Solute Transport Process in Intestinal Epithelial Cells

Abstract

In rat small intestine, the active transport of organic solutes results in significant depolarization of the membrane potential measured in an epithelial cell with respect to a grounded mucosal solution and in an increase in the transepithelial potential difference. The mucosal cell membrane depolarizes while the serosal cell membrane markedly hyperpolarizes on the active solute transport. Corresponding results are derived from the calculations of emf''s [electromotive forces] in a variety of intestines, using the data that have hitherto been reported. The hyperpolarization of serosal membrane induced by the active solute transport might be ascribed to activation of the serosal electrogenic Na pump. To determine the causative factors in mucosal membrane depolarization during active solute transport, cell water contents and ion concentrations were measured. The cell water content increased remarkably and, at the same time, intracellular monovalent ion concentrations significantly decreased with glucose transport. Net gain of glucose within the cell was estimated from the restraint of osmotic balance between intracellular and extracellular fluids. In contrast to the apparent decreases in intracellular Na+ and K+ concentrations, significant gains of Na+ and K+ occurred with glucose transport. The quantitative relationships among net gains of Na+, K+ and glucose during active glucose transport suggest that the coupling ratio between glucose and Na+ entry by the carrier mechanism on the mucosal membrane is approximately 1:1 and the coupling ratio between Na+-efflux and K+-influx of the serosal electrogenic Na pump is approximately 4:3 in rat samll intestine. In addition to the electrogenic ternary complex inflow across the mucosal cell membrane, there are: decreases in intracellular monovalent ion concentrations; the temporary formation of an osmotic pressure gradient across the cell membrane; and the streaming potential induced by water inflow through negatively charged pores of the cell membrane in the course of an active solute transport in intestinal epithelial cells; all possible causes of mucosal membrane depolarization.