1. Methods for studying the kinetics of movement of radioactive tracers across human distal colonic epithelium have been developed for use in vivo and applied to examination of trans-epithelial transfers of potassium.

2. During absorption of 43K from the lumen, two potassium pools, A and B, in the epithelium were identified, having mean turnover half-times of 16 and 87 min respectively. On average about 50% of the potassium lumen-to-plasma flux and most, if not all, of the potassium plasma-to-lumen flux took place through pool A. The transcellular pathway (equated with pool B) appeared to be of relatively little significance in transepithelial movement of potassium in the human distal colon.

3. Sodium and chloride movements were unaffected when lithium was present in the luminal solution, but the potassium secretion rate was nearly trebled due to increased plasma-to-lumen flux through the fast turnover pool A.

4. The permeability of the epithelium to potassium, as judged by the plasma-to-lumen fluxes, greatly exceeded that to sodium. This appeared to depend on a special highly selective potassium channel which differed from the shunt pathway in possessing definite capacity.

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