Concentrations Of Large Neutral Amino Acids Research Articles

Effects of large neutral amino acid concentrations on 6-[F-18]Fluoro-L-DOPA kinetics.

6-[F-18]Fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) has been used to measure the central dopaminergic function in many species, including humans and monkeys. For transport across the blood brain barrier (BBB), FDOPA competes with plasma large neutral amino acids (LNAA). In this article we evaluate the effects of normal physiological LNAA concentration variation on BBB transport (K1) and the FDOPA uptake measurement, Ki. We also investigate a method for reducing the dependency of FDOPA quantitation on LNAA. Adult vervet monkeys (Cercopithecus aethiops sabaeus, n = 19) were fasted overnight before FDOPA positron emission tomography scans. Blood samples were drawn for LNAA determination, metabolite analysis, and compartmental modeling. The estimated K1 and Ki were both negatively correlated with LNAA concentrations (r2 = 0.51 and 0.62, respectively). Using an adjustment to K1 and Ki based on these correlations, the LNAA dependency was reduced (SD of the data for K1 was reduced by 33%, for Ki by 40%). Experiments with amino acid loading on an additional six animals indicate that BBB transport can be described using Michaelis-Menten kinetics. Results show a clear dependence of FDOPA uptake on plasma LNAA concentrations, which can be removed to increase the precision of FDOPA quantitation.

Motor fluctuations during continuous levodopa infusions in patients with Parkinson's disease.

The cause of motor fluctuations occurring during constant-rate levodopa infusions is unknown. We examined whether known pharmacokinetic factors could explain the fluctuations and looked for clues to pharmacodynamic causes. Eleven subjects with stage III-V Parkinson's disease (PD) and a fluctuating response to levodopa underwent constant-rate infusions for 36-110 h. Levodopa, 3-O-methyldopa (3-OMD), and plasma large neutral amino acids (LNAAs) were measured at 2- to 6-h intervals and PD was monitored hourly from 07:00 to 22:00 h with tapping speed. Ten subjects had motor fluctuations during the infusions. Zero to 68% of the variability of tapping speed could be explained by variation in plasma LNAA concentrations in individual subjects. Fluctuations occurred more commonly later in the day, which may be related to the tendency for LNAAs to increase during the day. Motor fluctuations were not associated with minor variations in levodopa or 3-OMD concentrations. Fluctuations during constant infusions were more marked in patients using larger daily doses of oral levodopa; severity of PD did not predict fluctuations during the infusions. There was no trend for fluctuations or dyskinesia to decrease or increase during several days of constant-rate levodopa infusion. A portion of motor fluctuations occurring during constant levodopa infusions can be explained by peripheral pharmacokinetic mechanisms. Fluctuations are more prominent in subjects who have taken larger daily doses of levodopa, implicating pharmacodynamic factors as well.

Decreased plasma ratio of tryptophan to competing large neutral amino acids in human immunodeficiency virus type 1 infected subjects: Possible implications for development of neuro-psychiatric disorders

Those large neutral amino acids (LNAA) which compete with each other for the carrier mediated transport from plasma into the brain were determined in plasma in human immunodeficiency virus (HIV-1) seropositive subjects and seronegative controls. Previous findings of a decreased concentration of tryptophan were confirmed whereas no difference between HIV-1 seropositive subjects and controls were found in those LNAAs with which tryptophan competes for the transport into the brain. Thus, the ratio in plasma of tryptophan to the total LNAA concentration was decreased in HIV-1 seropositive subjects. This ratio is considered to, at least partly, regulate the availability of tryptophan in the brain. Since tryptophan is a precursor to the neurotransmitter 5-HT and since the enzymes involved in the 5-HT synthesis normally are not saturated, the decreased plasma ratio of tryptophan might cause a decrease in brain 5-HT synthesis and, thus, to an impaired function in brain 5-HT neurons. This mechanism might, as well as previously demonstrated accumulation within the brain of the neurotoxic tryptophan metabolite quinolinic acid, contribute to the development of dementia and other neuro-psychiatric disorders, often seen in AIDS patients. Treatment with 5-HT receptor agonists might prove effective to prevent neuro-psychiatric disorders.

The imbalance of brain large-chain aminoacid availability in amyotrophic lateral sclerosis patients treated with high doses of branched-chain aminoacids

Following the report of an increased mortality among patients with amyotrophic lateral sclerosis given high daily doses of branched-chain aminoacids, we assessed the plasma concentrations of large neutral aminoacids and glutamic acid and the large neutral aminoacid brain influx in 24 amyotrophic lateral sclerosis patients receiving placebo or branched-chain aminoacids ( l-leucine 12 g, l-isoleucine 6 g, l-valine 6 g daily), in 15 untreated amyotrophic lateral sclerosis patients and in 15 healthy volunteers. The branched-chain aminoacid plasma concentrations increased three- to six-fold in the treated group compared to the patients receiving placebo or no treatment and to the healthy controls. Plasma glutamic acid concentrations in healthy volunteers were 51.59±7.53 nmol/ml while in the amyotrophic lateral sclerosis patients receiving no treatment, placebo or branched-chain aminoacids were 92.33±12.15 nmol/ml, 91.21±15.86 nmol/ml and 95.08±17.96 nmol/ml respectively. The glutamic acid concentration was significantly higher ( P<0.01) in amyotrophic lateral sclerosis patients than in healthy individuals. Plasma phenylalanine and tyrosine were lower in the amyotrophic lateral sclerosis patients than in healthy controls, regardless of treatment, whereas tryptophan levels were not significantly different. The branched-chain aminoacid brain influx of the treated group was 110-140% of that measured in the patients receiving placebo and in the healthy controls. The aromatic aminoacid brain influx was lower in the treated group than in the placebo group or healthy controls. An impairment of brain large neutral aminoacid availability might possible contribute to enhancing the progression of symptoms in patients with amyotrophic lateral sclerosis.

Insulin-induced decline of plasma amino acid concentrations in obese subjects with and without non-insulin-dependent diabetes

Obesity-associated hyperaminoacidemia is traditionally interpreted as a consequence of insulin resistance. We performed two different experiments to investigate the effects of both obesity-associated insulin resistance and the insulin resistance of non-insulin-dependent diabetes mellitus (NIDDM) on amino acid metabolism. In the first experiment, we measured postabsorptive amino acid concentrations and their decline in response to an oral carbohydrate load in 19 obese nondiabetic women and 19 normal-weight nondiabetic controls. Obese subjects were more resistant to insulin with respect to its effects on glucose metabolism than normal-weight controls, as calculated by the method described by Matthews. However, postabsorptive plasma concentrations of the so-called large neutral amino acids (LNAA), namely phenylalanine, tyrosine, valine, leucine, and isoleucine, and their decrease in response to carbohydrate consumption were similar in both groups. In the second experiment, we compared the decrease of plasma concentrations of LNAA during a euglycemic, hyperinsulinemic clamp in obese subjects with and without NIDDM. Peripheral glucose uptake (PGU) was more impaired in NIDDM subjects compared with obese controls. Furthermore, hepatic glucose production (HGP) was less attenuated by insulin infusion in NIDDM than in control subjects. Postabsorptive plasma LNAA concentrations were not different in the two groups. Values obtained in either group were not different from the postabsorptive concentrations in the normal-weight control subjects of experiment 1. All amino acid levels decreased substantially in response to insulin infusion. The magnitude of the decrease was not significantly different in the two groups, except for a slightly greater decrease of the plasma isoleucine concentration in obese control subjects. These results indicate that insulin resistance in obesity and NIDDM has dissociated effects on insulin-mediated glucose and amino acid metabolism. Although obese subjects both with and without NIDDM were resistant to insulin with respect to its effects on glucose metabolism, postabsorptive plasma levels of LNAA and their decrease in response to carbohydrate consumption or insulin infusion were similar in obese subjects, irrespective of the presence of NIDDM, and normal-weight control subjects.

Is rat brain content of large neutral amino acids (LNAAs) a reflection of plasma LNAA concentrations?

Competition between the large neutral amino acids (LNAAs), including the monoamine precursors tyrosine and tryptophan, for the carrier-mediated transport from plasma into the brain is considered the major regulator of brain LNAA content. The plasma ratio of each LNAA to the sum of all LNAAs, previously, in standardized experiments, has proved an excellent predictor of brain LNAA content. To investigate how diurnal variations in plasma LNAAs are reflected in the brain, we have killed rats, in groups of six, every 1 h throughout one 24 h period. The concentration of each LNAA in both plasma and brain varied with time-of-day. Also the total concentration of LNAAs in brain was shown to vary diurnally. Obviously, variations in total brain LNAA concentration cannot be explained by changes in plasma LNAA ratios. Consequently, other factors, contributing to the regulation of the diurnal rhythm in brain LNAA content, must be considered.

Chronopharmacology of isoprenaline: the effects on rat plasma concentrations of large neutral amino acids depend on time of day for administration.

This investigation was undertaken in order to test our hypothesis, that the endogenous circadian rhythms in large neutral amino acids (LNAAs) in plasma are partly regulated by beta-adrenergic mechanisms. The beta-adrenoceptor agonist isoprenaline, which is known to decrease the concentrations of rat plasma LNAAs, was given at several time-points over a full 24-h period intraperitoneally to rats, which were killed 1 h later. Isoprenaline, administered in the evening, decreased the plasma amino acid concentrations more than it did in the same doses given in the morning. Also the potency of isoprenaline was in this respect higher in the evening than in the morning. This finding, of a time-of-day dependent relationship between the effects of a beta-adrenoceptor agonist and plasma LNAA concentrations, supports our hypothesis, that beta-adrenergic mechanisms are involved in the endogenous regulation of the plasma LNAA rhythms. We suggest that the known endogenous circadian variation in sympathetic activity, which gives rise to a reciprocal rhythm in beta-adrenoceptor responsiveness, could be one factor explaining that part of the circadian rhythm in plasma LNAA concentrations, which is not due to variations in food intake.

?-Adrenergic Effects on Plasma and Brain Large Neutral Amino Acids Are Unaltered by Chronic Administration of Antidepressants

Effects of isoproterenol (3 mg kg-1, i.p. for 60 min) and salbutamol (3, 10 mg kg-1, i.p. for 60 min) on large neutral amino acid concentrations in rat plasma and brain were assessed. Phenylalanine, leucine, isoleucine, and valine were measured by gas chromatography with electron-capture detection; tyrosine and tryptophan were measured by HPLC with electrochemical detection. These drugs induced increases in brain tryptophan, tyrosine, phenylalanine, and valine and decreases in plasma tryptophan, tyrosine, leucine, isoleucine, and valine. Effects of salbutamol (3 mg kg-1, i.p. for 60 min) were assessed following chronic administration of phenelzine sulfate and desipramine.HCl (each drug 10 mg kg-1 per day, s.c. via Alzet 2ML4 osmotic minipumps for 28 days). There were no effects of these antidepressants on basal levels of large neutral amino acids in brain and plasma. In both brain and plasma, salbutamol-induced changes in large neutral amino acids were unaffected by these antidepressants. The results indicate that beta-adrenoceptor-regulated availability of plasma and brain large neutral amino acids is unaffected by chronic administration of tricyclic or monoamine oxidase inhibitor antidepressants.

Dietary Disproportions of Amino Acids in the Rat: Effects on Food Intake, Plasma and Brain Amino Acids and Brain Serotonin

Food intake, growth, plasma and brain amino acid, and brain serotonin and 5-hydroxyindole-3-acetic acid (5-HIAA) concentrations were measured in rats fed low protein diets containing disproportionate amounts of large neutral amino acids (LNAA) devoid of tryptophan or histidine (tryptophan or histidine imbalance). Five-day food intakes and weight gains of rats fed the imbalanced diets were depressed. The concentration of the limiting amino acid was low in brains of rats fed diets containing LNAA that compete with either tryptophan or histidine for entry into brain. Correlations were observed between the brain concentrations of most individual LNAA and either the ratios of the plasma concentration of that LNAA to the sum of the other LNAA, or the predicted rates of influx of that LNAA. Cumulative food intakes were correlated with brain concentrations of the limiting amino acid, tryptophan or histidine. Food intakes were not consistently correlated with concentrations of serotonin and 5-HIAA because these compounds were altered only in brains of rats in the tryptophan study. Competition among amino acids for uptake into brain appears to be involved in the feeding response of the rat to dietary disproportions of amino acids, but this response is not directly related to changes in brain concentrations of serotonin and 5-HIAA.

Influence of fluctuations of plasma large neutral amino acids with normal diets on the clinical response to levodopa.

Plasma large neutral amino acids (LNAAs) compete with levodopa for entry into the brain. Fluctuations in plasma LNAA concentrations could therefore contribute to variability in clinical response to levodopa. The hourly plasma levodopa, plasma LNAAs and clinical response were investigated in 11 fluctuating Parkinsonian patients on a regular hospital diet. The fluctuations in plasma levodopa were 2 to 3 times greater than the fluctuations of plasma LNAAs. The correlation between clinical response and plasma levodopa was substantially improved in only one patient by considering plasma LNAAs and calculating relative levodopa flux into brain. Although plasma LNAAs significantly increased during the day, the patients' clinical status did not uniformly deteriorate and mean afternoon clinical scores correlated better with mean plasma levodopa and levodopa flux than with mean plasma LNAAs. Minimum effective concentrations of levodopa for clinical response did not correlate with 9 am LNAA concentrations. It is concluded that in most patients, the relatively small variation in plasma LNAAs in comparison with the large variations in plasma levodopa indicates that fluctuations in LNAA are not an important contributor to the fluctuating response to levodopa.

Psychobiological effects of carbohydrate- and protein-rich meals in patients with seasonal affective disorder and normal controls

Patients with seasonal affective disorder (SAD) frequently report carbohydrate craving and note that carbohydrate ingestion energizes them. Bright artificial light has been shown to reverse the symptoms of SAD, including carbohydrate craving. In this study, 16 depressed SAD patients and 16 matched controls were fed two different isocaloric meals, one rich in protein and one rich in carbohydrates, in a crossover design. Although their biochemical response in terms of plasma large neutral amino acid concentrations was identical, SAD patients reported activation following carbohydrate ingestion, whereas normal controls reported sedation. Marked ordering effects on psychological parameters were noted, suggesting that order should be taken into account as a methodological consideration in meal studies.

Effects of running the Boston marathon on plasma concentrations of large neutral amino acids.

Plasma large neutral amino acid concentrations were measured in thirty-seven subjects before and after completing the Boston Marathon. Concentrations of tyrosine, phenylalanine, and methionine increased, as did their "plasma ratios" (i.e., the ratio of each amino acid's concentration to the summed plasma concentrations of the other large neutral amino acids which compete with it for brain uptake). No changes were noted in the plasma concentrations of tryptophan, leucine, isoleucine, nor valine; however, the "plasma ratios" of acid patterns may influence neurotransmitter synthesis.

Diurnal rhythm in absolute and relative concentrations of large neutral amino acids in human plasma

In order to obtain detailed information on the diurnal plasma rhythmicity in those large neutral amino acids (LNAA) which compete with each other for carrier-mediated transport from plasma into the brain, blood samples were collected every hour for 24h from six healthy men, aged 25–50 yr. The well-known diurnal rhythm in the concentration of plasma amino acids was confirmed and is described in greater detail in this paper. These data might be helpful in clinical situations where LNAAs (e.g. L-dopa, α-methyldopa and tryptophan) are used as therapeutic agents, since these amino acids compete with endogenous LNAAs for carrier-mediated transport from plasma into the brain. Furthermore, it was found that the amplitude of variation differed between the individual LNAAs. In addition the relationships between them, expressed as ratios of the concentrations of each single LNAA to total LNAA concentration also possess diurnal rhythms, different from those observed in absolute LNAA concentrations. This rhythmicity in relative LNAA concentrations might be of importance for brain function, since altered relationships between LNAAs in plasma might bring about altered concentrations of LNAAs in the brain and consequently, in monoamine synthesis.

Rat brain concentration of administered amino acids: Dependence on time of day for administration

Several large neutral amino acids (LNAA) (e.g. L-DOPA and L-tryptophan) are used as therapeutic agents. To reach the brain they have to compete with the naturally occurring large neutral amino acids for the saturable, carrier mediated transport into the brain. Since the concentration of LNAA in plasma demonstrates a diurnal rhythm, this competition could be expected to vary accordingly. To investigate if this variation could influence the brain concentration of a certain administered amino acid we injected three groups of rats with L-DOPA, L-tryptophan or saline in the afternoon when the rat plasma concentration of LNAA is at its lowest. Three other groups of rats received the same treatments at 3 a.m., when the concentration of LNAA is reported to be at a maximal level. The brain concentrations of the administered amino acids were significantly higher and LNAA in plasma lower in the groups injected in the afternoon compared with those injected during the night. These findings support the hypothesis that the time of the day when an amino acid is administered is of importance to the concentration of the administered amino acid in the brain.

Immobilization decreases amino acid concentrations in plasma but maintains or increases them in brain.

Immobilization for 2 h significantly decreased plasma concentrations of 13 of 16 amino acids assayed, including the transmitter amine precursors tyrosine and total tryptophan. The level of plasma free tryptophan, however, was increased. Despite the reduced plasma levels, corresponding brain concentrations of many large neutral amino acids (LNAAs) were increased (tryptophan, phenylalanine, valine, leucine, and isoleucine). Brain concentrations of tyrosine and the other amino acids measured were unaltered. The results for the LNAAs were not explained by calculated brain influx rates. Therefore, altered influx kinetics or perhaps altered brain protein metabolism or efflux may be responsible. Comparison of calculated brain influxes and brain concentrations of LNAAs suggests that the rise in level of plasma free tryptophan during immobilization is not responsible for the increase in level of brain tryptophan and that the mechanism responsible for the maintenance of or increase in brain concentrations of the other LNAAs is probably involved. Maintenance of brain concentrations of basic amino acids is explicable by reduced competition for brain uptake.

Food Intake, Growth and Tissue Amino Acids in Rats Fed Amino Acid Analogues

The objective of this study was to examine the effects of dietary additions of analogues of large neutral amino acids (LNAA), previously shown to inhibit entry of natural LNAA into brain, on food intake, growth and tissue concentrations of specific amino acids in young rats. A mixture of norleucine, norvaline, α-amino-phenylacetate and α-aminooctanoate (atypical amino acids, AAA) markedly depressed food intake and growth of rats fed a 6% protein diet (LP) for 10 d but not of rats fed a 50% protein diet (HP). Except in rats fed HP, dietary AAA usually decreased concentrations of LNAA more than of small neutral amino acids (SNAA) or lysine, especially in brain. Concentrations of LNAA, especially in brain and muscle of rats adapted to LP or HP meals and fed one LP-AAA meal were lower than in similar rats fed one LP meal without AAA; feeding an HP-AAA meal to such rats generally prevented or lessened these changes. AAA-induced changes in SNAA and lysine were usually small in meal-fed rats. When AAA induced decreases in LNAA, the branched-chain amino acids were usually most affected; valine and isoleucine sometimes were undetected in brain and muscle. Serotonin and dopamine concentrations were not low in brain despite low levels of tryptophan and tyrosine. Changes in tissue LNAA concentrations would appear to reflect in part competition by large neutral AAA for transport of natural LNAA from the blood.

Effect of various oral protein doses on plasma neutral amino acid levels.

Six healthy fasting females each received, on different days, 0, 6, 12.5 or 25 g of albumin dissolved in 200 ml water. Blood was collected before and at intervals after the albumin ingestion and assayed for plasma levels of large, neutral amino acids (LNAA), insulin and glucose. Insulin increased significantly after ingestion of 12.5 and 25 g of albumin, whereas changes in serum glucose were small and inconsistent. Increases of plasma LNAA concentrations were dose-related and correlated significantly with the molar percentage in the albumin of the respective amino acids at 1 and 2 hours but not at 3 and 4 hours after consumption. Ratio in plasma of tyrosine to other LNAA increased by 20 to 60%, and still at 4 hours the plasma tyrosine ratio was significantly elevated above control level after all 3 albumin doses. The plasma ratio of tryptophan to other LNAA decreased by 30 to 50%, and at 4 hours the plasma tryptophan ratio was still significantly below control level after the 2 larger albumin doses. The marked changes in the plasma tryptophan and tyrosine ratio suggests that the central serotonin and catecholamine synthesis could possibly be affected following ingestion of pure protein.

Variables influencing the effect of a meal on brain tryptophan.

Previous work from our laboratory points to plasma free tryptophan being a useful predictor of brain tryptophan concentration in many circumstances. Other work, in particular various studies on the acute effects of food intake, has emphasized the roles of plasma total tryptophan and of plasma large neutral amino acids that compete with tryptophan for transport to the brain. We have now studied associations between the above variables under different dietary conditions. Rats were allowed to feed for restricted periods during a 12-h light-12-h dark cycle. In the first study, rats were given access to a carbohydrate diet for 2 h midway through the light cycle and following an 18-h fast. The resultant rise of brain tryptophan was explicable largely by the associated fall in large neutral amino acids. In a second study, rats were adapted to a regimen whereby they were allowed access to the standard laboratory diet for 4 h during the dark cycle for 3 weeks. A postprandial decrease in brain tryptophan was associated with a fall in free tryptophan and of its ratio to competing amino acids. The brain change could be attributed neither to changes in plasma total tryptophan (which increased) nor to changes of its ratio to the competers (which remained unchanged). Results as a whole are thus consistent with changes of plasma free tryptophan and large neutral amino acid concentrations affecting brain tryptophan concentration under different dietary circumstances. It is suggested that these influences serve to maintain brain tryptophan when dietary supplies are defective.

Indole amines and amino acids in various brain regions after infusion of branched chain amino acids into hepatectomized rats.

This study was designed to determine regional changes of amino acids and indole amines in the brain and possible interactions between amino acids and indole amines 18 h after hepatectomy in rats. Hepatectomy and glucose infusion alone resulted in a profound increase of most large neutral amino acids (LNAA) in plasma and in the brain except for the branched-chain amino acids (BCAA), which maintained normal or somewhat lower values in plasma. Hepatectomy and infusion of glucose combined with BCAA sharply reduced the plasma and brain amino acid concentrations of other LNAA. Simultaneously the concentrations of serotonin and 5-hydroxyindoleacetic acid were decreased in all brain regions. In both groups of hepatectomized rats there were regional variations of the amino acid and the indole amine concentrations in the brain, but the response to BCAA infusion was generally the same in all brain regions. No difference in survival between the 2 groups could be found.

"On-off" phenomenon in Parkinson's disease: correlation to the concentration of dopa in plasma.

To investigate the relation between "on-off" fluctuations in symptomatology and bioavailability of dopa in patients with Parkinson's disease, five Parkinsonian patients with pronounced "on-off" symptoms were studied. Continuously during the study the degree of disability in the patients was registered. Every one hour, and in addition, whenever there was a change from "on" to "off" or vice versa, a blood sample was collected for dopa determination. Since dopa is transported from plasma into the brain by a saturable carrier for which it has to compete with endogenous large neutral amino acids (LNAA), the concentrations of these competitors were measured too. In four of the patients there were considerable oscillations in the plasma dopa concentration during the day; in one of these patients the highest value was as much as 12 times higher than the lowest value. These dramatic fluctuations in the absolute concentration of dopa in plasma had a major influence on the relative dopa concentrations (calculated as the ratio dopa/sum of LNAA) as the fluctuations in the concentrations of LNAA in plasma were much less pronounced. Consequently, the absolute and the relative concentrations of dopa in plasma were highly parallelled. In four of the five patients "on"-periods began within one hour after a peak in the concentration of dopa in plasma and in the fifth patient five out of seven "on"-periods were preceded by a rise in plasma dopa concentration within the same time interval. From the present data it could be concluded that the "on-off" phenomenon in Parkinson's disease, at least partly, is due to oscillations in the concentration of dopa in plasma. A reduction in the variations of the concentration of dopa in plasma seems to be necessary to overcome the "on-off" problem. The introduction of a slow release preparation of dopa is therefore urgently warranted. The concentration of LNAA in plasma must, however, also be considered in this context.