title: Modulation of the voltage-sensitive Na+/H+ exchange in sea urchin spermatozoa through membrane potential changes induced by the egg peptide speract.
abstract: Sea urchin sperm motility can be activated by alkalinization of the internal pH, and previous studies have shown that the internal pH can be regulated by a voltage-sensitive Na+/H+ exchanger present in the flagellar plasma membrane. In this study, the effects of speract, a peptide purified from egg conditioned media, on the Na+/H+ exchange were investigated. Evidence presented indicates that speract activates K+ channels in the flagellar membrane and modulates the Na+/H+ exchange activity through resultant changes in membrane potential. In the presence of tetraphenylphosphonium, a lipophilic ion, or high external Na+, the isolated flagella were depolarized, and Na+/H+ exchanger was inhibited. Speract and valinomycin, a K+ ionophore, were able to reactivate 22Na+ uptake, H+ efflux, and alkalinization of intraflagellar pH under either of the depolarizing conditions. Membrane potential measurements using 3,3'-dipropylthiodicarbocyanide iodide indicated repolarization by either speract or valinomycin. The speract-induced voltage changes did not require Na+ but were sensitive to [K+]. Thus, speract induced a slight depolarization in Na+-free seawater with 10 mM K+ but a hyperpolarization with 2 mM K+. Further support for the activation of K+ channels in the flagella was the 2-5-fold stimulation of K+ efflux induced by speract as measured with a K+ electrode. The ionic selectivity of the speract-activated channel assessed by voltage measurements was K+ greater than Rb+ greater than Cs+. The half-maximally effective concentration of speract was about 0.2 nM. That the H+ and K+ efflux in response to peptide was receptor-mediated was confirmed by the use of speract or resact on intact sea urchin spermatozoa, where the peptides were found to stimulate K+ efflux and to reverse the tetraphenylphosphonium inhibition on H+ efflux only in the homologous spermatozoa. Modulation of the voltage-sensitive Na+/H+ exchange by egg peptides, therefore, appears to be indirect and is coupled through its action on membrane potential.
mesh_terms: D004305:Dose-Response Relationship, Drug; D005407:Flagella / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009842:Oligopeptides / Q000494:pharmacology*; D009861:Onium Compounds / Q000494:pharmacology; D009943:Organophosphorus Compounds / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D011522:Protons; D012617:Sea Urchins; D012964:Sodium / Q000378:metabolism*; D013094:Spermatozoa / Q000187:drug effects* / Q000378:metabolism
chemical_list: D007473:Ion Channels; D009842:Oligopeptides; D009861:Onium Compounds; D009943:Organophosphorus Compounds; D011522:Protons; C028364:speract; D012964:Sodium; D011188:Potassium; C013289:tetraphenylphosphonium