Bicarbonate-stimulated [14C]butyrate uptake in basolateral membrane vesicles of rat distal colon

Abstract

Background: The mechanism of short-chain fatty acid (SCFA) absorption by the colon is not known. The aim of these experiments was to identify the transport mechanisms present in the basolateral membrane to develop an overall model of colonic SCFA absorption. Methods: These studies determined the uptake of [14C]-butyrate (used as a model SCFA) by basolateral membrane vesicles prepared from rat distal colonic mucosa. Results: Significantly higher [14C]butyrate uptake under an acidic environment (extravesicular pH [pH0] = intravesicular pH [pH1] = 5.5) than that under alkaline environment (pH0 = pH1 = 7.5) indicates the presence of nonionic diffusion. In the absence of a pH gradient (pH0/pH1 = 7.5/7.5), outward gradients of bicarbonate significantly stimulated [14C]butyrate uptake. Additional presence of a pH gradient (pH0/pH1 = 6.0/7.5) further enhanced the bicarbonate gradient-stimulated [14C]butyrate uptake that was not inhibited by voltage clamping but was inhibited substantially by an anion exchange inhibitor 4,4′-diisothiocyanatostilbene-2,2′disulfonic acid (DIDS) (82%). Both increasing extravesicular butyrate concentration and intravesicular bicarbonate concentration saturated bicarbonate/pH gradient-stimulated [14C]butyrate uptake with an apparent Michaelis constant (Km) for butyrate of 6.9 mmol/L and an apparent Km for bicarbonate of 27.4 mmol/L. Conclusions: Butyrate uptake by basolateral membrane vesicles represents both nonionic diffusion and a carrier-mediated SCFA-bicarbonate exchange process that differs from the SCFA-bicarbonate exchange recently identified in apical membrane vesicles. Thus, two distinct carrier-mediated anion exchange processes located in apical and basolateral membranes mediate transcellular SCFA transport in colonocytes.