Transintestinal Acetate Transport in a herbivorous Teleost: Anion Exchange at the Basolateral Membrane

Purified basolateral membrane vesicles were prepared from the intestinal epithelium of the tilapia Oreochromis mossambicus, a herbivorous teleost. Characteristics of volatile fatty acid (VFA) transport were investigated using [3H]acetate as a representative anion. No significant change in [3H]acetate influx was observed in the presence of a cation gradient (K+ or Na+) compared to the influx observed in the absence of a cation gradient, indicating the lack of cation-dependent coupling for acetate transport. The time course of [3H]acetate uptake into vesicles preloaded with acetate or bicarbonate was enhanced compared to [3H]acetate uptake into vesicles preloaded with gluconate. A series of transstimulation and cis-inhibition studies involving both organic and inorganic anions indicated the presence of a highly specific anion-exchange carrier which readily exchanged [3H]acetate with the volatile fatty acids (formate, acetate, propionate and butyrate) and bicarbonate. Kinetic analysis of [3H]acetate influx as a function of external acetate concentration yielded a biphasic uptake curve which was interpreted as carrier-mediated transfer (Jmax=21,77±2.05 nmol mg−1 protein 10s−1; Km=12.70±2.95mmoll−1) plus apparent diffusion (P=0.17±0.02 nmol mg−1 protein 10s−1 mmol−1 acetate). [3H]Acetate uptake was also a hyperbolic function of internal bicarbonate concentration, displaying a relatively low HCO3−half-saturation constant (Km=0.41 mmol l−1). Intact intestinal sheets mounted in Ussing chambers demonstrated net absorptive fluxes of [3H]acetate when serosal acetate concentration was maintained at 1.0 mmol l−1 and the mucosal acetate concentration was varied from 1.32 to 10.0mmoll−1. At mucosal acetate concentrations lower than 1.32 mmol l−1, a net secretion of VFAs was observed. Transepithelial transport of [3H]acetate was significantly inhibited by the presence of acetazolamide. A transintestinal transport model for volatile fatty acids is proposed in which specific anion antiporters, located on the brush-border and basolateral poles of the cell, exchange luminal VFAs for serosal and intracellular bicarbonate, resulting in net transepithelial uptake of VFAs. This process is driven by a downhill lumen-to-blood VFA concentration gradient and the intracellular generation of bicarbonate by carbonic anhydrase.