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

Abstract

Ascending and descending segments of colon of normal and Na-depleted rats were perfused with solutions of differing KC1 concentration. Net K flux, electrical p.d. [potential difference] and, in some experiments, unidirectional K fluxes were measured. Variation of luminal K concentration over the range 0-40 mM did not affect p.d. or K efflux rate. K secretion rate fell about 30% when Na-free choline chloride solution was perfused. 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. 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. K influx was due to simple diffusion and K efflux to diffusion and active transport, both processes being increased by Na depletion.