PROGESTERONE TRIGGERS A WAVE OF INCREASED FREE CALCIUM DURING THE HUMAN SPERM ACROSOME REACTION

The mammalian sperm acrosome reaction (AR), an essential fertilization event, requires an influx of Ca2+. The Ca2+ increase occurring in the human sperm head during the AR initiated by progesterone, a putative in vivo AR initiator, was investigated using video-image analysis with fura-2, a fluorescent Ca2+ probe. Progesterone treatment of capacitat ed human sperm resulted in a wave-like increase in sperm head cytosoli c [Ca2+](i) that appears to increase fastest in a region near the equa torial segment and then spreads throughout the rest of the head. The p rogesterone-mediated Ca2+ increase in the sperm head was strongly inhi bited and the wave eliminated by picrotoxin, a blocker of GABAA recept or/Cl- channels and an inhibitor of the progesterone-mediated Cl- effl ux and progesterone-initiated AR of human sperm. These results are the first to detect a ligand-mediated Ca2+ wave in sperm and to suggest t hat Cl- efflux influences Ca2+ influx during the AR. (C) 1997 Academic Press.