APICAL MEMBRANE CL-BUTYRATE EXCHANGE - MECHANISM OF SHORT-CHAIN FATTY-ACID STIMULATION OF ACTIVE CHLORIDE ABSORPTION IN RAT DISTAL COLON

The cellular model of short chain fatty acid stimulation of electroneu tral Na-Cl absorption in large intestine proposes that SCFA, following its uptake across the apical membrane, recycles and is coupled to fun ctional Na-H and Cl-short chain fatty acid exchanges. To establish the presence of a Cl-butyrate exchange (used as a model short chain fatty acid), studies of Cl-36 and C-14-butyrate uptake across apical membra ne vesicles of rat distal colon were performed. An outward butyrate-gr adient stimulated transient accumulation of Cl-36 uptake that was not inhibited by pH clamping with valinomycin (a K ionophore) and FCCP (a proton ionophore). Outward butyrate-gradient-stimulated Cl-36 uptake w as inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DI DS) with a half-maximal inhibitory concentration (IC50) of 68.4 mu M, and was saturated by both increasing extravesicular Cl concentration ( K-m for Cl of 26.8 +/- 3.4 mM and a V-max of 12.4 +/- 0.6 nmol/mg prot ein . 9 sec) and increasing intravesicular butyrate concentration (K-m for butyrate of 5.9 mM and a V-max for Cl of 5.9 nmol/mg protein . 9 sec). Cl-36 uptake was also stimulated by outward gradients of other s hort chain fatty acids (e.g., propionate, acetate and formate). In con trast, an outward Cl gradient failed to enhance C-14-butyrate uptake. Extravesicular Cl more than extravesicular butyrate enhanced Cl-36 eff lux from apical membrane vesicles. These studies provide compelling ev idence for the presence of an electroneutral, pH-activated, Cl-butyrat e exchange which in concert with Na-H exchange is the mechanism by whi ch butyrate stimulates electroneutral Na-Cl absorption.