Nitric oxide regulates human sperm capacitation and protein-tyrosine phosphorylation in vitro

The aim of the present study was to investigate whether the generation of n itric oxide by human spermatozoa is associated with human sperm capacitatio n and with the tyrosine phosphorylation of sperm proteins. Human spermatozo a were capacitated in the presence or absence of nitric oxide-releasing com pounds or nitric oxide synthase inhibitors, and then the percentage of acro some loss induced by human follicular fluid or by calcium ionophore was det ermined. The presence of the nitric oxide-releasing compounds primed sperma tozoa to respond earlier to human follicular fluid whereas nitric oxide syn thase inhibitors decreased the percentage of acrosome reaction. Moreover, n itric oxide modulated tyrosine phosphorylation of sperm proteins. A tight c orrelation between capacitation and tyrosine phosphorylation regulated by n itric oxide was observed. Results indicate that nitric oxide is involved in human sperm capacitation and emphasize the importance of oxidoreduction re actions in the fine control of sperm physiology.