Stimulation of human sperm during capacitation in vitro by an adenosine agonist with specificity for A2 receptors

The effects of an adenosine agonist with specificity for A2 receptors, on human sperm prepared for in vitro fertilization (IVF), were examined to verify physiological effects and possible pharmacological use. 5'-N-ethyl-carboxamidoadenosine (NECA), when added at 100 mu M over 30 min in B2 medium, did not induce a spontaneous acrosome reaction after 0, 3, and 6 h capacitation in B2, nor did it modify sperm motility. However, NECA increased the number of capacitated spermatozoa able to respond (p < 0.05) to A23187 (10 mu M) after 6 h preincubation in B2 medium. This effect was associated with an increase in cAMP production, which was measured by RIA after 10 and 20 min incubation with NECA in uncapacitated sperm, and with changes in the kinetics of protein tyrosine phosphorylation as revealed by Western blot. Phosphorylation of a 95-kDa protein was enhanced by NECA in uncapacitated sperm and inhibited in capacitating sperm (incubated 1 h in B2), whereas phosphorylation of a 50-kDa protein was systematically enhanced whatever the preincubation time. NECA can stimulate uncapacitated human sperm via cAMP production and protein phosphorylation/dephosphorylation without inducing an acrosome reaction or influencing motility. Cyclic AMP-dependent protein kinase A seems to positively control protein phosphorylation involved in human capacitation. Adenosine present in the tubal fluid or produced by the spermatozoon itself may influence capacitation in vivo through sperm A2 receptors. In cases of male infertility, use of NECA in sperm handling for IVF should be evaluated as a means to improve capacitation without increasing the possibility of a premature spontaneous acrosome reaction.