Effects of sugar and amino acid transport on transepithelial fluxes of sodium and chloride of short circuited rat jejunum

Abstract

1. Using the Ussing Zerahn (1951) technique the relation between short circuit current (I(sc)) and unidirectional ion fluxes across the short circuited rat jejunum has been examined.2. These parameters of jejunal function have been measured in absence of sugars and amino acids and at 10 mM glucose, 28 mM glucose, 28 mM galactose and 20 mM proline. Sulphate has been used as substituent for chloride on both sides of the gut wall and potassium has been used on the serosal side to substitute for sodium. With regard to the effects of proline on I(sc) and ion fluxes, investigations were made to see whether sulphate and potassium substitutions significantly affected proline transport.3. The purpose of the study was to examine the I(sc) seen in sugars and amino acid free conditions and the Delta I(sc) induced by these substances. Further, it was our purpose to examine the validity of extending to amino acids and sugars in general the suggestion that jejunal transport of these substances stimulates an electroneutral secretion of sodium chloride.4. In the absence of sugars and amino acids at least two-thirds of an I(sc) of 50-70 muA/cm(2) are accounted for by a process of chloride secretion located in the deep parts of the epithelium. The remaining one third or less of the I(sc) can be accounted for in terms of net flux of sodium. The results further indicate that a state of hypoxia in the deep part of the epithelium is the cause of the very low I(sc), p.d. on electrical resistance usually found for sacs of everted rat jejunum.5. The increase in I(sc) induced by proline and glucose is best accounted for by the simultaneously induced increase in mucosa to serosa flux of sodium. There is no compelling evidence that proline should increase I(sc) by a stimulation of chloride secretion.6. Proline, glucose (and galactose) stimulate unidirectional fluxes of sodium and chloride. The effects on serosa to mucosa fluxes equal or exceed those on mucosa to serosa fluxes. The effect of proline on mucosa to serosa flux of thiourea exceeds by far that on serosa to mucosa flux. This and the effects of potassium and sulphate substitution lead to the conclusions in (7) and (8).7. Transport of sugars and amino acids induces or stimulates in rat jejunum in vitro an electroneutral secretion of sodium chloride.8. This effect suffices to explain why, in spite of marked increases in I(sc), increments in net flux of sodium are not induced by transport of proline and glucose. It also explains why the effects of sugar and amino acid transport on water transport in vitro are much less than in vivo.