Effects of potassium and osmolality on spermatozoan motility of salmonid fishes.

In salmonid fishes, rainbow trout and masu salmon, and the plecoglossid fish, ayu, seminal plasma had an osmolality around 300 mosmol kg-1 isotonic to the blood plasma, and contained a higher concentration of potassium than the blood plasma. Spermatozoa of salmonid fishes were motile when semen was diluted 1:100 with solutions of sodium chloride or mannitol, over the osmotic range of 0-300 mosmol kg-1. They were immotile in sodium chloride solution containing several mM potassium. This indicates that osmolality is not an essential determinant of sperm motility in the Salmonidae, and that sperm motility in these species is suppressed by the seminal potassium in the sperm duct, and initiated by a decrease in potassium concentration surrounding spawned spermatozoa when they are released into fresh water.