Intracellular Calcium Accumulation and Responsiveness to Progesterone in Capacitating Human Spermatozoa

Abstract

ABSTRACT: 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 μmol/L) nor by the GABA‐A receptor antagonists bicuculline (10 μmol/L) and picrotoxin (50 μmol/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. 1991 American Society of Andrology