Specificity of neutral amino acid uptake at the basolateral side of the epithelium in the cat salivary gland in situ.

Abstract

Amino acid uptake was measured in resting cat submandibular glands with a natural blood supply or perfused at constant flow with a Krebs-albumin solution. Following a bolus arterial injection of a 3H-labeled amino acid and D-[14C]mannitol (extracellular reference tracer), the venous effluent was immediately sampled sequentially. The maximal uptake, Umax, from the blood or perfusate was determined from the paired-tracer dilution curves using the expression: uptake % = {1-(3H/14C) .times. 100}. In glands with a natural blood supply, Umax values up to 46% were measured for short-chain (serine and alanine) and long-chain (valine, methionine, leucine, isoleucine, 1-amino-cyclopentane carboxylic acid, phenylalanine, tryptophan, tyrosine, histidine and glutamine) neutral amino acids. Umax was negligible for amino acids of the imino-glycine group (proline and glycine) and the non-metabolized amino acids, 2-aminoisobutyric acid (AIB) and methylaminoisobutyric acid (MeAIB). In glands with a natural blood supply addition of an unlabeled amino acid to the tracer injectate reduced Umax for the test amino acid by up to 80%. The pattern of these interactions suggested the presence of 2 transport systems for neutral amino acids, one preferring short-chain and the other long-chain amino acids. In glands perfused at constant flow rates with an amino acid-free Krebs-albumin solution high Umax values were measured: L-serine (66%), L-alanine (54%), L-leucine (43%), L-phenylalanine (42%) and L-tyrosine (51%). Only a low uptake was observed for L-proline (8%) and glycine (14%). There was no uptake of methylaminoisobutyric acid, which confirms the result obtained in glands with an intact circulation. Saturation of L-phenylalanine influx was observed in perfused glands as the perfusate concentration of unlabeled L-phenylalanine was increased from 0.5 to 20 mmol .cntdot. l-1. A Michaelis-Menten analysis based on a single entry system indicated an apparent Km of 6.4 .+-. 0.8 mmol .cntdot. l-1 and a Vmax of 1719 .+-. 94 nmol .cntdot. min-1 g-1. Since the fenestrated capillaries in the salivary gland are readily permeable to the test amino acid and D-mannitol, the amino acid carriers are probably in the basolateral side of the epithelium. The use of a paired-tracer dilution technique to measure uptake in a single circulatory passage enabled a detailed characterization of neutral amino acid transport in the salivary gland and has overcome the limitation of previous studies based on solute transfer from blood to saliva.