The gradient of electrical potential difference and of sodium and potassium of the gut contents along the caecum and colon of normal and sodium-depleted rats.

1. The Na, K and water content of stools, and of gut contents removed from the terminal ileum, caecum and colon were determined in normal and Na-depleted rats and the p.d. across the colon wall measured at the site of removal of each specimen.2. During passage through the caecum and colon, especially the ascending segment of colon, the faecal Na and water content fell considerably, K content being unchanged in the normal rats and falling in the Na-depleted. Na concentration of the faecal water fell but K concentration rose owing to water absorption.3. Feeding normal rats with a sulphonated polystyrene resin caused a considerable Na loss in the stool, the ratio Na/(Na+K) being consistently greater than in rats not taking resin. Resin induced little Na but much K loss in Na-depleted rats.4. The electrical p.d. across the colon wall varied little over the length of the caecum and colon in normal rats, rarely exceeding 20 mV, the serosa being +ve with respect to lumen. Potential difference measurements were greater in Na-depleted rats, and those of the caecum and descending colon were consistently higher than those of the ascending colon. There was a similar pattern in resin-fed rats but potentials tended to be higher.5. K concentration of the gut contents was always greater than could be accounted for if K were passively distributed across the colonic mucosa.6. It was concluded that: (i) active Na absorption was stimulated by Na depletion; (ii) K was probably actively transported into the colon lumen, and when unabsorbable anions were present in the gut K secretion was critically important in Na absorption; (iii) the elevation of p.d. associated with Na depletion was probably associated with the stimulated Na transport.