SPERM CHEMOTAXIS - EGG PEPTIDES CONTROL CYTOSOLIC CALCIUM TO REGULATE FLAGELLAR RESPONSES

Fragmentary evidence indicates that intracellular [Ca2+] (Ca-i) mediates sperm chemotaxis. However, neither correlations of swimming responses to chemoattractant-induced alterations of Ca-i nor explanations of how chemoattractant gradients control Ca-i exist. Here Ca-i increases produced by the egg peptide speract-not previously known to cause flagellar responses-were prolonged by treatment with 3-isobultyl-1-methylxanthine (IBMX). Flagellar waveform asymmetry then increased 40% and swimming paths became tightly circular. Moreover, both responses required external Ca2+ (as does sperm chemotaxis to eggs and egg products). Ca-i increases by the established chemotactic peptide resact also required external Ca2+ and were enhanced by IBMX. Therefore, diverse egg peptides may use fundamentally similar mechanisms to control Ca-i and thereby swimming behavior in chemotaxis. Repetitive increasing additions of speract produced adaptive membrane potential and Ca-i responses indicating that sperm can detect increasing gradients of egg peptide over a broad concentration range. We offer a model in which shallow or decreasing gradients elevate Ca-i and redirect swimming paths but sufficiently steep gradients keep Ca-i low and swimming linear until the egg is reached. A negative-feedback loop, initiated by cGMP-mediated activation of sperm K+ channels and terminated by subsequent inactivation of guanylyl cyclase, may coordinate gradient detection with control of Ca-i. Continued stimulation of more receptors by steeply increasing gradients of egg peptide thus maintains membrane hyperpolarization and suppresses Ca2+ entry and Ca-i elevation. The molecular basis for chemotaxis therefore is explained as translation of the spatial gradient of peptide concentration into changes in K+ channel activity in the time domain. (C) 1994 Academic Press, Inc.