A membrane potential-sensitive Na+-H+ exchange system in flagella isolated from sea urchin spermatozoa

Abstract

Sea urchin sperm motility is activated by a Na+-dependent increase of internal pH. A flagellar preparation was used in the present study to investigate this ionic mechanism. Using 22Na and a pH electrode, the stoichiometry of Na+ uptake to H+ release in the isolated flagella was found to be 1.09 ± 0.11. Reversing the Na+ gradient induced reacidification of the intraflagellar pH as measured by [14C]methylamine, while reversal of the H+ gradient resulted in a Na+ efflux. Furthermore, a parallel inhibition of both ionic movements was observed with increasing external [K+]. These results indicate that Na+ and H+ are coupled through an exchanger. Measurements of the membrane potential (ψ) with [3H]tetraphenylphosphonium showed depolarization by K+, suggesting its inhibitory effect on the exchanger is through changes in ψ. This is further supported by the following experiments. (a) Cs+ by itself had little effect on either ψ or the Na+/H+ exchange, but in the presence of the ionophore valinomycin it depolarized ψ and inhibited the exchange. (b) Tetraphenylphosphonium a highly permeant cation, at 2.5 mM caused depolarization and inhibition of the exchange, and these effects were reversible by repolarization of ψ with valinomycin. The inhibitory effect of depolarization was not due to the electrogenicity of the exchange since both directions of the exchange were inhibited. It is proposed that the flagellar exchange is basically a electroneutral process but has a charged regulatory component (a gate or a conformational change) which confers the observed potential sensitivity.