Intracellular Mg 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 ions, changes in ATP levels result in changes in intracellular Mg concentration ([Mg]), which can be followed using intracellular Mg 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] increased in all cell compartments. In contrast, when ATP increases, such as adding nutrients to starved sperm, [Mg] significantly decreases in all compartments. On the other hand, when the acrosome reaction was induced, either with progesterone or with ionomycin, [Mg] 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] were still observed when the sperm were incubated in high extracellular Mg suggesting that this decrease is not due to Mg efflux. Interestingly, the progesterone and ionomycin effects on [Mg] were abolished on sperm incubated in Ca-free media. Altogether, these results indicate that [Mg] is regulated in sperm during capacitation and acrosomal reaction, and suggest that these measurements can serve to evaluate ATP levels in real time.