Unidirectional uptake of L-alanine and L-serine by system ASC in the basolateral membrane of cat salivary epithelium. Selective inhibition of rapid alanine metabolism by aminooxyacetate

Effect of a protein-rich meal on urinary and salivary free amino acid concentrations in human subjects

The aim of the present study was to investigate whether in healthy volunteers acute changes in plasma free amino acid composition after a protein-rich test meal are reflected in the urinary and salivary concentrations of the corresponding amino acids. The ingestion of a protein-rich meal elicited a significant increase of plasma and urine amino acid concentrations. The postprandial salivary amino acid excretion showed only minor changes. For several amino acids (alanine, arginine, asparagine, glycine, threonine and valine) significant relations were observed between the increase in concentration of these amino acids in venous plasma and urine. In whole saliva, only threonine and valine showed a significant relationship with the corresponding plasma concentration. Our data suggest that the urinary amino acid excretion of several amino acids has the potential for estimating short-term changes in plasma concentrations. Determination of salivary amino acid concentrations seems less appropriate for this purpose.

Evidence for an alanine, serine, and cysteine system of transport in isolated brain capillaries

Brain capillaries isolated from 2-month-old male and female Sprague-Dawley rats were used to study the transport of neutral amino acids. The uptake of alanine, leucine, and alpha-methylaminoisobutyric acid (MeAIB) was a linear function of time for the first minute of incubation. Based on these observations, an incubation time of 1 min was used to measure transport activities. The intracellular water volume of the isolated capillaries was 2.2 microliters/mg protein. This value was significantly lower (1.8 microliter/mg protein) when measured in the absence of sodium. L-Alanine, L-serine, and L-cysteine were taken up from the abluminal surface of brain capillaries by a sodium- and energy-dependent, carrier-mediated system. This uptake, for the most part, was not inhibited by MeAIB. System ASC (alanine-serine-cysteine) appeared to be of primary importance for the transport of these amino acids in isolated brain capillaries. The apparent Km and Vmax values for L-alanine uptake by ASC transport, based on the Hofstee plot presentation, were 1.3 mM and 0.975 nmol/microliter water content/min, respectively. The results also indicate that the transport of MeAIB and 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH) was limited to the sodium-dependent system A (alanine) and the sodium-independent system L (leucine), respectively.

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.

Evidence for a lactate transport system in the sarcolemmal membrane of the perfused rabbit heart: kinetics of unidirectional influx, carrier specificity and effects of glucagon

The kinetics and specificity of L-lactate transport into cardiac muscle were studied during a single transit through the isolated perfused rabbit heart using a rapid (15 s) paired-tracer dilution technique. Kinetic experiments revealed that lactate influx was highly stereospecific and saturable with an apparent Kt = 19 +/- 6 mM and a Vmax = 8.4 +/- 1.5 mumol/min per g (mean +/- S.E., n = 14 hearts). At high perfusate concentrations (10 mM), the inhibitors alpha-cyano-4-hydroxycinnamate (Ki = 7.3 mM), pyruvate (Ki = 6.5 mM), acetate (Ki = 19.4 mM) and chloroacetate (Ki = 28 mM) reduced L-lactate influx, and Ki values were estimated assuming a purely competitive interaction of the inhibitors with the monocarboxylate carrier. The monocarboxylic acids [14C]pyruvate and [3H]acetate were themselves transported, and sarcolemmal uptakes of respectively 38 +/- 1% and 70 +/- 8% were measured relative to D-mannitol. Perfusion of hearts for 10-30 min with 0.15 or 1.5 microM glucagon increased myocardial lactate production and simultaneously inhibited tracer uptake of lactate, pyruvate and acetate. It is concluded that a stereospecific lactate transporter exhibiting an affinity for other substituted monocarboxylic acids is operative in the sarcolemmal plasma membrane of the rabbit myocardium.

Regulatory effects of insulin and experimental diabetes on neutral amino acid transport in the perfused rat exocrine pancreas. Kinetics of unidirectional L-serine influx and efflux at the basolateral plasma membrane

Relatively little is known about the hormonal regulation of amino acid transport in the normal and diabetic exocrine pancreas. In this study unidirectional influx and tracer efflux of L-serine at the basolateral interface of the rat pancreatic epithelium was investigated in the perfused exocrine pancreas using a rapid (less than 30 s) paired-tracer dilution technique. In the non-diabetic pancreas L-serine influx was saturable and stimulated by perfusion with exogenous bovine insulin (100 microU/ml). Transport of L-serine and methylaminoisobutyric acid was markedly elevated in pancreata isolated from streptozotocin diabetic rats and insulin partially reversed the stimulation of L-serine transport induced by experimental diabetes. These results suggest that insulin and diabetes modulate the epithelial transport activity for small neutral amino acids in the intact exocrine pancreas.

Rapid transcapillary exchange and unidirectional neuronal uptake of noradrenaline in the perfused rabbit heart

Capillary permeability and cellular uptake of noradrenaline by the isolated artificially perfused rabbit heart was measured using rapid (less than 30 s) single-circulation tracer-dilution techniques. In a single coronary circulation capillary extractions of L-[14C]noradrenaline and D-[3H]mannitol (extracellular reference) relative to an intravascular marker, 125I-labelled albumin, were similar and above 60%. The 'apparent' volume of distribution for tracer noradrenaline was 2.5-fold larger than that measured for D-mannitol (0.32 ml g-1) suggesting cellular uptake of the amine. Unidirectional noradrenaline uptake was estimated by directly comparing coronary sinus dilution profiles of L-[3H]noradrenaline and D-[14C]mannitol. Michaelis-Menten saturation kinetics based on a single-entry system were determined (Km = 2.8 +/- 1.5 microM, Vmax = 2.1 +/- 0.5 nmol min-1 g-1, n = 4) by perfusing hearts with varying concentrations of L-noradrenaline (1-10 microM). Various known inhibitors of noradrenaline uptake were investigated to determine whether uptake was mediated by neuronal (uptake1) and/or extraneuronal (uptake2) mechanisms. Desipramine (5 microM), imipramine (5 microM) and metaraminol (2 microM) resulted in a 66-94% inhibition of noradrenaline influx. In comparison, the steroids, 17 beta-oestradiol (1 microM) and corticosterone (10 microM), and the noradrenaline metabolite normetanephrine (5 microM) caused virtually no inhibitory effects. The beta-adrenergic antagonist propranolol (5 microM) was also relatively ineffective. These results together with the kinetic constants estimated suggest that the rapid noradrenaline uptake reflects transport into adrenergic neurones lying in the coronary interstitium. The high resolution of this paired-tracer dilution technique has permitted a 'non-invasive' study of neuronal uptake mechanisms and its application may be of clinical value.

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.

A paired-tracer dilution method for characterizing membrane transport in the perfused rat hindlimb. Effects of insulin, feeding and fasting on the kinetics of sugar transport

We have applied the paired-tracer dilution method to the study of transport processes in a mixed mammalian muscle preparation, the perfused rat hindlimb. The method is suitable for the characterization of the kinetic parameters of sugar and amino acid transport and its regulation by hormones, contractile activity, hypoxia, etc. Insulin stimulates sugar transport by increasing the Vmax. of the process 2-3 fold, but its affinity is unchanged. Starvation increases the affinity of sugar transport in perfused skeletal muscle.