On the mechanism of isoosmotic transport of fluid across the small intestine. The effect of the Staverman reflection coefficient of the solute used to increase the osmolality of the mucosal solution on the composition of the absorbate.

Abstract

The objective of the present study was to investigate the mechanism by which the fluid transported across the small intestine becomes isoosmotic with the mucosal solution. The osmolality of the mucosal solution of a unilateral preparation (the serosal side was not bathed) was increased with water soluble, nonelectrolyte solutes of different Staverman reflection coefficients (σ), and the composition of the undiluted absorbate was studied. The solutes ('hyperosmotic agents') used were formamide, urea, erythritol, and mannitol. In a series of separate experiments, utilizing a method based on potential difference depression, we found that the σ's of these solutes in the hamster small intestine were 0.26, 0.85, 0.93, and 1.00, respectively. Our data on transport across the unilateral preparation showed that when the intestine was incubated in an isotonic mucosal solution (Krebs–Ringer bicarbonate solution containing 10 mM glucose, 292 mosmol/kg), the absorbate was essentially isotonic, although its composition was significantly different from that of the mucosal solution. When the osmolality of the mucosal solution was increased to 342, 392, or to 442 mosmol/kg with any of the hyperosmotic agents, the absorbate was always isoosmotic with the mucosal solution. With formamide (low σ) as the hyperosmotic agent, the concentration of electrolytes and glucose in the absorbate was the same as that in the absorbate of the control preparation (incubated in the isotonic mucosal solution), and the transported fluid became isoosmotic with the mucosal solution exclusively due to the presence of formamide in the absorbate. When the osmolality of the mucosal solution was increased with urea, erythritol, or mannitol, the concentration of these hyperosmotic agents in the absorbate decreased linearly with the increase in their σ, and the absorbate became isoosmotic with the mucosal solution due to a linear increase in the concentration of electrolytes and glucose. In addition, we found that the transport of fluid, solutes, and fluid/solute (ml/mmol per gram dry intestine) decreased linearly with the increase in σ of the hyperosmotic agents. Further calculation of the data showed that the increase in the concentration of electrolytes and glucose with the increase in σ of the hyperosmotic agents was almost exclusively due to a linear decrease in fluid/solute transport.