Mitochondria are crucial organella for cell metabolism. Although this organellum is localized in the midpiece in mammalian sperm flagellum, its contribution to sperm motility as energy source under physiological conditions is unclear. Ca2+ plays a crucial role in fertilization in many species participating in the main functions of spermatozoa: maturation, motility, and the acrosome reaction. Recently, several reports indicate that the mitochondrion is an organelle involved in the regulation of cytosolic Ca2+ level in certain types of cells, but the role of mitochondria in Ca2+ homeostasis in spermatozoa has not been explored.
In this study, we measured mitochondrial Ca2+ level using a fluorescent Ca2+ indicator, Rhod-2, in mouse and human spermatozoa and compared it to cytoplasmic Ca2+ level determined with another Ca2+ indicator, Fluo-3. Restricted localization of sperm mitochondria facilitates us to analyze Ca2+ imaging of this organellum.
Our preliminary results indicated that the resting Ca2+ concentration in the mitochondria matrix is maintained quite low compared to cytoplasmic Ca2+ in both mouse and human spermatozoa although some variety in the results with human sperm was observed. We are further investigating calcium dynamics in mitochondria as response to different treatments.