Regression of high-affinity carrier-mediated intestinal transport of taurine in adult cats

Abstract

The sulfur-containing beta-amino acid taurine is an essential nutrient for cats. Owing to some metabolic peculiarities, cats are more dependent on dietary sources of taurine than other mammals. We therefore studied taurine uptake by intestinal brush-border membrane vesicles (BBMV) isolated from cat small intestine. Taurine uptake by feline BBMV was not influenced by a transmembrane Na+ gradient compared with Na(+)-free conditions. Kinetic analysis of initial taurine uptake yielded no evidence for a carrier-mediated transport process. These findings cannot be attributed to an overall inability of cumulative substrate uptake in our vesicle preparations because D-glucose uptake was strongly Na+ dependent, showing the overshoot phenomenon typically associated with active transport in vesicle experiments. Furthermore, L-leucine transport was mediated by a single Na(+)-dependent carrier mechanism (maximal transport velocity 1.2 nmol.mg protein-1.s-1, Km = 2.1 mM). In contrast to taurine uptake by feline BBMV, transport of taurine by pig intestinal BBMV was markedly enhanced by an inwardly directed Na+ gradient. Thus, differing from other mammalian species, taurine uptake across the intestinal brush-border membrane of the adult cat seems not to be mediated by a specific transport mechanism. Therefore taurine absorption from the gastrointestinal tract will possibly become a limiting factor for maintenance of taurine homeostasis in the cat under conditions of decreased dietary taurine intake.