Increases in concentrations of potassium and bicarbonate ions promote acquisition of motility in vitro by Japanese eel spermatozoa

We examined the effects in vitro of the ionic composition and pH of the medium on the acquisition of motility by testicular spermatozoa of the Japanese eel, Anguilla japonica. We obtained milt from the genital pore and testicular milt from the testes of artificially matured cultivated eels. About 45% of milt spermatozoa were motile when they were diluted with hyperosmotic solution. By contract, 1.4% of testicular spermatozoa were motile. We produced an artificial seminal plasma (ASP), which corresponded in terms of ionic constituents to the seminal plasma obtained from the milt, and incubated immotile testicular spermatozoa with the ASP. The spermatozoa. showed increased motility (44.0-60.7%) after a 60-min incubation at all pH values examined (pH 7.5-8.7). Absence of bivalent cations from the ASP did not impede the acquisition of motility at pH 8.1. However, testicular spermatozoa incubated with potassium-free ASP (pH 7.8-8.4) did not become motile, and the motility increased in proportion to the concentrations of potassium ions (0-30 mM) in ASP at pH 8.1. When testicular spermatozoa were incubated with HCO3--free ASP at pH 8.1 (the ASP including 15.2 mM KCI), spermatozoa did not become motile. Motility of the spermatozoa increased in proportion to the concentrations of HCO3- ions in ASP. Thus, acquisition of motility required potassium and bicarbonate ions and their concentrations regulated the percentage of motile spermatozoa. The pH of testicular milt (pH 7.5) was significantly lower than that of milt (pH 8.0), and the concentration of potassium ions in testicular plasma (44.3 mM) was significantly higher than that in seminal plasma (15.2 mM). Thus, it appears that immotile testicular spermatozoa become motile upon transfer from the testicular lumen to the sperm duct, with increases in pH and in the concentration of bicarbonate ions. (C) 1997 Wiley-Liss, Inc.