Role of the Na+/Ca2+ exchanger in calcium homeostasis and human sperm motility regulation

A number of cell functions, such as flagellar beating, swimming velocity, acrosome reaction, etc., are triggered by a Ca2+ influx across the cell membrane. For appropriate physiological functions, the motile human sperm maintains the intracellular free calcium concentration ([Ca2+](i)) at a Submicromolar level. The objective of this study was to determine the role of the Na+/Ca2+ exchanger (NCX) in the maintenance of [Ca2+](i) in human spermatozoa. Spermatozoa maintained in extracellular medium containing >= 1 mu M Ca2+ exhibited motility similar to that of the control. In addition to several calcium transport mechanisms described earlier, we provide evidence that the NCX plays a crucial role in the maintenance of [Ca2+](i). Three chemically unrelated inhibitors of the NCX (bepridil, DCB (3',4'-dichlorobenzamil hydrochloride), and KB-87943) all blocked human sperm motility in a dose and incubation time dependent manner. The IC50 values for bepridil, DCB, and KB-87943 were 16.2, 9.8, and 5.3 mu M, respectively. The treatment with the above-mentioned blockers resulted in an elevated [Ca2+](i) and a decreased [Na+](i). The store-operated calcium channel (SOCC) inhibitor SKF 96365 also blocked the sperm motility (IC50 = 2.44 mu M). The presence of the NCX antigen in the human spermatozoa was proven by flow cytometry, confocal laser scanning microscopy, and immunoblotting techniques. Calcium homeostasis of human spermatozoa is maintained by several transport proteins among which the SOCC and the NCX may play a major role.