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.
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