Role of protein tyrosine phosphorylation in the thapsigargin-induced intracellular Ca2+ store depletion during human sperm acrosome reaction

During human sperm capacitation, an increase in phosphotyrosine content of specific proteins results partially from an increase in the intracellular free Ca2+ concentrations. In the present study, the inter-regulation between protein phosphotyrosine content and the intracellular Ca2+ concentration during the thapsigargin treatment of capacitated human sperm was investigated. The involvement of a tyrosine kinase pathway in the thapsigargin-induced acrosome reaction was also investigated. In response to thapsigargin, two sperm subpopulations, called LR (low responsive) and HR (high responsive), according to their increase in intracellular Ca2+, were observed. In addition to their high increase in intracellular Ca2+, sperm from the HR population expressed a higher protein phosphotyrosine content, and a higher proportion (P < 0.05) of them underwent the acrosome reaction in response to thapsigargin, as compared with LR sperm. Although the tyrosine kinase inhibitor PP2 abolished the thapsigargin-induced increase in protein phosphotyrosine content, it did not affect the intracellular Ca2+ concentration or the percentage of acrosome-reacted sperm. The inability of an src-related tyrosine kinase inhibitor to block the thapsigargin-mediated Ca2+ increase and acrosomal exocytosis suggests that, during the acrosome reaction, the signalling pathway mediated by src-related tyrosine kinases is involved upstream of the capacitative Ca2+ entry.