Early persistent activation of sperm K⁺ channels by the egg peptide speract

Transduction by sperm of the instructive signal provided by the egg peptide speract involves rapid, complex changes in internal ion and cyclic nucleotide content. Here, investigations of hypotonically swollen sperm provide insight into the underlying processes and identify K⁺ channel activation as an initial ionic event in gamete recognition. A sustained hyperpolarization of swollen sperm is promoted by <2.5 pM speract and is followed (with >100 pM speract) by transient repolarization and (with >10 nM speract) by depolarization that is dependent on external Ca²⁺. Monophasic increases in pH_i are produced only by >25 pM speract, indicating that hyperpolarization may not directly promote alkalinization. Increased K⁺-selective (K⁺ > Rb⁺ > Cs⁺ > Na⁺) membrane permeability is found after all speract >2.5 pM, suggesting that hyperpolarization results from persistent activation of K⁺ channels and that repolarization has a different ionic basis. Supporting this contention, the K⁺ channel blocker tetraethylammonium (20 mM) inhibits the increased K⁺ permeability that follows treatment of swollen sperm (and of sperm in seawater) with 2.5 pM speract. Such induced activation of K⁺ channels is observed in patch-clamped swollen sperm examined in the cell-attached configuration, upon application of 5-50 pM speract to the bath medium. The efficacy of externally applied speract and its potency indicate that activation is indirect and probably involves an as yet unidentified diffusible mediator whose production is promoted by speract at concentrations 0.01-0.001 times those predicted from reported estimates of the K_d for the known speract receptor.