The Electrical Potential Difference

Studies of the physicochemical forces involved in the transfer of ions and water across biological membranes have received great impetus from the work on the isolated frog skin by Ussing, Zerahn and Koefoed-Johnson (1, 2). This type of approach would help to define the mechanisms involved in the transfer of fluid and electrolyte across the intestine in vivo. A previous study on the large intestine in vivo has stressed the striking concentration gradients developed between the lumen and the plasma (3). Evaluation of the driving forces for electrolyte and water transfer across the large intestine has not been possible, however, in the absence of measurements of transfer rates and electrical potential. Recent studies on the isolated large intestine of the guinea pig and frog have indicated that the absorption of sodium is active, and that of chloride is passive (4, 5). The maintenance of an electrical potential gradient was found to be dependent upon active sodium transport. However, since the intestine can secrete (6) as well as absorb, even these studies provide limited information. In the experiments reported here, the electrical potential difference across the large intestine of the anesthetized dog was measured. Simultaneously the net transfer of water, sodium, potassium, chloride and bicarbonate between the lumen and the blood were determined. The unidirectional movements of sodium and chloride were calculated from the rate of absorption of their radioisotopes. By correlating the electrical potential difference with observed chemical events, it was possible to obtain a more precise analysis of some of the forces influencing ionic and water transfer across the large intestine. Information was obtained which clearly demonstrates at least two active