Regulation of salmonid fish sperm motility by osmotic shock-induced water influx across the plasma membrane

The motility of salmonid fish sperm is initiated by a decrease in the extracellular K+ concentration. However, our previous studies revealed that salmonid fish sperm motility could be initiated in the presence of an inhibitory concentration of K+ by drastic osmotic shock induced by suspension in a hypertonic glycerol solution and subsequent dilution in a hypotonic solution (glycerol-treatment). In the present study, we examined if an osmotic shock-induced water influx is involved in the regulation of salmonid fish sperm motility. HgCl2, a common inhibitor of aquaporins (AQPs), decreased the duration of salmonid fish sperm motility. Dilution of sperm cells in a hypotonic solution increased the cellular volume, whereas HgCl2 inhibited such an increase in cellular volume. Furthermore, the expression of AQP 1a and 10 in rainbow trout testes was confirmed. In contrast, HgCl2 did not affect glycerol-treated sperm motility, indicating that AQPs are not involved in glycerol-treated sperm motility. We also explored the possibility of aquaporin-independent water influx in glycerol-treated sperm by assessing the sperm membrane permeability using propidium iodide. The plasma membrane of glycerol-treated sperm was considerably permeabilized. The cellular volume was decreased in a hypertonic glycerol solution and increased upon subsequent hypoosmotic shock, indicating an AQP-independent water flux across the plasma membrane upon glycerol-treatment. Taken together, these results showed that water influx across the plasma membrane via AQP is crucial for the maintenance of salmonid fish sperm motility under normal conditions, whereas water influx by osmotic shock-induced membrane permeation is critical for the initiation of glycerol-treated sperm motility.