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 Ca²⁺ concentration ([Ca²⁺]_i). Using a fluorescent Ca²⁺-sensitive dye, Fluo-4 AM, we measured the changes in sperm [Ca²⁺]_i in response to asterosap. KB-R7943 (KB), a selective inhibitor of Na⁺/Ca²⁺ 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 Ca²⁺ entry into sperm.