Inhibition of guinea pig sperm capacitation, the acrosome reaction and sperm-egg fusion by dl-propranolol in vitro

dl-Propranolol (Pro), known to have spermicidal effect, was evaluated for its ability to influence capacitation, the acrosome reaction (AR) and fertilization of guinea pig spermatozoa at non-spermicidal dose levels. A concentration-dependent decrease in AR and the whiplashed motility occurred in Pro as low as 0.05 mmol/L. Pro 1.0 mmol/L completely abolished AR, followed by the loss of sperm capacity to penetrate into zona-free hamster egg. Pro exerted its inhibitory effect of AR primarily by restraining the capacitation stage of sperm but at Pro 0.5 mmol/L by blocking both capacitation and AR stages. % of AR of sperm preincubated with ionophore A-23187 0.2 mumol/L was significantly higher than that of the control, but markedly lower at Pro greater than or equal to mmol/L, implying that Pro prevented sperm Ca2+ influx. Prenylamine was also found to inhibit AR potentially and enhance the action of Pro against ionophore A-23187. Furthermore, Pro inhibition of the fertilizing ability of preincubated spermatozoa was antagonized by cAMP. Pro 0.5 mmol/L even caused the egg vesiculation. The reversibility of the action depends on the dose and time of sperm exposure to Pro. These findings suggest that the inhibitory effects of Pro on guinea pig sperm capacitation, the acrosome reaction and fertilization may be the mechanisms for its antifertility action.