Tyrosine kinase inhibition reduces the plateau phase of the calcium increase in response to progesterone in human sperm

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Abstract
Progesterone (P) has previously been shown to induce a rapid increase in [Ca2+]i as well as tyrosine phosphorylation of proteins in human spermatozoa. Both these effects are essential for induction of the acrosome reaction by P. We investigated a possible relationship between the P-induced calcium increase and tyrosine kinase activation, by evaluating the effect of the tyrosine kinase inhibitor genistein on these two effects. We found that preincubation with genistein abolished P-induced tyrosine phosphorylation of two sperm proteins of 97 and 75 kDa molecular weight and significantly inhibited the plateau phase of P-induced [Ca2+]i increase without affecting the peak phase. Conversely, the plateau phase was enhanced by the tyrosine phosphatase inhibitor Na3VO4. The effect of genistein was specific for P, since no inhibition was observed on the [Ca2+]i increase induced by thapsigargin, an inhibitor of endoplasmic Ca2+-ATPase previously shown to mobilize Ca2+ in spermatozoa. These results indicate that tyrosine kinase activation is involved in the generation of the plateau phase of Ca2+ influx induced by P, and suggest the possibility that two different pathways are involved in the induction of Ca2+ entry by P in human sperm.