Intracellular calcium accumulation and responsiveness to progesterone in capacitating human spermatozoa

Progesterone induced a rapid, long-lasting, dose-dependent increase of intracellular free calcium concentration ([Ca2+]i) in human sperm capacitated overnight. This effect was not counteracted by the cytosolic progesterone receptor antagonist RU486 (1 mumol/L) nor by the GABA-A receptor antagonists bicuculline (10 mumol/L) and picrotoxin (50 mumol/L). Also, the rank order of potency of several progestative steroids on [Ca2+]i differed from that previously reported for uterine intracellular progesterone receptor or for P-GABA interaction in the central nervous system, indicating a different pathway for progesterone stimulation of human sperm. Modifications of basal and progesterone-stimulated [Ca2+]i during sperm capacitation were also studied. A progressive, parallel increase of basal and progesterone-stimulated [Ca2+]i in capacitating spermatozoa was found. In particular, progesterone-stimulated [Ca2+]i increased from a basal concentration of 147% +/- 17% at 10 minutes to 327% +/- 65% after 120 minutes of incubation in capacitating medium. This increase was well correlated with basal [Ca2+]i (r = 0.93). In contrast, basal and progesterone-stimulated [Ca2+]i concentrations were constantly low in spermatozoa incubated in noncapacitating medium. In capacitated spermatozoa, initial responsiveness to progesterone and basal [Ca2+]i was higher than in capacitating and noncapacitated samples, and remained constant throughout the duration of the experiment. The progressive, parallel increase of [Ca2+]i and response to progesterone observed during in vitro capacitation of human spermatozoa might be physiologically relevant in vivo during capacitation of sperm in the female genital tract.