Removal of ambient K⁺ inhibits net Na⁺ transport in toad bladder by reducing Na⁺ permeability of the luminal border

Abstract

Sodium crosses epithelia in two steps: an influx into the cell through the luminal membrane down an electrochemical gradient, and an energy-requiring efflux across the serosal border against a gradient. Koefoed-Johnsen and Ussing¹ originally proposed that cellular efflux of Na⁺ at the serosal border of frog skin is coupled to K⁺ influx by an exchange pump. It was, therefore, assumed that the inhibition of active Na⁺ transport following the removal of K⁺ from the serosal side was due to a reduced pool of K⁺ available for the coupled exchange. Indirect evidence suggests that this is not correct. (1) K⁺ influx from the serosa does not correlate to any significant degree with active Na⁺ transport^2–5. (2) Removal of Na⁺ as well as K⁺ from the serosal medium substantially reduces the inhibitory effect of K⁺ removal on active transport^2,6. (3) One would predict that if K⁺ removal did inhibit active transport only at the serosal pump site, then as active transport decreased the ‘active transport pool’, that is, the mucosally derived cell Na⁺, would increase as it does with ouabain. Studies by Robinson and Macknight⁷, however, showed that the mucosally derived pool does not change on removal of external K⁺. Essig and Leaf² suggested that removal of serosal K⁺ inhibited Na⁺ transport by decreasing the permeability of the luminal membrane. They showed that when K⁺ was removed from the serosa, there was a decrease in the amount of ²²Na taken up into the tissue after exposure of the luminal surface to the isotope for 5 min. We have recently found that tissue uptake of ²²Na after exposure of toad bladders to the isotope is linear with time for only up to 24 s. Because only the linear portion of the uptake is a measure of the Na⁺ flux across the luminal border⁸, it is clear that longer periods of uptake are more representative of the steady-state isotope distribution in the tissue. This value is controlled not only by the luminal permeability but also by the serosal permeability and the rate of net transport. These issues prompted us to re-investigate the effect of low external K⁺ on Na⁺ movement across the luminal border. We report here that the Na⁺ transport inhibition seems to be due to a reduced Na⁺ permeability at the luminal border.