Abstract
Two recent studies reported that Na-Ca exchange in the outer segments of tiger salamander rod photoreceptors (Cervetto, L., Lagnado, L., Perry, R. J., Robinson, D. W., and McNaughton, P. A. (1989) Nature 337, 740-743) and of bovine rod photoreceptors (Schnetkamp, P. P. M., Basu, D. K., and Szerencsei, R. T. (1989) Am. J. Physiol. 257, C153-157) requires and transports K+ in a 4Na/(1Ca+1K) stoichiometry. In this study, we have examined the effects of K+ ions and membrane potential on the kinetics of Na-Ca and Ca-Ca exchange in rod outer segments isolated from bovine retinas. The objective was to establish the ion selectivity and voltage dependence of the different cation binding sites on the Na-Ca-K exchange protein. Potassium ions activated Na-Ca exchange when present on the Ca2+ side, although the extent of activation decreased with decreasing Na+ concentration. Potassium ions inhibited Na-Ca exchange when present on the Na+ side; inhibition arose from competition between Na+ and K+ for a common single cation-binding site. Activation of Na-Ca exchange by K+ displayed a different ion selectivity than that observed for inhibition of Na-Ca exchange by K+. The results are interpreted in terms of a three-site model for the rod Na-Ca-K exchanger. The rate of forward Na-Ca exchange decreased by 1.75-fold for a 60 mV depolarization of the plasma membrane but only at lower Na+ concentrations. The rate of Ca-Ca exchange was not affected by changes in membrane potential.
Highlights
We have examined the effects of K+ ions and membrane potential on the kinetics of Na-Ca and CaCa exchange in rod outer segments isolated from bovine retinas
The advantage of ROS for the study of NaCa exchange has been illustrated by measurements of Na-Ca exchange currents inROS (Hodgkin et al, 1987; Hodgkinand Nunn, 1987), by measurements of Na+-induced Ca” fluxes measured with 45Ca(Schnetkamp, 1980) or with optical dyes (Schnetkamp, 1986; Schnetkamp and Bownds, 1987; Schnetkamp et al, 1989),and by purification of the exchanger from bovine ROS (Cook and Kaupp, 1988; Nicoll and Applebury, 1989)
In aprevious study we demonstrated that Na-Ca exchange in bovine ROS requires K+ and Ca2+ on one side of the membrane and Na+ on the other (Schnetkamp et al, 1989)
Summary
1) and inhibited forward Na-Ca exchange veryeffectively (Fig. 7), no gradient for ammonium ions across the ROS plasma membrane was maintained formore than a second due to permeation of the neutral species ammonia and acetic acid (Schnetkamp, 1985b).
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