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

Measurement of blood-brain barrier permeability of rats with alpha-aminoisobutyric acid during microdialysis: possible application to behavioral studies

The purpose of this study was to determine the suitability of alpha-aminoisobutyric acid (AIB) as a marker of the integrity of the blood-brain barrier (BBB) during studies in which amino acid content in dialysates collected by microdialysis probes and behavior of the rat are studied concurrently. AIB (200 mg/kg) was injected intraperitoneally at 1800 h into 24-h fasted, ketamine-acepromazine anesthetized rats on either 10 or 30 days following guide cannula implantation. Dialysates from the medial preoptic area and blood from the tail vein were collected 1 h before and after the AIB injection. Analysis of amino acids, including AIB, in the collections was conducted by reverse-phase HPLC. In 21 of 24 rats, AIB in dialysates averaged less than 3% of plasma at 10, 30, and 50 min after AIB injection. In those rats unidirectional blood-to-brain transfer constant, K(i), of AIB was constant and a good relationship was found between dialysate amino acid concentrations and those predicted from calculations of transport based on the brain uptake index (BUI) for some amino acids. AIB concentration in dialysates was greater than 10% of plasma at 30 min in only 3 of 24 rats. We conclude that AIB can be used as a marker to monitor the integrity of the BBB during serial measurements of dialysate concentrations of amino acids and has application in behavioral studies.

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 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.

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.