Sodium-coupled amino acid and sugar transport byNecturus small intestine

Summary

Necturus small intestine actively absorbs sugars and amino acids by Na-coupled mechanisms that result in increases in the transepithelial electrical potential difference (ψ ^ms) and the short-circuit current (I _sc) which can be attributed entirely to an increase in the rate of active Na absorption. Studies employing conventional microelectrodes indicate that the addition of alanine or galactose to the mucosal solution is followed by a biphasic response. Initially, there is a rapid depolarization of the electrical potential difference across the apical membrane (ψ ^ms) which reverses polarity (i.e. cell interior becomes positive with respect to the mucosal solution) and a marked decrease in the ratio of the effective resistance of the mucosal membrane to that of the serosal membrane (R ^m/R ^s); these events do not appear to be dependent on the availability of metabolic energy. These initial, rapid events are followed by a slow increase in (R ^m/R ^s) toward control values which is paralleled by a repolarization ofψ ^ms and increases inψ ^ms andI _sc; this slow series of events is dependent upon the availability of metabolic energy.

The results of these studies indicate that: (i) the Na-coupled mechanisms that mediate the entry of sugars and amino acids across the apical membrane are “rheogenic” (conductive) and result in a decrease inR ^m and a depolarization ofψ ^ms; and (ii) the subsequent increase in (R ^m/R ^s) and repolarization ofψ ^ms are the results of a decrease inR ^s which is associated with an increase in the activity of the Na pump at the basolateral membrane.

The physiologic implications of these findings are discussed and an equivalent electrical circuit model for “rheogenic” Na-coupled solute transport processes is analyzed.