Effect of Changes in the Osmolality of the Luminal Fluid on Intracellular Concentration of Solutes in the Hamster Jejunum

Abstract

Paired experiments were performed in vitro using two adjacent segments of hamster jejunum. The intracellular Na, K, glucose, and water concentrations of the segment incubated in isotonic solution on the mucosal side (mucosal solution) (control) were compared to the intracellular concentrations of the same solutes and water in the segment incubated in hypo- or hypertonic mucosal solutions (experimental). The solution bathing the serosal side (serosal solution) was always isotonic and remained so throughout the experimental period of 45 min. Results indicate that hypotonicity of the mucosal solution significantly increased the total and intracellular water content of the intestine. Intracellular Na, K, and glucose concentrations significantly decreased, while, except for glucose, the intracellular solute content remained unaltered. Hyperosmolality of the mucosal solution, on the other hand, decreased the total and intracellular water content, with an increase in the concentration of all intracellular solutes. The intracellular solute content, however, did not change. Calculation of the data shows that the approximate total intracellular solute concentration (2Na+ + 2K+ + glucose) increases linearly with the increase in osmolality of the mucosal solution. The approximate total intracellular solute concentration was always higher than that of the mucosal solution. In muscle tissue, all of the K+ but only 30% of the Na is osmotically active. If this percentage of free Na+ is applied to the epithelial cell, our calculations indicate that the intracellular fluid of the intestinal cell became approximately isoosmotic with the mucosal solutions used. This osmotic equilibrium between the mucosal solution and the epithelial cell may be a prerequisite for the isoosmotic transport of fluid across the intestinal wall.