Regulation of amino acid influx and efflux at the basolateral plasma membrane of the salivary gland epithelium: effects of parasympathetic nerve stimulation.

Basolateral amino acid transport systems in the salivary epithelium of resting and secreting cat submandibular glands were characterized by means of a rapid paired-tracer dilution technique. Amino acid uptake was measured by comparison of venous tracer concentration profiles for a labeled amino acid and D-mannitol (an extracellular tracer of similar size) following an intra-arterial bolus injection of both radioactive molecules. Unidirectional uptake of 21 amino acids, dopamine, noradrenaline, and serotonin was quantified in non-secreting glands. During 8 Hz parasympathetic nerve stimulation, significant epithelial uptakes were measured for L-[3H] alanine and L-[3H] phenylalanine, but less than 0.2% of the injected amino acid was recovered in the collected saliva. In non-secreting glands, cross-inhibition studies of L-[3H] alanine, L-[3H] phenylalanine, and L-[3H] lysine uptake by unlabeled amino acid competitors and detailed kinetic influx experiments indicated that epithelial uptake was mediated by three distinct parallel transport systems: ASC (short-chain neutral), L (branched-chain and aromatic neutral), and y+ (cationic). Rapid metabolism of alanine was inhibited by aminooxyacetate, and the metabolic uncoupler dinitrophenol selectively accelerated the efflux of transported large neutral amino acids and L-lysine. Concurrent autoradiographic experiments suggest that transport sites for small and large neutral amino acids are localized in the basolateral plasma membrane of acinar, demilunar, and striated ductal cells.