Na+ /Ca2+ exchanger contributes to asterosap-induced elevation of intracellular Ca2+ concentration in starfish spermatozoa.

Asterosap, a group of equally active isoforms of sperm-activating peptides from the egg jelly of the starfish Asterias amurensis , functions as a chemotactic factor for sperm. It transiently increases the intracellular cGMP level of sperm, which in turn induces a transient elevation of intracellular Ca 2+ concentration ([Ca 2+ ] i ). Using a fluorescent Ca 2+ -sensitive dye, Fluo-4 AM, we measured the changes in sperm [Ca 2+ ] i in response to asterosap. KB-R7943 (KB), a selective inhibitor of Na + /Ca 2+ exchanger (NCX), significantly inhibited the asterosap-induced transient elevation of [Ca 2+ ] i , suggesting that asterosap influences [Ca 2+ ] i through activation of a K + -dependent NCX (NCKX). An NCKX activity of starfish sperm also shows K + dependency like other NCKXs. Therefore, we cloned an NCKX from the starfish testes and predicted that it codes for a 616 amino acid protein that is a member of the NCKX family. Pharmacological evidence suggests that this exchanger participates in the asterosap-induced Ca 2+ entry into sperm.