Transport of amino acids by rabbit choroid plexus in vitro

Glutamine transport at the blood-brain and blood-cerebrospinal fluid barriers

Glutamine has multiple physiological and pathophysiological roles in the brain. Because of their position at the interface between blood and brain, the cerebral capillaries and the choroid plexuses that form the blood-brain barriers (BBB) and blood-cerebrospinal fluid (CSF) barriers, have the potential to influence brain glutamine concentrations. Despite this, there has been a paucity of data on the mechanisms and polarity of glutamine transport at these barrier tissues. In situ brain perfusion in the rat, indicates that blood to brain L-[14C]glutamine transport at the blood-brain barrier is primarily mediated by a pH-dependent, Na(+)-dependent, System N transporter, but that blood to choroid plexus transport is primarily via a pH-independent System N transporter and a Na(+)-independent carrier that is not System L. Transport studies in isolated rat choroid plexuses and primary cultures of choroid plexus epithelial cells indicate that epithelial L-[14C]glutamine transport is polarized (apical uptake>basolateral) and that uptake at the apical membrane is mediated by pH dependent System N transporters (identified as SN1 and SN2 by polymerase chain reaction) and the Na(+)-independent System L. Blood-brain barrier System N transport is markedly effected by cerebral ischemia and may be a good marker of endothelial cell dysfunction. The multiple glutamine transporters at the blood-brain and blood-CSF barriers may have role in meeting the metabolic needs of the brain and the barrier tissues themselves. However, it is likely that the main role of these transporters is removing glutamine, and thus nitrogen, from the brain.

Transport of 14C-gamma-aminobutyric acid into brain, cerebrospinal fluid and choroid plexus in neonatal and adult rats

In general blood to brain entry of amino acids is greater in the neonatal rats compared to the adults. gamma-Aminobutyric acid (GABA), a neurotransmitter amino acid, shows limited transport across the blood-brain barrier (BBB) in the adult rat. Characteristics of GABA entry into the immature rat brain is yet to be addressed. This investigation was set to study the entry of GABA into brain of the neonatal rat compared to the adult. Using the bilateral in situ brain perfusion technique, the entry of 14C-GABA into brain, cerebrospinal fluid (CSF) and lateral ventricles choroid plexuses was studied in the adult and neonatal rats. 14C-GABA uptake into neonatal rat brain after 20 min perfusion was 0.116+/-0.014 ml g(-1), approximately twice that of the adults (P<0.01). Half saturation constant, K(m), did not change with age (P>0.05), whereas maximal transport into the brain, V(max), was reduced from 0.152 to 0.068 nmol min(-1) g(-1) showing a significant reduction with age (P<0.05). In the neonate the entry of GABA into the CSF was dominant when compared to that into the brain, this could be due to a greater diffusional component, K(d), which was detected to be high in the neonate. In conclusion, the uptake of 14C-GABA into brain of the immature rats exceeded that in the adults which is thought to be due to both greater maximal transport and greater diffusion in the neonate compared to the adult.

Conditionally immortalized cell lines as a new in vitro model for the study of barrier functions

Conditionally immortalized brain and retinal capillary endothelial and choroid plexus epithelial cell lines were established from a transgenic rat (Tg rat) and mouse (Tg mouse) harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen. These cell lines exhibit temperature-sensitive cell growth due to the expression of ts SV 40 large T-antigen. Mouse brain (TM-BBB) and rat brain (TR-BBB) and rat retinal (TR-iBRB) capillary endothelial cell lines appear to have a spindle-fiber shaped morphology and exhibit the typical endothelial markers, such as von Willebrand factor and acetylated low-density lipoprotein uptake. These cell lines express in vivo influx and efflux transporters, such as P-glycoprotein (P-gp) and GLUT1, which is capable of 3-O-methyl-D-glucose transport. TM-BBB cells are able to undergo efflux transport of cyclosporin A, which is a substrate for P-gp transport activity. They may also express oatp2 and exhibit dehydroepiandrosterone sulfate and digoxin uptake activity. TR-BBB cells express the mRNA of multidrug resistance associated protein 1 (MRP1) and a large neutral amino acid transporter, which consists of LAT1 and 4F2hc. TR-iBRB cells exhibit pH-dependent L-lactic acid transport activity and express the mRNA of monocarboxylate transporter (MCT) 1 and 2. The choroid plexus epithelial cell line (TR-CSFB) has polygonal cell morphology, expresses the typical choroid plexus epithelial cell marker, transthyretin, and has Na+, K+-ATPase located on the apical side. TR-CSFB cells also exhibit amino acid transport activity which has been observed in vivo. These barrier cell lines established from the Tg rat and Tg mouse have in vivo transport functions and are good in vitro models for drug transport to the brain and retina and as a screen for drugs which might be capable of delivery to the brain and retina.

Characterization of the amino acid transport of new immortalized choroid plexus epithelial cell lines: a novel in vitro system for investigating transport functions at the blood-cerebrospinal fluid barrier

PURPOSE

To establish and characterize a choroid plexus epithelial cell line (TR-CSFB) from a new type of transgenic rat harboring the temperature-sensitive simian virus 40 (ts SV 40) large T-antigen gene (Tg rat).

METHODS

Choroid plexus epithelial cells were isolated from the Tg rat and cultured on a collagen-coated dish at 37 degrees C during the first period of 3 days. Cells were subsequently cultured at 33 degrees C to activate large T-antigen. At the third passage, cells were cloned by colony formation and isolated from other cells using a penicillin cup.

RESULTS

Five immortalized cell lines of choroid plexus epithelial cells (TR-CSFB 1 approximately 5) were obtained from two Tg rats. These cell lines had a polygonal cell morphology, expressed the typical choroid plexus epithelial cell marker, transthyretin, and possessed Na+, K+-ATPase on their apical side. TR-CSFBs cells expressed a large T-antigen and grew well at 33 degrees C with a doubling-time of 35 approximately 40 hr. [3H]-L-Proline uptake by TR-CSFB cells took place in an Na+-dependent, ouabain-sensitive, energy-dependent, and concentration-dependent manner. It was also inhibited by alpha-methylaminoisobutylic acid, suggesting that system A for amino acids operates in TR-CSFB cells. When [3H]-L-proline uptake was measured using the Transwell device, the L-proline uptake rate following application to the apical side was five-fold greater than that following application to the basal side. In addition, both Na+-dependent and Na+-independent L-glutamic acid (L-Glu) uptake processes were present in TR-CSFB cells.

CONCLUSIONS

Immortalized choroid plexus epithelial cell lines were successfully established from Tg rats and have the properties of choroid plexus epithelial cells, and amino acid transport activity was observed in vivo.

Choroid plexus histidine transport

System-N transport plays an important role in l-glutamine uptake into isolated rat choroid plexus but its role in the transport of another System-N substrate, l-histidine, has yet to be determined. Similarly, the possible effects on System-N mediated l-histidine transport of changes in pH and extracellular l-glutamine, such as occur in cerebral ischemia and hepatic encephalopathy, have yet to be examined. In the absence of competing amino acids, l-[3H]histidine uptake in isolated rat choroid plexus was mediated by both Na+-independent and Na+-dependent transport. The former was inhibited by 2-amino-2-norbornane carboxlic acid, indicating System-L transport, while the latter appears System-N mediated as it was inhibited by three System-N substrates but not substrates for System-A and -ASC. The Na+-dependent uptake had a Km of 0.2 mM and a Vmax of 1.4 nmol/mg/min. It accounted for 30% of l-histidine uptake in the presence of physiological concentrations of amino acids. Reductions in pH markedly inhibited Na+-dependent but not Na+-independent transport indicating that, as in liver but not neurons, System-N mediated transport at the choroid plexus is pH sensitive. Increases in l-glutamine concentration in the pathophysiological range reduced l-histidine uptake via both System-L and -N.

Acidic amino acid accumulation by rat choroid plexus during development

Acidic amino acid accumulation by the choroid plexuses of the lateral ventricles was investigated using 1, 2, 3 week and adult (7-10 weeks old) rats. The accumulation from both blood and CSF sides of the choroid plexuses were investigated. The uptake from blood side was studied using the bilateral in situ brain perfusion, and time-dependent uptake profiles (2, 10, 20, and 30 min) of 14C-labelled aspartate, glutamate, and NMDA were measured. [3H]Mannitol was also included in perfusion fluid as a baseline for [14C]amino acid uptake into choroidal tissue. Uptake of [14C]aspartate and [14C]glutamate declined with age, while [14C]NMDA showed no significant uptake at any age. Twenty min [3H]mannitol uptake in the 1-week-old rat was significantly greater than the adult (P < 0.05). The K(m) for [14C]aspartate and [14C]glutamate obtained from multiple time uptake profiles also showed reduction with development but it was greater than that for mannitol. [14C]Aspartate declined from 69.8 +/- 21.1 microliters.min-1.g-1 in the neonate to 40.6 +/- 4.0 microliters.min-1.g-1 in the adult (P < 0.05), while glutamate showed a sharper decline from 78.9 +/- 24.2 microliters.min-1.g-1 to 17.7 +/- 5.4 microliters.min-1.g-1 (P < 0.01). Accumulation of 14C-labelled aspartate and glutamate by the choroid plexus from CSF side was also measured using ventriculo-cisternal perfusion. The accumulation in the adult was found to be 2-3 times greater than that in the neonatal rat (P < 0.05) for both amino acids. The uptake from either side was found to be saturable, stereospecific, not inhibited by neutral amino acid analogues, and shared by both aspartate and glutamate.

High activities of glutamine transaminase K (dichlorovinylcysteine beta-lyase) and omega-amidase in the choroid plexus of rat brain

Certain halogenated hydrocarbons, e.g., dichloroacetylene, are nephrotoxic to experimental animals and neurotoxic to humans; cysteine-S-conjugate beta-lyases may play a role in the nephrotoxicity. We now show that with dichlorovinylcysteine as substrate the only detectable cysteine-S-conjugate beta-lyase in rat brain homogenates is identical to glutamine transaminase K. The predominant (mitochondrial) form of glutamine transaminase K in rat brain was shown to be immunologically distinct from the predominant (cytosolic) form of the enzyme in rat kidney. Glutamine transaminase K and omega-amidase (constituents of the glutaminase II pathway) activities were shown to be widespread throughout the rat brain. However, the highest specific activities of these enzymes were found in the choroid plexus. The high activity of glutamine transaminase K in choroid plexus was also demonstrated by means of an immunohistochemical staining procedure. Glutamine transaminase K has a broad specificity toward amino acid and alpha-keto acid substrates. The omega-amidase also has a broad specificity; presumably, however, the natural substrates are alpha-ketoglutaramate and alpha-ketosuccinamate, the alpha-keto acid analogues of glutamine and asparagine, respectively. The high activities of both glutamine transaminase K and omega-amidase in the choroid plexus suggest that the two enzymes are linked metabolically and perhaps are coordinately expressed in that organ. The data suggest that the natural substrate of glutamine transaminase K in rat brain is indeed glutamine and that the metabolism of glutamine through the glutaminase II pathway (i.e., L-glutamine and alpha-keto acid-->alpha-ketoglutarate and L-amino acid + ammonia) is an important function of the choroid plexus. Moreover, the present findings also suggest that any explanation of the neurotoxicity of halogenated xenobiotics must take into account the role of glutamine transminase K and its presence in the choroid plexus.

The uptake of anionic and cationic amino acids by the isolated perfused sheep choroid plexus

The carrier-mediated uptake of anionic and cationic amino acids by the basolateral face (blood side) of the isolated perfused sheep choroid plexus was demonstrated using the paired-tracer dilution technique. The uptake of these two classes of amino acid was higher than at the blood-brain barrier with no cross-competition between them. In addition the uptake was markedly stereospecific with a high degree of selectivity and no interaction with the L-system amino acids.

The steady-state amino acid fluxes across the perfused choroid plexus of the sheep

The steady-state flux of labelled amino acids was investigated across the isolated perfused choroid plexus of the sheep. The extraction of anionic, cationic, small and large neutral amino acids by the blood side of the choroid plexus was demonstrated. However, there was no uptake of the analogue MeAIB, confirming the absence of the 'A' carrier system on this side of the blood--CSF barrier. The direction of the net flux of amino acids across the tissue varied depending on the amino acid and its concentration. At a concentration of 0.01 mM the net movement for phenylalanine, serine, aspartate and glycine was from blood to CSF. When the concentration of amino acid was increased to 0.1 mM, the net flux of phenylalanine and serine remained from blood to CSF whereas the net flux of the transmitters, aspartate and glycine, was in the opposite direction, from CSF to blood. When the level was raised further, to 1 mM, all four amino acids showed a net CSF to blood flux. The concentration of amino acids is newly formed CSF was calculated from the blood to CSF fluxes and was found to be between 2 and 200 microM, similar to that found in mammalian bulk phase CSF.

Release from live choroid plexus of apical fragments and electrophoretic characterization of their synthetic products

Protein synthesis and secretion by the choroid plexus (CP) has been implicated as a major source of certain proteins in cerebrospinal fluid (CSF), such as transthyretin. The suggestion that proteins are elaborated from CP through apocrine secretion has been borne out by the presence of newly labeled proteins in apical protrusions from CP (Agnew et al.: Cell and Tissue Research 208:261-281, 1980a). When the protrusions (aposomes) separate from the cells, they continue to incorporate labeled amino acids (Gudeman et al.: Tissue and Cell 19:101-109, 1987). In the present work the formation of aposomes in live CP explants indicated that these spheroids were not the result of fixation. Aposomes were also identified within rat CSF by immunohistochemistry with monoclonal directed against aposomes as well as with anti-transthyretin serum. The protein product of aposomes was characterized by 2-dimensional SDS-PAGE and compared to the protein products of whole CP tissue. Paradoxically, transthyretin, a heavily labeled protein in the tissue, was virtually undetected in the aposome synthetic profile. However, four other proteins were expressed in relatively equivalent amounts by the aposomes. The presence of mRNA in aposomes was detected with a poly dT probe, and the presence of actin was revealed by phalloidin staining of aposomes. These studies provide a more comprehensive definition of aposomes, but the functions of their secreted proteins remains to be determined.

Neutral amino acid uptake by the isolated perfused sheep choroid plexus

1. The uptake of neutral amino acids from the blood into the cells of the choroid plexus was studied by means of the rapid (less than 40 s) single-circulation paired-tracer dilution technique in the isolated perfused choroid plexus of the sheep. 2. The study provides the first direct evidence for the carrier-mediated entry of neutral amino acids from blood into the cells of the choroid plexus. 3. In the terms of Christensen's classification the presence of L-amino acid carrier systems for large neutral amino acids with bulky side chains has been demonstrated. 4. No measurable uptake of [14C]methyl amino isobutyric acid ([14C]MeAIB) during a single passage through the choroid plexus circulation was demonstrated which indicates the probable absence of a significant 'A' transport system. 5. The uptake of small neutral amino acids such as glycine and L-alanine was shown to be carrier-mediated. Results suggest that these amino acids are mainly transported by the glycine and ASC carrier systems, respectively. 6. The results suggest that there is a similarity between the transport systems for neutral amino acids on the blood side of both the blood-brain barrier and blood-cerebrospinal fluid barrier, the exception being for the presence of a glycine carrier on the blood side of the choroid plexus.

Steady-state distribution of cycloleucine and alpha-aminoisobutyric acid between plasma and cerebrospinal fluid

Estimates of the steady-state distribution ratios of two nonmetabolizable amino acids, alpha-aminoisobutyric acid and aminocyclopentane carboxylic acid (cycloleucine), between plasma and cerebrospinal fluid were made with a view to establishing whether or not the low values found with metabolizable amino acids, such as glycine or leucine, could be accounted for by uptake and metabolism by the brain. The estimates, based on the ratios found after i.p. injections either in bolus form or by implantation of "osmotic pumps" containing the labeled amino acids, were comparable with those found for metabolizable amino acids.

Permeability of the blood-cerebrospinal fluid and blood-brain barriers to thyrotropin-releasing hormone

The permeability of the blood-cerebrospinal fluid (CSF) barrier to 3H-labelled thyrotropin-releasing hormone (TRH), was studied at the blood-tissue interface of the isolated perfused choroid plexus of the sheep, using a rapid (less than 30 s), single circulation paired-tracer dilution technique, in which D-[14C]mannitol serves as an extracellular marker. Arterio-venous loss of 14C radioactivity reflects the percentage of the D-mannitol dose that crosses the blood-CSF barrier using a non-specific pathway. This loss suggests that the choroidal epithelium is moderately leaky. Cellular uptake of TRH, estimated by directly comparing venous dilution profiles of [3H]TRH and D-[14C]mannitol was independent of this leakiness. The unidirectional transport of TRH could not be saturated with unlabelled TRH at a concentration as high as 10 mM, but was markedly reduced by 10 mM proline and by the inhibitor of amidase and aminopeptidase activity, bacitracin (2 mM). Permeability of the blood-brain barrier to [3H]TRH was studied in the adult rat, employing the intracarotid injection technique of Oldendorf in which [14C]butanol served as an 'internal standard'. Brain-uptake of 3H radioactivity corrected for residual vascular space indicated a low extraction from the blood of TRH during a 15 s period of exposure to the peptide. Self-inhibition of [3H]TRH uptake by unlabelled TRH (10 mM) could not be demonstrated, but L-proline (10 mM) and bacitracin (2 mM) strongly inhibited this uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Neutral amino acid transport by plasma membrane vesicles of the rabbit choroid plexus

Uptakes of neutral L-amino acids into rabbit choroid plexus apical membrane vesicles were studied using L-[14C]amino acids. Apical membrane vesicles were prepared using Ca2+ precipitation and uptakes of 14C-labeled substrates were measured by a rapid mixing and filtration procedure. Na-dependent, concentrative uptake was observed for proline, histidine and methylaminoisobutyric acid (MeAIB). Phenylalanine and D-glucose uptakes showed no significant Na dependence. Proline uptake was a saturable function of the proline concentration with Jmax 3.5 nmol/(mg min) and Kt 0.3 mM. Competition experiments in the presence of Na indicated that proline and MeAIB are mutually competitive. Proline uptake was also inhibited 25% by phenylalanine, but MeAIB uptake was relatively unaffected. Neither proline nor MeAIB transport was significantly inhibited by glycine. We conclude that proline uptake across rabbit choroid plexus apical membrane vesicles in via an Na-dependent pathway which is shared by MeAIB and, to a minor extent, by phenylalanine, but from which glycine is excluded. Histidine uptake was inhibited by glycine, GABA, phenylalanine, proline and MeAIB. This suggests that histidine may utilize a different pathway in addition to that shared with proline and MeAIB. These transporters should play an active role in the regulation of amino acids in the CSF.

Transport of alpha-aminoisobutyric acid across brain capillary and cellular membranes

The transport of alpha-aminoisobutyric acid (AIB), N-methyl-AIB (MeAIB), and diethylenetriaminepentaacetic acid (DTPA) from blood to brain was measured over different experimental periods in eight regions of the rat brain. Unidirectional transfer rate constants were determined from multiple-time/graphical and single-time analysis of the experimental data; values of 0.0018, 0.00057, and 0.000021 ml g-1 min-1, respectively, were obtained for the thalamus by graphical analysis. The initial distribution volume of AIB and MeAIB in brain tissue was several-fold greater than that of DTPA and the tissue plasma volume, and this difference was not accounted for by red blood cell uptake. This discrepancy could be due to rapid transport of AIB and MeAIB into brain endothelial cells in addition to the relatively rapid uptake by choroidal, meningeal, and ependymal associated tissues that was demonstrated by autoradiography. Thus, it may be misleading and erroneous to consider the blood-brain barrier (BBB) to be a simple, single-membrane structure when analyzing the blood-brain transfer data of solutes such as amino acids. The data from the ventriculocisternal perfusion experiments and previously published AIB uptake data in mouse brain slices were used to estimate the transfer rate constants across brain cell membranes. These studies indicated that the transport of AIB into brain cells was approximately 110 to 265 times greater than that across normal brain capillaries per unit mass of brain tissue, and that the BBB limits blood-to-brain cell transport of this amino acid. These observations (low rate of transport across normal brain capillaries and rapid concentrative uptake by brain cells) indicate that AIB is a good marker for measuring moderate to large increases in BBB permeability by experiments that require unidirectional flux of the tracer.

Ventriculo-cisternal perfusion of twelve amino acids in the rabbit

The clearances of twelve amino acids from the ventricles during ventriculo-cisternal 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.

Culture and characterization of epithelial cells from bovine choroid plexus

Epithelial cells were isolated from choroid plexus, which plays a major role in cerebrospinal fluid production and regulation. Incubation of bovine choroid plexuses with pronase released cells which attached to plastic dishes with a plating efficiency of 5%. The cells were predominantly polygonal as judged by phase-contrast microscopy. These polygonal cells undergo limited cell division and survive for 1-2 weeks in culture before being overgrown by fibroblasts. The fibroblastic cells could be selectively removed from the cultures but the addition of 100 microgram/ml cis-hydroxyproline to the medium for several days. The specific activities of three membrane-bound enzymes, gamma-glutamyl transpeptidase, alkaline phosphatase, and leucine aminopeptidase were compared in selective cultures of polygonal cells and fibroblasts. Polygonal cells were found to have 4-5 times the gamma-glutamyl transpeptidase of fibroblasts, whereas fibroblasts have 2-3 times the alkaline phosphatase of polygonal cells. Leucine aminopeptidase levels in the two cultures were roughly equivalent. The polygonal cells rapidly lost gamma-glutamyl transpeptidase activity over a 4-day period in culture but acquired increased levels of leucine aminopeptidase. Alkaline phosphatase remained roughly constant. Under similar conditions fibroblasts showed a 3- to 4-fold increase in the specific activities of all three enzymes; these changes coincided with a substantial increase in cell density. Based on morphology, resistance to cis-hydroxyproline, absence of antihemophilic factor antigen, and enzymatic characteristics, we believe the polygonal cells to be of epithelial origin.

Accumulation of peptides by choroid plexus in vitro: Tyr-D-Ala-Gly as a model

Accumulation of Tyr-D-Ala-Gly (TAG) in rat choroid plexus was studied in vitro. Choroid plexus of the lateral ventricles and the fourth ventricle accumulated TAG against a concentration gradient. This accumulation was inhibited by some metabolic inhibitors and some peptides, but not by amino acids. The change and stereo-configuration of peptides had great influence on the accumulation. Metenkephalin was one of the strongest inhibitors. Absence of sodium ions in the medium did not affect the accumulation, but increase or decrease of potassium ions reduced it. Injection of reserpine for chemical denervation of sympathetic nerves or bilateral removal of the superior cervical ganglion had no effect. These results indicate that choroid plexus has different transport systems for amino acids and peptides, which are not affected by denervation of the sympathetic nerves that innervate choroid plexus.

Polarity of the blood-brain barrier: neutral amino acid transport into isolated brain capillaries

Capillary endothelial cells isolated from rat brain exhibit Na+-dependent uptake of the neutral amino acid analog alpha-(methylamino)isobutyric acid. Since studies in vivo demonstrate that this transport system is not present on the blood side of brain capillaries we conclude that Na+-dependent neutral amino acid transport is located on the brain side. Therefore, the luminal plasma membrane and the antiluminal plasma membrane appear to be functionally distinct. This polarity should permit brain capillary endothelial cells to actively regulate the internal milieu of the brain.

Movement of glycine across the blood-brain barrier of the rabbit

The single pass clearance of glycine from the cerebral capillary bed was measured in the rabbit with the indicator diffusion method. It was shown that glycine was extracted to the same degree as fructose (ca..07). The concentration of glycine injected into the carotid artery was varied some 700-fold without significantly affecting the clearance of this amino acid in transit through the brain. Cross inhibition of the unidirectional movement of glycine was not demonstrable. These present results and most previous studies suggest that this putative neurotransmitter does not cross the blood-brain barrier by a carrier mediated process.

A comparison of the rate equations, kinetic parameters, and activation energies for the initial uptake of L-lysine, L-valine, gamma-aminobutyric acid, and alpha-aminoisobutyric acid by mouse brain slices

At substrate concentrations, in medium, of 0.2 to 20 mM and at temperatures of 25 and 37 degrees C, the initial concentrative influx of the amino acids L-lysine (30 and 37 degrees C), L-valine, and gamma-aminobutyric acid into incubated mouse-cerebrum slices follows the rate equation for the initial influx of alpha-aminoisobutyric acid (Cohen, J. Physiol. 228:105, 1973), v equals Vmax/(1+Kt/S)+kuS. Kinetic constants at 37 degrees C are: Vmax equals 0.089 mumoles/g final wet wt of slices, min, Kt equals 0.69 mM, ku equals 0.037 mumoles/g final wet wt, mM-substrate, min for L-lysine; Vmax equals 0.60, Kt equals 1.30, ku equals 0.067 for L-valine; and Vmax equals 1.71, Kt equals 1.58, ku equals 0.094 for gamma-aminobutyric acid. The linear term, kuS, is due to an unsaturable process of concentrative uptake, not diffusion. Comparison of temperature coefficients reveals a "reference" pattern for typical low affinity transport of amino acids into brain slices. Its characteristics are: Activation energies associated with Vmax and ku are in range 14 to 20 kcal/mole; K, varies only slightly with temperature, L-Lysine and alpha-aminoisobutyric acid fit this pattern; L-valine and gamma-aminobutyric acid deviate in part. The Akedo-Christensen plot (J. Biol. Chem. 237:118, 1962) does not distinguish between the rateequation v equals Vmax/(1+Kt/S)+kuS for saturable uptake plus first-order unsaturable concentrative uptake, and the rate equation v equals Vmax/(1 + Kt/S)+kd(S minus Si) for saturable uptake plus first-order nonconcentrative "passive diffusion".