Abstract
Before dilution in hypoosmotic media sperm of freshwater fish are maintained quiescent by a range of factors including osmolality, K^+^ and pH, and the onset of motility is generally associated with an increase in cytoplasmic Ca^2+^. In contrast, Ca^2+^ in conjunction with osmolality was found to inhibit motility of intact bluegill sperm. Consistent with seminal plasma composition, 0.16 mmol/L Ca^2+^ and greater, in conjunction with an osmotic concentration of 290 mOsm/kg, inhibited the onset of bluegill sperm motility; sperm diluted in saline at 290 mOsm/kg without Ca^2+^ became motile. Cations Mn^2+^ and Sr^2+^, in conjunction with osmolality, had an inhibitory effect on initiation of sperm motility similar to that of Ca^2+^. Sperm motility was inhibited by Ca^2+^ channel blockers nimodipine and nifedipine, the mitochondrial Ca^2+^ uniporter inhibitor ruthenium red and the calmodulin inhibitors W‐7 and trifluoperazine dihydrochloride. These results provide evidence that elevated cytoplasmic Ca^2+^ inhibits sperm motility and yet low levels permit or promote motility. This study demonstrates a unique inhibitory action of Ca^2+^ on the motility of intact fish sperm at physiologically relevant levels. J. Exp. Zool. 307A:590–599, 2007. © 2007 Wiley‐Liss, Inc.