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

Publisher Website
Google Scholar
Abstract
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.