Stimulation of active and passive sodium absorption by sugars in the human jejunum

Abstract

The effects of glucose and fructose on water and sodium absorption in the human jejunum were compared to assess the relative contribution of active and passive sugar stimulation of sodium transport. The effect of fructose is assumed to be entirely passive, and the difference between the effects of fructose and glucose is assumed to be a measure of sugar-stimulated, active sodium absorption. Water and sodium movement with mannitol was the base line. Three sets of test solutions with differing sugar concentrations were studied. Fructose stimulated 66-100 per cent as much net sodium and water absorption as glucose. Fructose stimulated potassium absorption, whereas glucose stimulated potassium secretion. Urea absorption was stimulated by both sugars. Glucose and fructose stimulated sodium absorption when chloride was the major anion, but they had relatively little effect on net sodium movement when chloride was replaced by bicarbonate or sulfate. It is concluded that glucose stimulates passive and active sodium transport in the human jejunum. Stimulated active sodium absorption generates an electrical potential across the mucosa that causes sodium (and potassium) secretion and partly or completely nullifies the effect of active sodium transport on net sodium movement. Net sodium absorption sitmulated by glucose is mainly (66-100 per cent) the passive consequence of solvent flow. The accompanying anion determines the degree to which sugars stimulate sodium absorption (C1 greater than SO-4 greater than HCO3). The effects of bicarbonate and sugars on jejunal sodium absorption are not additive.

PIP: The carrier interaction and solvent drag components of sugar-stimulated sodium absorption were evaluated by comparing the effects of mannitol, fructose, and glucose on jejunal absorption of water, sodium, potassium, and urea. Using water and sodium movement with mannitol as the baseline, 3 sets of test solutions with differing sugar concentrations were studied. Fructose stimulated 66-100% as much net sodium and water absorption as glucose; in addition, fructose stimulated potassium absorption, whereas glucose stimulated potassium secretion. Both sugars stimulated urea absorption. When chloride was the major anion in the solution, glucose and fructose stimulated sodium absorption, however, the sugars had little effect on net sodium movement when chloride was replaced by bicarbonate or sulfate. From these observations it was concluded that glucose stimulates both passive and active sodium transport in human jejunum. Active sodium absorption generates an electrical potential causing secretion. Net sodium absorption stimulated by glucose was mainly the passive consequence of solvent flow. The effects of bicarbonate and sugars on jejunal sodium absorption were not additive.