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Journal of Experimental Biology 156,41-61 (1991)
Published by Company of Biologists 1991

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

E. TITUS ¹ and G. A. AHEARN ¹

¹ Department of Zoology, University of Hawaii, Manoa, Honolulu, HI 96822, USA

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 trans-stimulation 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±2.95 mmoll^-1) plus apparent diffusion (P=0.17±0.02 nmol mg^-1 protein 10s^-1 mmoll^-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 mmoll^-1). Intact intestinal sheets mounted in Ussing chambers demonstrated net absorptive fluxes of [³H]acetate when serosal acetate concentration was maintained at 1.0 mmoll^-1 and the mucosal acetate concentration was varied from 1.32 to 10.0 mmoll^-1. At mucosal acetate concentrations lower than 1.32 mmoll^-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.

Key words: volatile fatty acids, anion exchange, intestine, transport, basolateral, tilapia, herbivore, Oreochromis mossambicus

Accepted on November 7, 1990

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