title: High affinity L-aspartate transport in chick small intestine.
abstract: Epithelial cells isolated from chick small intestine were used to study the mechanism of L-aspartate transport. Two kinetically distinct uptake systems of high (Km' = 16 microM) and low (Km'' = 2.7 mM) affinity are observed. This paper examines the cation dependence and membrane potential sensitivity of the high affinity system. Unidirectional influx studies indicate that extracellular Na+ is an absolute requirement for transport function. Flux is optimal when K+ is present intracellularly, however this cation is not required for Na+-dependent L-aspartate uptake. In the absence of K+, flux enhancement is observed when the intracellular pH is acidic. In contrast, acidic intracellular pH is inhibitory in cells that are preequilibrated with K+. Sodium ([Na+]o greater than [Na+]i gradients, and potassium ([K+]o less than [K+]i) or proton ([H+]o less than [H+]i) gradients can independently energize the Na+-dependent accumulation of L-aspartate above equilibrium levels, suggesting that Na+ and L-aspartate cotransport occurs with concomitant K+ or H+ antiport. L-Aspartate influx is insensitive to membrane potential changes created by inwardly directed anion gradients in the presence or absence of intracellular K+. A model is presented that is consistent with electroneutral Na+-coupled transfer with an ion antiport site of low specificity.
mesh_terms: D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism*; D001692:Biological Transport; D002645:Chickens; D004848:Epithelium / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007408:Intestinal Absorption; D007421:Intestine, Small / Q000378:metabolism*; D007700:Kinetics; D011188:Potassium / Q000378:metabolism
chemical_list: D001224:Aspartic Acid; D011188:Potassium