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 > NH4 > Rb > 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.

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