The Influence of the Intracellular Potential on Potassium Uptake by Beetroot Tissue

Intracellular potentials were measured in beetroot tissue during the steady-state uptake of K⁺ from various solutions. In solutions containing bicarbonate, the membrane potential becomes up to 70 mv more negative than the estimated equilibrium potential for K⁺. The uptake of K⁺ from such solutions is correlated with variations in the potential, both when the bicarbonate concentration is changed and also when the metabolic activity of the tissue is changed by washing in water for various periods. However, the estimated permeability to K⁺ varies from 0.4 x 10^-7 to 1.5 x 10^-7 cm·sec^-1. It is postulated that the change of potential arises from the metabolic transport of HCO₃^- into the cell or H⁺ outwards, and that the associated uptake of K⁺ is partly or entirely by passive diffusion across the cell membrane. In contrast, K⁺ uptake from KCl solutions is not accompanied by any significant change in the membrane potential, which remains relatively close to the K⁺ equilibrium potential. In solutions containing both KHCO₃ and KCl, it appears that an amount of K⁺ equal to the influx of Cl^- is taken up independently of the potential, while the component of K⁺ uptake which is not balanced by Cl^- uptake is related to the potential in the manner described. These results suggest that K⁺ uptake is linked to Cl^- uptake in an electrically neutral active transport process.