Measurement of potassium turnover in rod photoreceptors in toad isolated retina using ion-selective microelectrodes

Ion-selective microelectrodes (ISMs) were used to measure the turnover of intracellular K+ (Ki+) in rods in the isolated retina of the toad, Bufo marinus. The light-evoked hyperpolarization of rods decreases their passive K+ efflux, which in combination with active K+ uptake, decreases extracellular K+ concentration, Ko+.Rb+ substitutes for K+ in these processes. The turnover of Ki+ was measured as Rb+ and K+ were exchanged, using ISMs that were approximately five times more sensitive to Rb+ than to K+. When Ko+ was replaced by Rbo+, the light-evoked decrease in K+ efflux produced only a small change in ISM voltage, delta VISM, owing to the background of Rbo+. As Rbi+ replaced Ki+, the efflux shifted from K+ to Rb+ and delta VISM grew in amplitude. After loading the rods with Rbi+, Rbo+ was replaced by Ko+. The light-evoked decrease in Rb+ efflux lead transiently to a large delta VISM, since the change in Rbo+ was superimposed upon a background of Ko+. As Ki+ replaced Rbi+, the amplitude of delta VISM declined. When measured using this technique, the turnover of Ki+ was 95% complete in approximately 15 min. In low Ca2+ solutions, transmembrane fluxes of K+ (Rb+) increased and turnover of Ki+ occurred more rapidly. During background illumination, transmembrane fluxes of K+ (Rb+) decreased and turnover of Ki+ was slowed. These experiments have provided independent corroboration of earlier observations concerning rod K+ fluxes. This ISM-based technique also may be useful in measuring K+ turnover in other cell types.