Intestinal electrolyte absorption by parallel determination of unidirectional sodium and water transfers

¹ U.S. Naval Radiological Defense Laboratory, San Francisco, California

Plasma-to-lumen, lumen-to-plasma and net transfer rates for both sodium and water have been determined on dogs with Thiry-Vella loops, using a single transit perfusion. The net and lumen-to-plasma transfer rates of sodium are proportional to the concentration of sodium in the intestinal perfusate; net absorption of sodium ion occurs at all luminal sodium concentrations above 32 mEq/l. Water net transfer rate is inversely proportional to luminal osmolality; net absorption of water takes place at all luminal osmolalities up to 400 mOs/l. Hence, water must be actively transported and not transported exclusively as a passive osmotic consequence of sodium movement. Lumen-to-plasma water-transfer rates do not show a clear-cut osmotic dependency, owing to correlated sodium and water transfers. The regression slope of sodium on water transfer rates indicates the one-way transfer of an extremely dilute solution (42 mEq/l.), and this is true regardless of the composition of solution in the lumen. Such a phenomenon is a clear description of a fluid circuit mechanism.