Effect of passive water absorption on transepithelial movement of extracellular solutes in rat intestine

It was shown previously that intravenous infusion of saline solution decreases net absorption of sodium and water by rat small intestine and that there is an associated increased movement of inulin from blood into intestinal lumen. It was proposed that decreased absorption may be due to swelling of intercellular channels with widening of the apical cell junction so as to increase inulin movement and extrusion of absorbate back into the intestinal lumen. If this proposal is correct, than a similar increase in permeability to solutes would be expected during passive absorption of hypotonic solution which has been shown to enlarge intercellular channels. In the present study, segments of rat intestine were perfused with isotonic or hypotonic solution. The administration of hypotonic solution increased net water absorption and resulted in a twofold to fourfold increase in the movement of inulin ¹⁴C ferrocyanide or ¹⁴C mannitol from blood into intestinal lumen. The increased permeability for these substances was approx the same and, therefore, nonselective. Perfusion with a solution with similarly low electrolyte content but made isotonic with mannitol caused no change in permeability. The results are consistent with the view that distension of intercellular spaces occurs during saline infusion or passive absorption of hypotonic solution and this may increase the back flow of absorbate from intercellular channels into intestinal lumen.