Disruption of Calcium Homeostasis in Human Spermatozoa: Implications on Mitochondrial Bioenergetics, ROS Production, Phosphatidylserine Externalization, and Motility

The etiology of male infertility is linked to oxidative stress, which is an imbalance caused by an excess of reactive oxygen species (ROS) that can negatively impact sperm function. It is known that a strong stimulus to induce excessive ROS production by spermatozoa is an intracellular calcium (Ca²⁺) overload; however, the link between Ca²⁺ dysregulation, ROS production, and impaired sperm function is still an area requiring further research. This investigation aimed to characterize the intracellular Ca²⁺ overload detrimental effects on human sperm quality. The intracellular Ca²⁺ overload was achieved by dose-dependent incubation with ionomycin, followed by analysis of key functional sperm parameters. Ca²⁺ overload caused an increase in cytosolic and mitochondrial ROS production, dissipation of mitochondrial membrane potential (ΔΨm), reduction in ATP content, cAMP levels, and motility. Furthermore, Ca²⁺ overload promoted phosphatidylserine externalization and a decrease in sperm viability. This study provides novel insights into the interplay between ROS and Ca²⁺ signaling, highlighting that disruption of homeostasis induces OS, leading to impairment of sperm quality. These findings not only contribute to the understanding of the mechanisms underlying male infertility but also provide an in vitro model for future research aimed at optimizing human sperm quality in patients with seminal OS.