1. Two methods for measurement of rectal potential difference in man were compared. A saline-filled catheter technique gave more reproducible results and was better tolerated by patients than the solid probe technique. 2. Infusion of amiloride at a concentration of 4 x 10(-6) mol/l in saline produced a variable fall of potential difference in six normal subjects. Dose-response curves in two subjects showed that complete inhibition of rectal difference occurred at 4 x 10(-5) mol/l and 4 x 10(-6) mol/l respectively. 3. This finding provides additional evidence that rectal potential difference in man results from electrogenic ion transport across the mucosal epithelial cell.

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