A POSSIBLE ROLE FOR CA2-ATPASE IN HUMAN SPERM CAPACITATION()

Mammalian spermatozoa require extracellular Ca2+, some of which must b e internalized, to undergo capacitation and acrosomal exocytosis. The mechanisms controlling the intracellular Ca2+ concentration are unclea r, but current evidence suggests that a Ca2+-ATPase may be involved. U sing treatments that potentially modulate enzyme activity, we investig ated this possibility in human spermatozoa; the capacitation state and acrosomal integrity were monitored by chlortetracycline fluorescence. Incubation of cells in the presence of quercetin, a Ca2+-ATPase inhib itor, significantly accelerated the transition from the uncapacitated F pattern of chlortetracycline fluorescence to the capacitated, acroso me-intact B pattern within 1 h. This was followed by an increase in th e number of cells displaying the capacitated, acrosome-reacted AR patt ern. Since most Ca2+-ATPases in somatic cells are sensitive to calmodu lin, we also investigated the effect of the calmodulin antagonist W-7 on chlortetracycline patterns. At 1-125 mu mol l(-1), W-7 significantl y stimulated capacitation and acrosomal exocytosis. Furthermore, W-7 a t 1 mu mol l(-1) proved to be more effective than W-5, a less potent a ntagonist, suggesting that the observed responses in human spermatozoa did reflect a calmodulin-sensitive mechanism. When the glucose concen tration in the culture medium was varied (from 0 to 5.56 mmol l(-1)) t o alter the availability of ATP for enzyme activity, it was found that a reduced concentration of glucose promoted capacitation more rapidly than did the standard concentration of 5.56 mmol glucose l(-1). Howev er, maximal changes, particularly in promoting the shift from the B to the AR pattern of chlortetracycline fluorescence, required millimolar concentrations of glucose during the last few minutes before assessme nt. Finally, the addition of partially purified mouse sperm decapacita tion factor (proposed to activate a Ca2+-ATPase and thus maintain a lo w intracellular Ca2+ concentration) to capacitated human sperm suspens ions caused a significant reversal in the capacitation state of cells (from the B to the F pattern). The F pattern of chlortetracycline fluo rescence predominates in conditions favouring low concentrations of in tracellular Ca2+. From these results, we suggest that a Ca2+-ATPase ma y play an important role during human sperm capacitation. A time-depen dent decrease in endogenous enzyme activity would allow the intracellu lar concentration of Ca2+ to rise to a critical value necessary for in itiation acrosomal exocytosis and subsequent successful fertilization.