Role of Calcium and Calcium Channels in Progesterone Induced Acrosome Reaction in Caprine Spermatozoa

There are several physiological and pharmacological evidences indicating that opening of voltage dependent Ca channels play a critical role in induction of acrosome reaction in mammalian sperm. We determined the intracellular free Ca concentration in ejaculated goat sperm using a fluorescent, Ca-specific probe, Fura2/AM, after the suspension of sperm in KRB medium, capable of sustaining capacitation and the acrosome reaction. We used nifedipine, D-600 and diltiazem, the Ca channel antagonists belonging to the classes of dihydropyridines, phenylalkylamines and benzothiazepines, to investigate the possibility that L-type voltage gated Ca channels play a role in the progesterone-stimulated exocytotic response. Progesterone promoted a rise in intracellular Ca in goat sperm and addition of nifedipine (100 nM) just prior to progesterone induction, significantly inhibited both intracellular Ca rise and exocytosis suggesting that Ca channels are involved in the process. However, the intracellular Ca increase during the process of capacitation was not affected with the addition of nifedipine suggesting a role of focal channel for Ca during capacitation. Studies using monensin and nigericin, two monovalent cation ionophores showed that an influx of Na also may play a role in the opening of Ca channels. These results strongly suggests that the entry of Ca channels with characteristics similar to those of L-type, voltage-sensitive Ca channels found in cardiac and skeletal muscle, is a crucial step in the sequence of events leading to progesterone induced acrosome reaction in goat sperm. (Asian-Aust. J. Anim. Sci. 2002. Vol 15, No. 7 : 949-956)