A method is described which allows rapid measurement of the electrical potential difference across colonic mucosal epithelium to be carried out during routine sigmoidoscopy. The potential differences measured had a mean value of 25 mV (range 4 to 51 mV) in 27 subjects with normal bowel. Six hours after two intravenous injections of 0·5 mg aldosterone the potential difference had risen to 60 mV (range 37 to 101 mV). The time course of responses studied after a single injection of aldosterone showed that the potential allowance rose within four hours and had fallen again after 18 hours. Urinary sodium concentrations and sodium/potassium ratios fell after aldosterone injections, the time course of the changes being similar to that of the potential differences of the colon. Sodium concentration of stool fluid also fell. The concentration of chloride in the stool fluid was consistent with a passive distribution of chloride according to the electrochemical gradient, but that of potassium was considerably greater than expected from a passive distribution, suggesting that potassium is actively secreted into the lumen of the colon.
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