Ion selectivity of the cation transport system of isolated intact cattle rod outer segments: evidence for a direct communication between the rod plasma membrane and the rod disk membranes.

The ion selectivity of cation transport through the plasma membrane of isolated intact cattle rod outer segments (rods) is investigated by means of 45Ca-exchange experiments and light-scattering experiments. These techniques appear to provide complementary information: the 45Ca experiments (45Ca fluxes in rods) describe electroneutral antiport, whereas the light-scattering experiments (shrinkage and swelling of rods upon hypertonic shocks with various electrolytes) reveal electrogenic uniport. Electroneutral symport of ions (salt transport) does not take place without addition of external ionophores and application of salts of weak acids. 1. Intact rods recover from a hypertonic shock in the presence of FCCP when lithium, sodium and potassium acetate are applied, but not when ammonium chloride, calcium and magnesium acetate are used. This indicates that the plasma membrane of isolated intact cattle rods is relatively permeable to net transport of Na+, Li+ and K+, and relatively impermeable to net transport of Cl-, Mg2+ and Ca2+ under conditions that do not give rise to diffusion potentials. 2. Rapid (t1/2 < 1 min) efflux of 45Ca from preloaded intact rods is observed when Na+, Ca2+, Sr2+, and under certain conditions also Ba2+, are added to the external medium. Li+, K+, Rb+, Cs+, Mg2+ and Mn2+ are ineffective in this respect as well as protons at pH 7.4. It is concluded that 45Ca efflux reflects electroneutral exchange diffusion of internal 45Ca with external Na+, Ca2+, Sr2+ and Ba2+, respectively. 3. All tested cations lower the rate of 45Ca uptake. The latter can be described by a single rate constant indicating a homogeneous rod preparation and a homogeneous endogenous Ca2+ pool. However, only those cations which stimulate 45Ca efflux from preloaded rods lower the final equilibrium of 45Ca uptake. Except for the effects of K+, Rb+ and Cs+ the reduction of the rate of 45Ca uptake by external cations appears to arise from competition for a common site on the plasms membrane. The observed affinities for this site do not correlate with actual transport (as indicated by the ability to stimulate 45Ca efflux). 4. K+ increases the affinity of the exchange diffusion system to Ca2+ from 1 microM to 0.15 microM and changes the relative affinities with respect to Ca2+ for the other cations (Na+, Mg2+, Mn2+, Sr2+, Ba2+). Furthermore, the maximal rate of Ba-Ca exchange is strongly stimulated by K+, whereas the maximal rate of Ca-Ca exchange is reduced at saturating Ca2+ concentrations. 5. The exchange diffusion transport mode can be turned off by external Na+ in a process that is not of a stochastic nature, which implies interdependence of individual transport entities and which results in an inhomogeneity of the endogenous Ca2+ pool. K+ acts as antagonist of Na+ in this effect. The revelence of these findings is discussed in relation to the generally accepted view, that a diffusable transmitter in the rod cytosol communicates the photochemical event in the disk membrane to the electrical properties of the plasma membrane...