title: Low-affinity intestinal L-aspartate transport with 2:1 coupling stoichiometry for Na+/Asp.
abstract: Epithelial cells isolated from chick small intestine were used to define the ionic and electrical characteristics of a low-affinity (Km = 4.1 mM) L-aspartate transport system. L-Glutamate and D-aspartate, but not D-glutamate, were found to inhibit L-aspartate influx, suggesting that this uptake system has a substrate specificity similar to that previously described for a high-affinity (Km = 16 microM) acidic amino acid transporter in the same cells. Low-affinity uptake is Na+ dependent with a Hill coefficient (n) of 1.4. Intracellular K+ moderately enhances but is not required for aspartate influx, and this response is modulated by changes in intracellular pH. The Na+-dependent uptake of aspartate is electroneutral, as evidenced by insensitivity to pronounced changes in delta psi induced by anion gradients or valinomycin in the presence of K+ gradients. Because the above characteristics can be consistent with several transport models, direct measurement of delta Na+-delta Asp coupling stoichiometry were performed. The coupling ratio was determined to be approximately 2.0. A model for intestinal Na+-dependent L-Asp transport is suggested in which each transport cycle involves inward transfer of 2Na+:1Asp+ and outward transfer of K+ or H+ in a net electroneutral set of events.
mesh_terms: D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism* / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D002645:Chickens; D004848:Epithelium / Q000378:metabolism; D005971:Glutamates / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007421:Intestine, Small / Q000378:metabolism*; D007700:Kinetics; D008353:Mannitol / Q000494:pharmacology; D013237:Stereoisomerism; D014634:Valinomycin / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D005971:Glutamates; D014634:Valinomycin; D001224:Aspartic Acid; D008353:Mannitol