On the Mechanism of Isosmotic Transport Across the Small Intestine. The Composition of the Absorbate Transported from a Mucosal Solution made Hypertonic by the Addition of Mannitol

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Abstract
Experiments were performed to investigate the mechanism by which the fluid transported across the small intestine becomes isosmotic with the mucosal solution when the latter is made hypertonic by the addition of a poorly absorbed substance. The mid small intestine of the hamster was isolated and everted. The mucosal surface of the control preparation was bathed with an isotonic Krebs–Ringer bicarbonate solution containing 10 mM glucose (KRBSG) (292 mOsmol/kg). The mucosal surface of the experimental preparations was bathed with one of the hypertonic solutions. These hypertonic solutions were prepared by the addition of 1, 50, 100, or 150 mM mannitol to KRBSG. No bathing solution was placed on the serosal side of these preparations, so that the fluid transported across the intestine, undiluted by a bathing solution (absorbate), could be collected on the serosal side. Results show that the absorbate was always isosmotic with the mucosal solution. In the experimental preparations, as compared to the absorption of other solutes, mannitol was poorly absorbed, even though its concentration in the absorbate sometimes reached 35% of its concentration in the mucosal solution. The small amount of mannitol found in the absorbate was never sufficient to render the absorbates isosmotic with hypertonic mucosal solutions. The absorbate became isosmotic with the mucosal solution due to an increased concentration of other solutes transported from the mucosal side. Increasing the osmolality of the mucosal solution progressively from 292 to 442 mOsmol/kg caused a linear increase in the concentration of Na, K, glucose, and mannitol in the absorbate, but, except for mannitol, the net amount of these substances transported (per gram dry tissue) to the serosal side decreased linearly. The transport of fluid also decreased. Our data further indicate that the ratio of fluid and solute transport progressively decreased with the increase in osmolality of the mucosal solution. Therefore, it appears that when the mucosal solution is made hypertonic by the addition of a poorly absorbed substance, the absorbate becomes isosmotic with the hypertonic mucosal solution by means of a greater diminution of fluid transport than of solute transport.

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