Uptake of Amino Acids by Brain Micro vessels Isolated from Rats with Experimental Chronic Renal Failure

The neurological disorders seen in patients with chronic renal failure and liver cirrhosis are analogous. Previous in vivo studies have shown that the impaired blood-brain amino acid transport seen in rats with chronic renal failure is similar to that of rats with portocaval anastomosis. To elucidate whether a comparable underlying pathogenic mechanism plays a role in both pathological conditions, blood and brain amino acid levels together with amino acid transport by isolated brain microvessels have been studied in rats with chronic renal failure and in sham-operated rats. Brain microvessels isolated from rats with experimental chronic renal failure showed that the uptake of labeled large neutral amino acid, i.e., leucine or phenylalanine, but not of lysine or alpha-methylaminoisobutyric acid, was significantly increased with respect to sham-operated rats; conversely, the uptake of glutamic acid in rats with chronic renal failure was significantly lower compared with values in controls. Kinetic analysis indicated that this was mainly due to increased exchange transport activity (Vmax) of the L-system, rather than to changes in the affinity (Km) of the carrier system for the relative substrate. These data, together with the significant rise of brain glutamine levels and an increased brain-to-plasma ratio of the sum of large neutral amino acids, are analogous to what was previously observed in rats with portocaval anastomosis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyroxine-Induced Changes in the Development of Neutral α-Amino Acid Transport Systems of Rat Brain

Abstract: Transport of representative neutral α-amino acids was measured in brain slices after injecting thy-roxine into donor rats of various ages from 1 to 23 days old. The hormone did not alter uptake in slices from 1-day-old rats even when treatment was begun on pregnant rats as much as 10 days before delivery. Injecting thy-roxine until age 6 days, however, decreased the activity of transport system A (the major sodium-dependent system in most mammalian cells) and caused appearance of a new transport system used by the model amino acids, 2-aminoisobutyric acid and 2-(methylamino)isobutyric acid. Uptake at 6 days was similar to that found in slices from older, untreated rats (e.g., those 14 days old). These results strongly suggest that one action of thyroxine is to accelerate the development of neutral α-amino acid transport systems of brain over the first six days after birth. Thyroxine treatment of rats from birth to age 14 days also appears to increase the activities of both system A and the second transport system used by the two model amino acids in brains from 14-day-old rats.     

The depletion of tryptophan and serotonin in the brain of developing hyperphenylalaninemic rats is abolished by the additional administration of lysine

In suckling hyperphenylalaninemic (hyper-Phe) rats, all essential amino acids including tryptophan are depleted in the blood. The inadequate supply of Trp to the developing brain leads to a decline of Trp, of serotonin, and of 5-hydroxyindoleacetic acid. The exhaustion of amino acids in both blood and brain can be restored by administration of Lys. Even though Phe is still elevated in blood and brain, Trp, serotonin and 5-hydroxyindoleacetic acid, are no longer depleted in the brain. This observation contradicts the idea that the serotonin deficit in the developing hyper-Phe brain is caused by competitive uptake inhibition of tryptophan or by the interference of Phe metabolites with the formation of serotonin. Increased accumulation of all large neutral amino acids in peripheral tissues and an impeded intestinal amino acid transport in suckling hyper-Phe rats appear to be responsible for the deficient supply of other amino acids, including Trp, to the developing brain. The availability of Lys for developing extraintestinal tissues seems to be involved in the regulation of intestinal amino acid transport.     

Uptake of Amino Acids by Brain Microvessels Isolated from Rats after Portacaval Anastomosis

Abstract: The uptake of amino acids by microvessels isolated from brains of rats was studied. Previous studies have demonstrated alterations in blood-brain amino acid transport after portacaval shunt in rats. In order to elucidate whether such changes in the blood-brain barrier were located in the microvessels, brain microvessels were isolated from both rats with portacaval shunt and controls. Brain microvessels from rats 2 weeks after shunt operations took up significantly greater amounts of ¹⁴C-labeled neutral amino acids, but not of glutamic acid. lysine, or α-methylaminoisobutyric acid than microvessels from sham-operated controls. Measurement of uptake kinetics showed a higher V _max for phenylalanine and leucine uptake and a lower V _max for lysine uptake in microvessels from shunted rats compared with control, whereas the respective K _m's of uptake were similar in both preparations. The results suggest that changes in brain microvessel transport activity account for altered brain neutral amino acid concentrations after portacaval shunt and that such changes can be studied in vitro in isolated microvessels.     

Neutral amino acid transport in isolated rat pancreatic islets

The neutral amino acid transport systems of freshly isolated rat pancreatic islets have been studied by first examining the transport of L-alanine and the nonmetabolizable analogue 2-(methylamino)isobutyric acid (MeAIB). By comparing the uptake of MeAIB and L-alanine for their pH dependency profile, choline and Li+ substitution for Na+, tolerance to N-methylation, and competition with other amino acids, the existence in pancreatic islets of both A and ASC amino acid transport systems was established. The systems responsible for the inward transport of five natural amino acids was studied using competition analysis and Na+ dependency of uptake. These studies defined three neutral amino acid transport systems: A and ASC (Na+-dependent) and L (Na+-independent). L-Proline entered rat islet cells mainly by system A; L-leucine by the Na+-independent system L. The uptake of L-alanine, L-serine, and L-glutamine was shared by systems ASC and L, the participation of system A being negligible for these three amino acids. An especially broad substrate specificity for systems L and ASC is therefore suggested for the rat pancreatic islet cells. The regulation of amino acid transport was also investigated in two conditions differing as to glucose concentration and/or availability, i.e. islets from fasted rats and islets maintained in tissue culture at high or low glucose concentrations. Neither alanine nor MeAIB transport was altered by fasting of the islet-donor rats. On the other hand, pancreatic islets maintained for 2 days in tissue culture at high (16.7 mM) glucose transported MeAIB at twice the rate of islets maintained at low (2.8 mM) glucose. Amino acid starvation of pancreatic islets during 11 h of tissue culture resulted in a 2-fold increase in MeAIB transport.     

Kinetic Analysis of Cerebrovascular Isoleucine Transport from Saline and Plasma

The concentration dependence of regional isoleucine transport across the blood-brain barrier was determined in anesthetized rats with the in situ brain perfusion technique of Takasato et al. [Am. J. Physiol. 247, H484-493 (1984)]. This technique allows, for the first time, accurate measurements of cerebrovascular amino acid transport in the absence of competing amino acids using saline perfusate, and in the presence of physiological concentrations of amino acids using plasma perfusate. Cerebrovascular isoleucine transport from saline perfusate followed Michaelis-Menten saturation kinetics where Vmax = 9 - 11 X 10(-4) mumol X s-1 X g-1 and Km = 0.054-0.068 mumol X ml-1 in six brain regions. A component of nonsaturable transport was not detected in any brain region even though perfusate isoleucine concentration was increased to greater than or equal to 150 times the normal plasma concentration. Isoleucine influx during plasma perfusion was only 8% of that predicted from the saline perfusion data due to transport inhibition by competing amino acids in plasma. Competitive inhibition increased the apparent Km for isoleucine transport from plasma by greater than or equal to 24-fold to 1.5-1.7 mumol X ml-1. These data provide accurate new estimates of the kinetic constants that describe amino acid transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transfer-site affinity (1/Km) for isoleucine is approximately fivefold greater than previously reported with the brain uptake index technique.     

Amino acid uptake systems in lizard and chick brain cells

The uptake of seven amino acids, -aminoisobutyric acid (AIB), cyclo-leucine (cyclo-Leu), -aminobutyric acid (GABA), glycine (Gly), glutamic acid (Glu), lysine (Lys), and taurine (Tau), representatives of different amino acid transport systems, was studied in slices of brain from Tokay lizards and White Leghorn chicks. In descending order, the rate of the initial uptake of the amino acids in both species was Glu>Gly>GABA>Cyclo-Leu>AIB>Lys>Tau. The substrate specificities and the differences in sodium and temperature dependence of the uptake of the amino acids indicate the presence of several distinct amino acid transport systems, some sodium-dependent and some sodium-independent. The structural specificity of amino acid transport classes in the brain of these species is similar to that in other vertebrate brain preparations.Special issue dedicated to Dr. Santiago Grisolia.     

Perinatal changes of transport systems for amino acids in slices of mouse brain

Perinatal changes in the uptake of amino acids were measured in slices of fetal (15- and 19-day) and newborn (4-, 24-, and 48-hr-old) mouse brain. Uptake increased with age; smaller changes occurred with basic and neutral amino acid transport systems, and the largest changes occurred in fetal brain with amino acids of putative neurotransmitter function (taurine, glycine, GABA, and the acidic amino acids). The pattern of increase in uptake was similar at high and at low external amino acid concentrations. Developmental changes in tissue content of Na⁺, K⁺, or ATP were small during this period, and so are unlikely to be responsible for the observed changes in uptake. It appears that by the 15th day of fetal life, the transport systems for essential amino acids are fairly well developed in the brain, and the transport systems for neurotransmitter amino acids are not so well developed, but undergo a rapid increase in the 15–19-day period. From birth to adulthood, the concentrative capacity of slices of mouse brain for nonessential (putative neurotransmitter) amino acids is much greater than for essential amino acids.This research was supported in part by NIH Grant No. RR05707.     

Transport of threonine and tryptophan by rat brain slices: relation to other amino acids at concentrations found in plasma

Threonine content of brain decreases in young rats fed a threonine-limiting, low protein diet containing a supplement of small neutral amino acids (serine, glycine and alanine), which are competitors of threonine transport in other systems (Tews et al., 1977). Threonine transport by brain slices was inhibited more by a complex amino acid mixture resembling plasma from rats fed the small neutral amino acid supplement than by mixtures resembling plasma from control rats or from rats fed a supplement of large neutral amino acids. Greater inhibition was seen with mixtures containing only the small neutral amino acids than with mixtures containing only large neutral amino acids. On an equimolar basis, serine and alanine were the most inhibitory; large neutrals were moderately so; and glycine and lysine were without effect. Threonine transport was also strongly inhibited by α-amino-n-butyric acid and homoserine, less so by α-aminoisobutyric acid, and not at all by GABA. The complex amino acid mixtures strongly inhibited α-aminoisobutyric acid transport by brain or liver slices but, in contrast to effects in brain, the extent of the inhibition in liver was not much affected by altering the composition of the mixture. Tryptophan accumulation by brain slices was effectively inhibited by other large neutral amino acids in physiologically occurring concentrations. Threonine, or a mixture of serine, glycine and alanine only slightly inhibited tryptophan uptake; basic amino acids were without effect and histidine stimulated tryptophan transport slightly. These results support the conclusion that a diet-induced decrease in the concentration in brain of a specific amino acid may be related to increased inhibition of its transport into brain by increases in the concentrations of transport-related, plasma amino acids.     

INHIBITION OF AMINO ACID UPTAKE BY THE ABSENCE OF Na+ IN SLICES OF BRAIN

—The Na+ requirement of amino acid transport was measured in brain slices. The tissue was first washed free of Na+ and then Na+ was replaced by one of the following: choline, Li+, Rb+, or mannose. Amino acid uptake was measured at different times (5–120 min) and at low (10^-7–10^-5m ) and high (10^-3m ) concentrations. Most of the Na+ could be washed out of the tissue; this also decreased K+ levels despite increased K+ in the medium. K+ tissue levels were partially restored when Na+ was added. The absence of Na+ abolished the uptake of Glu, Asp, GABA, Gly, Tau and Pro. Most of the neutral amino acids (Ala, Val, Trp, His) were very strongly inhibited by the absence of Na+ under most experimental conditions. Basic amino acids (Arg, Lys) were not completely inhibited, in that 30 per cent of the equilibrium uptake remained and some of the basic amino acid influx was independent of the Na+ tissue level. The uptake of amines (tyramine, cadaverine, putrescine) did not require Na+, and often was greater in the absence of Na+. We conclude that amino acid uptake in brain slices is Na+ dependent, although the absence of Na+ may affect transport indirectly.     

Amino acid transport in a polyaromatic amino acid auxotroph of Saccharomyces cerevisiae

The initiation of growth of a polyaromatic auxotrophic mutant of Saccharomyces cerevisiae was inhibited by several amino acids, whereas growth of the parent prototroph was unaffected. A comparative investigation of amino acid transport in the two strains employing (14)C-labeled amino acids revealed that the transport of amino acids in S. cerevisiae was mediated by a general transport system responsible for the uptake of all neutral as well as basic amino acids. Both auxotrophic and prototrophic strains exhibited stereospecificity for l-amino acids and a K(m) ranging from 1.5 x 10(-5) to 5.0 x 10(-5) M. Optimal transport activity occurred at pH 5.7. Cycloheximide had no effect on amino acid uptake, indicating that protein synthesis was not a direct requirement for amino acid transport. Regulation of amino acid transport was subject to the concentration of amino acids in the free amino acid pool. Amino acid inhibition of the uptake of the aromatic amino acids by the aromatic auxotroph did not correlate directly with the effect of amino acids on the initiation of growth of the auxotroph but provides a partial explanation of this effect.     

Acidic amino acid clearance from CSF in the neonatal versus adult rat using ventriculo-cisternal perfusion

The acidic amino acids aspartate and glutamate are excitatory neurotransmitters in the CNS. The clearance of this group of amino acids from CSF of adult and neonatal (7-day-old) rats was investigated. Ventriculo-cisternal perfusions with 14C-amino acids and 3H-dextran were carried out for up to 90 min. Uptake of the amino acids by the whole brain was measured, and the loss to blood was calculated. 3H-Dextran was included in the perfusate for measurement of CSF secretion rate. After 90-min perfusion, both aspartate and glutamate showed a similar uptake into the whole brain, and this did not change with age (p>0.05). However, clearance from CSF was greater in the adult, as was entry into blood from CSF. Addition of 5 mM excess unlabelled amino acid resulted in reduction in the brain uptake of both 14C-amino acids in the adult rat. In the neonate, addition of aspartate also reduced brain aspartate uptake, whereas addition of glutamate increased brain neonatal [14C]glutamate uptake. The rate of CSF secretion was significantly greater in the adult, 1.26+/-0.18 microl x min(-1) x g(-1), than in the neonate, 0.62+/-0.08 microl x min(-1) x g(-1), and the turnover of CSF was greater in adults (p<0.01). In summary, both aspartate and glutamate showed greater clearances from CSF in the adult than the neonate. This clearance was found to be by carrier-mediated mechanisms.     

Phenylalanine Transport Across the Blood-Brain Barrier as Studied with the In Situ Brain Perfusion Technique

Unidirectional L-phenylalanine transport into six brain regions of pentobarbital-anesthetized rats was studied using the in situ brain perfusion technique. This technique allows both accurate measurements of cerebrovascular amino acid transport and complete control of perfusate amino acid composition. L-Phenylalanine influx into the brain was sodium independent and could be described by a model with a saturable and a nonsaturable component. Best-fit values for the kinetic constants in the parietal cortex equaled 6.9 X 10(-4) mumol/s/g for Vmax, 0.011 mumol/ml for Km, and 1.8 X 10(-4) ml/s/g for KD during perfusion with fluid that did not contain competing amino acids. D-Phenylalanine competitively inhibited L-phenylalanine transport with a Ki approximately 10-fold greater than the Km for L-phenylalanine. There were no significant regional differences in Km, KD, or Ki, whereas Vmax was significantly greater in the cortical lobes than in the other brain regions. L-Phenylalanine influx during plasma perfusion was only 30% of that predicted in the absence of competing amino acids. Competitive inhibition increased the apparent Km during plasma perfusion by approximately 20-fold, to 0.21 mumol/ml. These data provide accurate new estimates of the kinetic constants that describe L-phenylalanine transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transfer site affinity (1/Km) for L-phenylalanine is three- to 12-fold greater than previously estimated in either awake or anesthetized animals.     

Effects of cyclic AMP and dibutyryl cyclic AMP on amino acid transport in the isolated rat ovary.

Prepubertal rat ovaries were incubated in medium containing the non-utilizable amino acids alpha-aminoisobutyric acid (AIB-14C) or 1-aminocyclo-pentane-carboxylic acid (cycloleucine-14C). The rate of uptake of the two amino acids was studied in the isolated ovaries after different incubation periods. Addition of 5mM cyclic AMP (cAMP) caused a slight stimulation of the AIB-transport but in higher concentrations (10-25 mM) an inhibition was noted. With dibutyrl cyclic AMP (dbcAMP) a dose-dependent increase was seen with 0.5-5 mM concentrations with no further effect of higher concentrations. Time course studies were performed with both AIB and cycloleucine in presence of 10 mM dbcAMP and increased uptake values were noted at each time studied (30-240 min). The phosphodiesterase inhibitor aminophyline in lower concentrations did not influence AIB-transport but 5-10 mM caused increased uptake values in the ovaries. The stimulatory action of dbcAMP on amino acid transport was augmented by a low concentration of aminophylline (0.5 mM). Experiments were in addition carried out in the presence of puromycin and under these circumstances it was still possible to enhance amino acid transport by addition of dbcAMP. The results are discussed in relation to earlier reported effects of gonadotropins on ovarian amino acid transport.     

THE UPTAKE OF AMINO ACIDS BY THE INTACT OLFACTORY BULB OF THE MOUSE: A COMPARISON WITH TISSUE SLICE PREPARATIONS

Abstract— Intact olfactory bulbs from 8- to 15-day-old mice were compared to slices of olfactory bulb and cerebral hemisphere with respect to uptake of amino acids, respiratory rate, levels of ATP, retention of sodium and potassium, and extracellular space. The uptake of amino acids was lower in intact bulbs than in slice preparations, both in regard to initial rates of uptake and to final steady state levels, at external amino acid concentrations from 0·2 to 2·0mM. Uptake was lower in bulbs attached to brain than in those separated from it and somewhat higher in the half of the bulb closer to the cut surface. In all preparations the uptake of glutamic acid and glycine was highest, uptake of histidine and valine was intermediate, and uptake of lysine was lowest. These differences between intact bulbs and slices could not be correlated with differences in respiratory rate, levels of ATP, or changes in levels of Na⁺ or K⁺ ions. Increases in dextran and inulin spaces, however, were greatest in preparations having the highest rates of amino acid uptake. Although for several amino acids the maximal velocity of uptake (V_max) was 4-fold higher in slices of bulb than in intact bulbs, the affinity of amino acids to their carrier systems ( K _m) was similar, an indication that the same transport process was operative in both cases. On the basis of these results we propose that intact olfactory bulbs incubated in vitro possess a regulatory mechanism for the limitation of amino acid uptake that is absent or diminished in slices.     

Portacaval Anastomosis: Brain and Plasma Metabolite Abnormalities and the Effect of Nutritional Therapy

Abstract: Rats with portacaval shunts were used as a model of hepatic encephalopathy and compared to sham-operated controls. First, the changes in intermediary metabolites and amino acids in blood and whole brain were characterized and found to be similar at 4 and 7 weeks after shunting. Second, the effects of nutritional therapy on selected metabolites and tryptophan transport into brain were assessed in rats 5 weeks after surgery. Ordinary food was removed and the rats were treated with glucose given either by mouth or intravenously, or intravenous glucose plus branched chain amino acids. Several abnormalities in plasma amino acid concentrations were reversed by treatment. The abnormally high brain uptake index of tryptophan, a consequence of portacaval shunting, was not lowered by any of the treatment regimens; it was even higher in the groups given glucose by mouth and glucose plus amino acids. Calculated competition for entry of tryptophan, phenylalanine, and tyrosine into brain was unchanged (glucose plus amino aicds), or reduced (glucose alone). Brain glutamine content was brought to near normal by all treatments. Infusion of glucose plus branched chain amino acids normalized brain content of tryptophan, phenylalanine, and tyrosine, even though the brain uptake index of tryptophan was higher in this group. Thus, partial or complete reversal of several abnormalities found after portacaval shunting was achieved by removal of oral food and administration of glucose. The addition of branched chain amino acids to the glucose infusion restored brain content of three aromatic amino acids to near normal, by a mechanism which appeared to be unrelated to transport across the blood-brain barrier.     

Characterization of tryptophan transport in human placental brush-border membrane vesicles.

The characteristics of tryptophan uptake in isolated human placental brush-border membrane vesicles were investigated. Tryptophan uptake in these vesicles was predominantly Na+-independent. Uptake of tryptophan as measured with short incubations occurred exclusively by a carrier-mediated process, but significant binding of this amino acid to the membrane vesicles was observed with longer incubations. The carrier-mediated system obeyed Michaelis-Menten kinetics, with an apparent affinity constant of 12.7 +/- 1.0 microM and a maximal velocity of 91 +/- 5 pmol/15 s per mg of protein. The kinetic constants were similar in the presence and absence of a Na+ gradient. Competition experiments showed that tryptophan uptake was effectively inhibited by many neutral amino acids except proline, hydroxyproline and 2-(methylamino)isobutyric acid. The inhibitory amino acids included aromatic amino acids as well as other system-1-specific amino acids (system 1 refers to the classical L system, according to the most recent nomenclature of amino acid transport systems). The transport system showed very low affinity for D-isomers, was not affected by phloretin or glucose but was inhibited by p-azidophenylalanine and N-ethylmaleimide. The uptake rates were only minimally affected by change in pH over the range 4.5-8.0. Tryptophan uptake markedly responded to trans-stimulation, and the amino acids capable of causing trans-stimulation included all amino acids with system-1-specificity. The patterns of inhibition of uptake of tryptophan and leucine by various amino acids were very similar. We conclude that system t, which is specific for aromatic amino acids, is absent from human placenta and that tryptophan transport in this tissue occurs via system 1, which has very broad specificity.     

Hyperammonaemia causes many of the changes found after portacaval shunting.

1. Portacaval shunting in rats results in several metabolic alterations similar to those seen in patients with hepatic encephalopathy. The characteristic changes include: (a) diminution of cerebral function; (b) raised plasma ammonia and brain glutamine levels; (c) increased neutral amino acid transport across the blood-brain barrier; (d) altered brain and plasma amino acid levels; and (e) changes in brain neurotransmitter content. The aetiology of these abnormalities remains unknown. 2. To study the degree to which ammonia could be responsible, rats were made hyperammonaemic by administering 40 units of urease/kg body weight every 12 h and killing the rats 48 h after the first injection. 3. The changes observed in the urease-treated rats were: (a) whole-brain glucose use was significantly depressed, whereas the levels of high-energy phosphates remained unchanged; (b) the permeability of the blood-brain to barrier to two large neutral amino acids, tryptophan and leucine, was increased; (c) blood-brain barrier integrity was maintained, as indicated by the unchanged permeability-to-surface-area product for acetate; (d) plasma and brain amino acid concentrations were altered; and (e) dopamine, 5-hydroxytryptamine (serotonin) and noradrenaline levels in brain were unchanged, but 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of 5-hydroxytryptamine, was elevated. 4. The depressed brain glucose use, increased tryptophan permeability-to-surface-area product, elevated brain tryptophan content and rise in the level of cerebral 5-HIAA were closely correlated with the observed rise in brain glutamine content. 5. These results suggest that many of the metabolic alterations seen in rats with portacaval shunts could be due to elevated ammonia levels. Furthermore, the synthesis or accumulation of glutamine may be closely linked to cerebral dysfunction in hyperammonaemia.     

Ventriculo–cisternal perfusion of twelve amino acids in the rabbit

The clearances of twelve amino acids from the ventricles during ventriculocisternal perfusion in the rabbit have been measured; uptake by the brain was also measured and this permitted the separate computation of loss to brain and loss to blood during the perfusion. Clearance under carrier-free conditions was greater than when a concentration of 5mM unlabeled amino acid was present in the perfusion fluid. Brain uptake was also usually reduced by the presence of unlabeled amino acid due presumably to suppression of accumulation by brain cells. Reduction of transport across the blood-brain barrier would tend to increase brain uptake, and there was some evidence for a balance between the two opposing tendencies. Inhibition of clearance of a given labeled amino acid could be brought about by unlabeled amino acids of different molecular species. In general, the amino acids fell into three categories: neutral, acidic, and basic, and there was some overlap between them; of the neutral amino acids the A- and L-classification of Christensen was valid, although once again there was some overlap. If, during ventriculo-cisternal perfusion of a labeled amino acid, the activity of this labeled amino acid in the blood was raised well above that in the inflowing perfusion fluid, the labeled amino acid continued to be cleared from the perfusion fluid, suggesting uphill transport. On this basis it was suggested that the normally low concentrations of amino acids in the cerebrospinal fluid (CSF), by comparison with those in plasma, were due to an active transport from the CSF to the blood. Substrate-facilitated transport, whereby the penetration of labeled amino acid into the perfusion fluid from blood could be accelerated by adding unlabeled amino acid to the perfusion fluid, or vice versa, was demonstrated.     

Rates of exchange of free amino acids between plasma and brain in mice

The rate of appearance of label in the brain in mice following the intraperitoneal or intravenous injection of tracer doses of amino acids was measured in short time periods (1–8 min). Amino acid flux varied between 2 and 10 nmol/min per g brain for the amino acids used. Defining half-life as the uptake of labeled amino acid amounting to 50% of endogenous levels, a short half-life (between 3 and 30 min) was found for the essential amino acids. The half-life of the nonessential amino acids varied between 2 and 24 h, depending on their level in brain. Flux (exchange) of an amino acid was increased when the level of amino acids belonging to the same transport class was increased by intracerebral injection. Protein-free diet resulted in decrease in some amino acids, increase in others; flux was altered parallel to changes in brain levels in animals on this diet. The stercospecificity of exchange and the substrate specificity of effects of altered brain amino acids indicate that exchange occurs via mediated transport. Mediated exchange was present in immature brain. Heteroexchange (flow of one amino acid causing the counterflow of a related amino acid) may play an important part in cerebral homeostasis.