Sodium-coupled amino acid and sugar transport by Necturus small intestine. An equivalent electrical circuit analysis of a rheogenic co-transport system

Necturus small intestine actively absorbs sugars and amino acids by Na-coupled mechanisms that result in increaes in the transepithelial electircal potential difference ( psi super(ms)) and the short-circuit current (I sub(sc)) which can be attributed entirely to an increase in the rate of active Na absorption. 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 in R super(m) and a depolarization of psi super(mc); and (ii) the subsequent increase in (R super(m)/R super(s)) and repolarization of psi super(mc) are the results of a decrease in R super(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-copuled solute transport processes is analyzed.