Mechanism of short-chain fatty acid uptake by apical membrane vesicles of rat distal colon.

In this study, the presence of a bicarbonate gradient-dependent, carrier-mediated anion exchange process for butyrate (a representative short-chain fatty acid) uptake in apical membrane vesicles isolated from rat distal colon is described. An outward gradient of both butyrate- and bicarbonate-stimulated [14C]butyrate uptake and resulted in transient accumulation (an "overshoot" phenomenon). Butyrate gradient-stimulated [14C]butyrate uptake was not altered either by an imposed pH gradient or at different pH values. In contrast, bicarbonate gradient-stimulated [14C]butyrate uptake was stimulated severalfold by an additional imposition of an outward pH gradient (pHi = 7.5; pH0 = 6.0). This bicarbonate- and pH gradient-stimulated butyrate uptake was not inhibited by either voltage clamping, with equimolar intravesicular and extravesicular K+ and valinomycin, or 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an anion-exchange inhibitor. Increasing butyrate concentrations saturated the bicarbonate- and pH gradient-stimulated butyrate uptake with a half-maximal concentration (Km) of 26.9 +/- 1.6 mmol/L. Butyrate uptake was substantially inhibited by 20 mmol/L propionate (45%) and acetate (60%) but was not inhibited by oxalate, inorganic anions (SO4(2-) and NO3-), and transport inhibitors (amiloride, acetazolamide, furosemide, and ouabain). It is concluded from these results that bicarbonate gradient-stimulated butyrate uptake in apical membrane vesicles of rat distal colon occurs via a carrier-mediated anion-exchange process that differs from other DIDS-sensitive anion exchanges [e.g., the Cl- -OH- (HCO3-) process].