In both herbivorous tilapia (Oreochromis mossambicus) and carnivorous rockfish (Sebastes caurinus) intestinal and pyloric cecal brush-border membrane vesicles (BBMV), [14C]glycylsarcosine ([14C]Gly-Sar) uptake was stimulated by a transmembrane proton gradient. A transmembrane K(+)-diffusion potential (inside negative) stimulated [14C]Gly-Sar uptake above that observed with short-circuited vesicles, whereas an inwardly directed Na+ gradient in both fishes had no effect on peptide uptake. In tilapia, [14C]Gly-Sar influx occurred by the combination of 1) a high-affinity, saturable, proton gradient-dependent carrier system [Kt [concentration that equals one-half of maximum influx (Jmax)] = 0.56 +/- 0.08 mM; Jmax = 1,945.0 +/- 174.6 pmol.mg protein-1.10 s-1]; 2) a low-affinity, nonsaturable (within 1-10 mM), proton gradient-dependent carrier system (nonsaturable carrier-mediated transport component = 4,514.0 +/- 28.1 pmol.mg protein-1.10 s-1.mM-1); and 3) a diffusional component accounting for < 10% of total influx within the concentration range tested. Influx (10 s) of 1-10 mM [14C]Gly-Sar in tilapia intestine was significantly (P < 0.01) inhibited by 10 mM diethylpyrocarbonate, a specific inhibitor of proton-coupled peptide transport systems. [14C]Gly-Sar influx into tilapia BBMV showed cis-inhibition and trans-stimulation by Gly-Pro, suggesting that [14C]Gly-Sar and Gly-Pro shared the same mucosal peptide transporter in fish. These observations strongly suggest that intestinal transport of peptides in herbivorous and carnivorous fishes is proton gradient dependent, electrogenic, sodium independent, and qualitatively resembles the peptide transport paradigm proposed for mammals.
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