ANGIOTENSIN-II INDUCES ACROSOMAL EXOCYTOSIS IN BOVINE SPERMATOZOA

Ejaculated mammalian spermatozoa must reside in the female genital tra ct for some time before gaining the ability to fertilize the egg. Duri ng this time, spermatozoa undergo some physiological changes that coll ectively are called capacitation. Capacitation of mammalian spermatozo a is a prerequisite for acrosome reaction, which is an exocytotic even t occurring before fertilization. The specific biophysical and biochem ical changes that accompany sperm capacitation and the agonists induci ng acrosome reaction are not fully understood. Using SDS-gel electroph oresis and immunoblotting, we demonstrate the existence of a class of angiotensin receptors (AT(1)) in bovine spermatozoa. In capacitated sp erm, we show that angiotensin II (ANG II) AT1 receptors are localized in the head and tail, whereas in noncapacitated cells the receptors ar e localized in the tail only. We find that ANG II markedly stimulates acrosomal exocytosis of capacitated bovine spermatozoa in vitro in a c oncentration range of 0.1-10 nM. No effect of ANG II was found in nonc apacitated cells. The ability of ANG II to stimulate the acrosome reac tion depends on the presence of calcium ions in the incubation medium. The ANG II-induced acrosome reaction was markedly inhibited by a sele ctive AT1 receptor antagonist, losartan (DUP 753). PD-123319, a select ive antagonist of the ANG II AT(2) receptor, had no effect on the ANG II-induced acrosome reaction. Thus ANG II via activation of AT(1) rece ptors may play a regulatory role in the induction of the acrosome reac tion.