1. The mechanism of potassium transport across human distal colon was investigated in twenty-two individuals without evidence of bowel disease, by using a dialysis method in conjunction with measurements of the transepithelial potential difference (p.d.). 2. Whether potassium was absorbed or secreted depended on its initial concentration in the lumen. The potassium net flux was approximately zero when the luminal potassium concentration was between 30 and 50 mmol/l. 3. Potassium secretion rate was little affected by sodium absorption rate, or by the luminal sodium concentration or by the osmolality of the luminal solution. 4. Potassium secretion rate was increased by partial substitution of other cations for sodium, in the descending order Li greater than NH4 greater than Rb greater than Na. Potassium concentration increased on average to over 80 mmol/l when lithium was in the lumen. 5. The observed transepithelial p.d. was inadequate to account for the intraluminal potassium concentrations attained, the discrepancy being most marked when ammonium or lithium was in the lumen. It is suggested that some potassium is secreted by the epithelial cells and this component of the total potassium flux into the lumen is increased when rubidium, ammonium or lithium is substituted for sodium.

The effect of aldosterone on the ionic composition of colonic epithelial cells and on lithium absorption rate was studied by experiments on rats in vivo. Aldosterone considerably increased the rate of sodium absorption without measurably altering the sodium and potassium content of the epithelium. Aldosterone did not alter the electrical resistance of the tissue. With lithium in the lumen, the net sodium fluxes in the colon were similar in normal and aldosterone-stimulated rats but the rate of diffusion of lithium across the epithelium was greater in the aldosterone-stimulated group. The amount of lithium accumulating in the epithelial layer was also considerably increased by aldosterone stimulation. Aldosterone appears to increase the permeability of the mucosal (luminal) barrier allowing increased entry of lithium into the colonic epithelial cells.

Edmonds, 1971). Since the procedures are atraumatic and can be done in the course of routine examinations, it is possible to make repeated observations and so study changes produced by hormones, drugs, or pathological processes (Edmonds, 1971;Rask-Madsen, and Jensen, 1973;Edmonds and Pilcher, 1973). Similar methods should be suitable for study of the epithelium of the terminal ileum in a patient with an ileostomy and the object of the present work was to examine the feasibility of the procedures in this situation. MethodsEleven patients (seven males, four females, 24-54 years), were studied. They had all been treated for ulcerative colitis by total colectomy and had normally functioning ileostomies. The studies were done at between two weeks and six years after formation of the ileostomy. In all the ileum appeared healthy and was functioning satisfactorily.