Fig. 8.

A. Model proposing that under NUTR conditions a basal Ca²⁺ entry occurs, the presence of intracellular ATP concentration allows the proper function of intracellular Ca²⁺ clearance mechanisms such as PMCA4 and SERCA pumps. During NUTR incubation also the Na⁺/K⁺ ATPase and the NCX exchanger work normally and all mechanisms functioning together maintain low levels of [Ca²⁺]_I, and Em and pHi are maintained stable. B. Once sperm are exposed to STRV conditions, the intracellular ATP concentrations decrease and inhibit the normal operation of pumps and transporters, increasing the [Ca²⁺]_i in sperm. [Ca²⁺]_i elevation in sperm somehow activates K⁺ channels and/or inhibits Na⁺ permeable channels that contribute to Em hyperpolarization, which in turn activate the Na⁺/H⁺ transporter that would increase pH_i and stimulate SLO3 and CatSper channels increasing, even more the [Ca²⁺]_i accumulation.