Transient Ca2+ mobilization caused by osmotic shock initiates salmonid fish sperm motility

Salmonid fish sperm motility is known to be suppressed in millimolar concentrations of extracellular K(+), and dilution of K(+) upon spawning triggers cAMP-dependent signaling for motility initiation. In a previous study, however, we demonstrated that suspending sperm in a 10% glycerol solution and subsequent dilution into a low-osmotic solution induced motility independently of extracellular K(+) and cAMP. In the present study, we further investigated the glycerol-induced motility mechanism. We found that treatment with solutions consisting of organic or inorganic ions, as well as glycerol, induced sperm motility in an osmolarity-dependent manner. Elimination of intracellular Ca(2+) by BAPTA-AM significantly inhibited glycerol-treated sperm motility, whereas removal of extracellular Ca(2+) by EGTA did not. Monitoring intracellular Ca(2+), using fluo-4, revealed that intracellular Ca(2+) increased when sperm were suspended in hypertonic solutions, and a subsequent dilution into a hypotonic solution led to a decrease in intracellular Ca(2+) concomitant with motility initiation. In addition, upon dilution of sperm into a hypertonic glycerol solution prior to demembranation, the motility of demembranated sperm was reactivated in the absence of cAMP. The motility recovery suggests that completion of axonemal maturation occurred during exposure to a hypertonic environment. As a result, it is likely that glycerol treatment of sperm undergoing hypertonic shock causes mobilization of intracellular Ca(2+) from the intracellular Ca(2+) store and also causes maturation of axonemal proteins for motility initiation. The subsequent dilution into a hypotonic solution induces a decrease in intracellular Ca(2+) and flagellar movement. This novel mechanism of sperm motility initiation seems to act in a salvaging manner for the well-known K(+)-dependent pathway.