Epithelial potassium transport: Tracer and electrophysiological studies in choroid plexus

Summary

Transport of potassium by bullfrog choroid plexus was studied using tracers and ion-selective microelectrodes.

• 1) Tracers: We measured unidirectional uptakes of⁴²K across each surface of the plexus, efflux of⁴²K from loaded tissues, intracellular potassium pools, and [³H]-ouabain binding.⁴²K uptake across the brush border membrane was composed of diffusional and saturable components. The saturable component exhibited kinetics of a two-site, model withK _m's of 0.3mm and aV _max of 8 μmol cm⁻² hr⁻¹. Ouabain inhibited uptake across the brush border membrane with aK _i of 1×10⁻⁷ m. Ethacrynic acid, phloretin, amiloride, and low sodium concentrations inhibited uptake, whereas bicarbonate ions increased transport up to 100%. The rate of ouabain-sensitive uptake across the serosal surface was only 6% of that across the ventricular surface. The efflux of potassium across the brush border membrane could account for most of the efflux from the epithelium. Potassium was accumulated within the plexus to a concentration in excess of 100mm.⁴²K in the extracellular compartments exchanged with 40–55% of the intracellular potassium. Ouabain bound to the brush border membrane with aK _m of 8×10⁻⁷ m, ak ₁ of 3.8×10⁴ mol⁻¹ min⁻¹ and ak ₋₁ of 3×10⁻² min⁻¹. Ouabain binding was blocked by cymarin and gitoxigin.

• 2) Electrodes: Under control conditions the intracellular electrical potential,E ^vc, was −45 mV and the apparent intracellular K⁺-concentration, K ⁺ _c , was 90mm. K⁺ ions appeared to be actively accumulated within the epithelium.E ^vc and K ⁺ _c were followed under three experimental conditions: (i) treating the tissue with ouabain; (ii) varying the ventricular K⁺ concentration; and (iii) passing transmural currents. It is concluded that the permeability of the ventricular membrane to potassium (1–5 × 10⁻⁵ cm sec⁻¹)_was much greater than the permeability of the serosal membrane and that the rate of K pumping into the epithelium was 0.35–0.55×10⁻⁹ mol cm⁻² sec⁻¹. 96% of the transmural current was paracellular, and the resistance of the serosal membrane was nine times greater than that of the ventricular membrane. Increasing K ⁺ _c produced Nernstian changes inE ^vc andE ^vc_K , but the cells only depolarized by 0.13 mV for each mV increase in the chemical potential of the ventricular solutions.

Both types of experiments are consistent with a model of the epithelium where the cells are predominantly K⁺-permeable (at the ventricular membrane) and there is a large paracellular shunt. K⁺ is actively transported into the cells by a brush border Na/K-pump, only to leave the cell passively across the same membrane. The pump, which is electrogenic, transports two K ions into the cell for every three Na ions pumped out. There are about 10×10⁶ pumps/cell, and each pump turns over 10 times/sec. The choroid epithelium behaves as predicted by the model proposed first by Koefoed-Johnsen, V., Ussing, H.H. (1958)Acta Physiol. Scand. 42:298.