ROLE FOR CA2-TRANSDUCTION PATHWAY LEADING TO ACROSOMAL EXOCYTOSIS IN HUMAN SPERMATOZOA( CHANNELS IN THE SIGNAL)

Progesterone interaction with human spermatozoa promotes a rise in int racellular Ca2+ and can trigger acrosomal exocytosis in capacitated ce lls. We have used nifedipine, a 1,4-dihydropyridine Ca2+ channel antag onist, to investigate the possibility that Ca2+ channels play a role i n the progesterone-stimulated exocytotic response. Cells were assessed biochemically for the generation of diacylglycerol (DAG) and microsco pically for acrosome loss using chlortetracycline fluorescence. When m otile cells were preincubated for 5 hr using culture conditions simila r to those used for successful human in vitro fertilization, a short e xposure to progesterone significantly stimulated DAG formation and acr osomal exocytosis. The addition of nifedipine (10 and 100 nM), either at time O or just prior to progesterone introduction, significantly in hibited both DAG formation and exocytosis, suggesting that Ca2+ channe ls are involved in the responses observed. Treatment of capacitated ce lls with a synthetic permeant DAG stimulated exocytosis irrespective o f whether nifedipine was present, indicating that Ca2+ channels functi on prior to DAG generation. The possibility that an influx of Na+, as well as Ca2+, might be involved in the exocytotic pathway was investig ated using the monovalent cation ionophores monensin and nigericin. Bo th significantly stimulated DAG generation and acrosome loss, but the prior inclusion of nifedipine significantly inhibited all responses. T hese results strongly suggest that the entry of Ca2+ through Ca2+ chan nels, with characteristics similar to those of L-type, voltage-sensiti ve Ca2+ channels found in cardiac and skeletal muscle, is a crucial st ep in the sequence of events leading to exocytosis in progesterone-sti mulated human spermatozoa. An influx of Na+ also may play a role, but at a point prior to the opening of Ca2+ channels. (C) 1996 Wiley-Liss, Inc.