Transport of potassium by the colon of normal and sodium‐depleted rats

Ascending and descending segments of colon of normal and Nadepleted rats were perfused with solutions of differing KCl concentration. Net K flux, electrical p.d. and, in some experiments, unidirectional K fluxes were measured.2. Variation of luminal K concentration over the range 0-40 mM did not affect p.d. or K efflux rate.3. K secretion rate fell about 30% when Na-free choline chloride solution was perfused.4. Net flux was a linear function of the luminal K concentration, and fell as the latter increased. Na depletion increased K secretion rate and passive permeability of the mucosa to K. Adrenalectomy had the reverse effect. The luminal K concentration, associated with zero net K flux was much greater than expected if the colonic mucosa behaved passively with respect to K.5. Unidirectional fluxes determined when 5 mM-KCl was in the lumen showed that the ratio influx/efflux was much less than predicted by Ussing's flux ratio equation.6. It was concluded that K influx was due to simple diffusion and K efflux to diffusion and active transport, both processes being increased by Na depletion.