Intracellular Mg²⁺ concentrations are differentially regulated in the sperm head and mid-piece in acrosome reaction...

The sperm ability to fertilize involves the regulation of ATP levels. Because inside cells, ATP is complexed with Mg²⁺ ions, changes in ATP levels result in changes in intracellular Mg²⁺ concentration ([Mg²⁺]_i), which can be followed using intracellular Mg²⁺ 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 [Mg²⁺]_i increased in all cell compartments. In contrast, when ATP increases, such as adding nutrients to starved sperm, [Mg²⁺]_i significantly decreases in all compartments. On the other hand, when the acrosome reaction was induced, either with progesterone or with ionomycin, [Mg²⁺]_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 [Mg²⁺]_i were still observed when the sperm were incubated in high extracellular Mg²⁺ suggesting that this decrease is not due to Mg²⁺ efflux. Interestingly, the progesterone and ionomycin effects on [Mg²⁺]_i were abolished on sperm incubated in Ca²⁺-free media. Altogether, these results indicate that [Mg²⁺]_i is regulated in sperm during capacitation and acrosomal reaction, and suggest that these measurements can serve to evaluate ATP levels in real time.