ATP levels result in changes in intracellular Mg(2+) concentration ([Mg(2+)](i)), which can be followed using intracellular Mg(2+) sensors such as Mag-520. In this work, we tested conditions known to decrease sperm ATP such as starvation and capacitation. As expected, in these conditions [Mg(2+)](i) increased in all cell compartments. In contrast, when ATP increases, such as adding nutrients to starved sperm, [Mg(2+)](i) significantly decreases in all compartments. On the other hand, when the acrosome reaction was induced, either with progesterone or with ionomycin, [Mg(2+)](i) was differentially regulated in the head and mid-piece. While Mag-520 fluorescence increased in the sperm mid-piece, it decreased in the head. These changes were observed in capacitated as well as in starved sperm but not in sperm incubated in conditions that do not support capacitation. Changes in [Mg(2+)](i) were still observed when the sperm were incubated in high extracellular Mg(2+) suggesting that this decrease is not due to Mg(2+) efflux. Interestingly, the progesterone and ionomycin effects on [Mg(2+)](i) were abolished on sperm incubated in Ca(2+)-free media. Altogether, these results indicate that [Mg(2+)](i) is regulated in sperm during capacitation and acrosomal reaction, and suggest that these measurements can serve to evaluate ATP levels in real time.