Unesterified cholesterol content of human sperm regulates the response of the acrosome to the agonist, progesterone

Human sperm become responsive to inducers of the acrosome reaction if they are washed free of seminal plasma and incubated in an appropriate medium. We tested the hypothesis that sperm must lose cholesterol during incubation in order to become responsive to the agonist, progesterone. Freshly ejaculated sperm contained 2.92 +/- 0.202 nmol unesterified cholesterol/10(7) sperm (mean +/- SEM, n = 18), When incubated for 24 h in vitro, sperm suspensions lost 29 +/- 6% of their free cholesterol (n = 23). Sperm lost cholesterol slightly faster than they became acrosomally responsive. Adding cholesterol to the medium prevented sperm from losing cholesterol and from becoming responsive. Varying the cholesterol content of the medium had similar effects on loss of sperm cholesterol (ED(50) = 406 nM) and acrosomal responsiveness (ED(50) = 388 nM), Incubating sperm with a 1:150 dilution of seminal plasma (containing 5.18 mu M cholesterol) also prevented sperm from losing cholesterol and from becoming responsive. Incubating sperm 24 h in medium containing 0.5 mg/ml phosphatidylcholine increased the amount of cholesterol lost and the number of sperm that became responsive, Our results support a model in which sperm unesterified cholesterol (or a molecule in equilibrium with it) suppresses acrosomal responsiveness. Sperm must lose unesterified cholesterol to become responsive to progesterone.