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

Abstract

Ion-selective microelectrodes (ISMs) were used to measure the turnover of intracellular K+[Formula: see text] 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, [Formula: see text]. Rb+ substitutes for K+ in these processes. The turnover of [Formula: see text] was measured as Rb+ and K+ were exchanged, using ISMs that were approximately five times more sensitive to Rb+ than to K+. When [Formula: see text]was replaced by [Formula: see text], the light-evoked decrease in K+ efflux produced only a small change in ISM voltage, ΔVISM, owing to the background of [Formula: see text]. As [Formula: see text] replaced [Formula: see text], the efflux shifted from K+ to Rb+ and ΔVISM grew in amplitude. After loading the rods with [Formula: see text], [Formula: see text] was replaced by [Formula: see text]. The light-evoked decrease in Rb+ efflux lead transiently to a large ΔVISM, since the change in [Formula: see text], was superimposed upon a background of [Formula: see text]. As [Formula: see text] replaced [Formula: see text], the amplitude of ΔVISM declined. When measured using this technique, the turnover of [Formula: see text] was 95% complete in approximately 15 min. In low Ca2+ solutions, transmembrane fluxes of K+ (Rb+) increased and turnover of [Formula: see text] occurred more rapidly. During background illumination, transmembrane fluxes of K+ (Rb+) decreased and turnover of [Formula: see text] 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.