Biochemical parameters regulating forward motility initiation in vitro in goat immature epididymal spermatozoa

An investigation was carried out to analyse the biochemical parameters influencing forward motility (FM) initiation in vitro in the goat caput-epididymal immature spermatozoa. Forward motility was induced in approximately 55% of caput-sperm upon incubation in an alkaline (pH 8.0) modified Ringer's solution containing theophylline (30 mM) (an inhibitor of cyclic AMP phosphodiesterase), dialysed epididymal plasma (EP) and bicarbonate. Both EP and bicarbonate induced sperm motility in a dose-dependent manner, and at saturating doses EP (0.6 mg protein mL(-1)) and bicarbonate (25 mM) induced FM in approximately 38% and 44% of the cells, respectively. The motility-promoting efficacy of EP was attributed to a heat-stable protein termed 'forward motility protein' (FMP). Bicarbonate served as an initiator as well as a stabilizer of FM and its action was not dependent on FMP. FMP can induce FM in the caput-sperm, but it is not essential for sperm motility initiation. Alteration of the medium pH from 6.60 to 8.00 caused a marked increase in the EP or bicarbonate-dependent sperm FM initiation, as well as intrasperm pH. At the physiological pH, bicarbonate served as a much more potent motility activator than FMP, although both the motility promoters showed maximal efficacy at alkaline pH (approximately 7.8). EP as well as bicarbonate elevated the intrasperm cyclic AMP level. Unlike EP, bicarbonate is capable of increasing intrasperm pH. The intrasperm pH increased from 6.54 +/- 0.02 to 6.77 +/- 0.03 during sperm transit from caput to cauda. The data are consistent with the view that FMP activates sperm forward motility by enhancing the intrasperm cyclic AMP level and that extracellular bicarbonate and pH play a vital role in the initiation of sperm FM during the epididymal transit.