Biomedical subjects

J E Preston

Publications and source records attributed to J E Preston.

28 records · Page 2Linked to original sources

PubMed1992

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.

Animals↗

PubMed1990

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.

Amino Acids↗

PubMed1990

Kinetics and Na independence of amino acid uptake by blood side of perfused sheep choroid plexus.

The kinetic constants and sodium independence of amino acid uptake by the basolateral face (blood side) of the isolated perfused choroid plexus of the sheep were investigated. Uptake of 3H-labeled L-alanine, glycine, L-glutamine, L-leucine, L-glutamate, and L-lysine was not significantly inhibited when the sodium level of the perfusate was lowered to less than 6 mM. The cerebrospinal fluid (CSF) secretion rate was, however, reduced by 65.6%. The Michaelis constants for L-serine, glycine, L-phenylalanine, L-glutamate, and L-arginine were measured and varied between 25.4 microM for L-arginine and 2.6 microM for L-glutamate. The Vmax for the amino acids varied by about the same range as Km, with L-serine having the highest Vmax of 28.8 nmol.min-1.g-1, and L-phenylalanine having the lowest at 4.6 nmol.min-1.g-1. At normal plasma concentrations of 30-155 microM for individual amino acids, the carriers for glycine, L-phenylalanine, and L-glutamate would be almost fully saturated (greater than 90%), while those for L-serine and L-arginine would be greater than 60% saturated. This restricted uptake of amino acids by the basolateral face of the choroid plexus, coupled to clearance of these substances out of the CSF (5), could account for the low levels in the CSF compared with that in the plasma.

Amino Acids↗

PubMed1990

Comparison between blood and saline perfusion on the uptake of amino acids by choroid plexus of the sheep.

Uptake by the isolated perfused sheep choroid plexus of labelled naturally occurring amino acids, and the analogue MeAIB (methyl amino isobutyric acid) was measured using the single-pass, indicator dilution method. The uptake (Umax) of natural amino acids at the blood face of the choroid plexus, relative to the extracellular marker mannitol, ranged between 26 and 47% when using a saline perfusate. However, when the perfusion fluid was changes to one containing whole blood, uptake of the leucine, phenylalanine, proline, aspartate glutamate and lysine were significantly reduced with Umax now between -1.6 and 19.5%. In contrast the uptake of alanine, glutamine and glycine remained high despite the change in perfusion fluid. All amino acids, which showed uptake from the blood, also demonstrated self-inhibition when an additional 1 mM of the same unlabelled amino acid was added to the perfusion fluid. This indicates saturation of a carrier-mediate uptake process. There was no detectable uptake of the amino acid analogue, MeAIB during perfusion with either medium, confirming the absence of the "A" type amino acid carrier system for which this analogue is specific.

Amino Acids↗

PubMed1989

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.

Amino Acids↗

PubMed1987

The effects of aluminum chloride on the rate of secretion of the cerebrospinal fluid.

The claim that AlCl3 could produce 100% inhibition of cerebrospinal fluid secretion was investigated using ventriculocisternal perfusion in the rabbit, and it was shown that a large part of this inhibition was an artefact due to a pH sensitivity of Blue Dextran, used as an indiffusible marker, caused by AlCl3. Thus the true inhibition found by us, using [3H]labeled markers, was about 33% and usually only partly reversible. Penetration of 22Na from blood into the perfused ventricles was partially inhibited by AlCl3. The effects of some other acid buffer systems, namely acetate and phosphate, on rate of secretion were measured; the results were highly variable. When mean arterial pressure was measured, it was found to be unaffected by AlCl3 but elevated with acetate and phosphate buffer mixtures.

Acetazolamide↗

PubMed1974