Inhibition of bovine spermatozoa by caudal epididymal fluid: I. Studies of a sperm motility quiescence factor.

Spermatozoa from all portions of the bovine epididymis are essentially quiescent when examined in vitro without dilution. However, sperm from the caudal epididymis, particularly the distal portion, develop full motility when they are diluted into seminal plasma or simple isotonic buffers. Dilution of the sperm into neat cauda epididymal fluid (CE fluid) does not result in the initiation of motility. The initiation of motility upon dilution into buffers is complete within 10-20 min, while the inhibition induced by CE fluid is nearly instantaneous. CE fluid concentration, but not sperm concentration, controls sperm motility. Therefore, an inhibitory component of this fluid, but not sperm-sperm interactions, is responsible for the inhibition. CE sperm, which have been diluted into isotonic buffers and are consequently motile, become quiescent when resuspended in CE fluid; thus, this process is fully reversible. No elevations in sperm cyclic AMP levels can be detected concomitant with the induction of motility but high concentrations of cyclic AMP phosphodiesterase inhibitors can overcome the quiescence induced by CE fluid. The inhibitors of CE sperm motility reported for other species, e.g., the high-viscosity mucin, immobilin ; carnitine; calcium; or glycerylphosphorylcholine , do not appear to be of importance in the bovine caudal epididymis. The quiescence produced by bovine CE fluid is strongly dependent upon the extracellular pH; i.e., motility is inhibited at pH 5.5 but not at pH 7.6.