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 (J_max=21,77±2.05 nmol mg⁻¹ protein 10s⁻¹; K_m=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 (K_m=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.