Discrimination of parallel neutral amino acid transport systems in the basolateral membrane of cat salivary epithelium.

Transport of short-chain and long-chain neutral amino acids across the basolateral membrane of the epithelium in the perfused cat salivary gland has been studied using a rapid (less than 30 s) single circulation paired-tracer dilution technique. Amino acid uptake was measured by comparing the venous dilution profiles for a tritiated amino acid and D-[14C]mannitol (extracellular reference) following a bolus intra-arterial injection of a mixture containing both molecules. Unidirectional influx (v) was estimated from the maximal tracer uptake (Umax), the perfusate flow (F) and the perfusate amino acid concentration (Ca): v = [-F . ln (1-Umax)] . Ca. L-alanine influx was saturable and apparently mediated by a single entry system (Km = 0.83 +/- 0.11 mM and Vmax = 655 +/- 32 nmol/min . g). These kinetic constants were considerably lower than our previously reported values for L-phenylalanine: Km = 6.4 mM and Vmax = 1719 nmol/min . g. In cross-inhibition experiments performed over a wide range of concentrations (0.05-24 mM), influx of L-alanine and L-phenylalanine could be further discriminated, since both L-phenylalanine (Ki = 22 mM) and L-alanine (Ki = 19 mM) behaved as poor competitors. Removal of Na+ from the perfusate resulted in a selective inhibition of L-alanine and L-serine influx, whereas influx of the long-chain neutral amino acids L-leucine, L-phenylalanine and L-tryptophan remained unaffected. Although prolonged perfusion of glands with dinitrophenol (0.8 mM for 20-30 min) caused a variable but net inhibition of unidirectional uptake, it markedly enhanced the tracer efflux of L-leucine, L-phenylalanine, L-tyrosine and the basic amino acid L-lysine. It appears that at least two separate neutral amino acid transport systems are operative at the blood-tissue interface of the salivary epithelium: (i) a Na+-dependent alanine-serine-cysteine preferring type of carrier exhibiting a high affinity for amino acids with short, polar or linear side chains and (ii) a Na+-independent leucine preferring type of carrier selective for large neutral amino acids.