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O E Pratt

Publications and source records attributed to O E Pratt.

At least 37 records · Page 2Linked to original sources
PubMed1987

Valine entry into rat brain after diet-induced changes in plasma amino acids.

Passage of amino acids across the blood-brain barrier is assumed to be modified by amino acid composition of the blood. To gain a better understanding of the effects of protein intake on brain amino acid uptake, we examined associations among diet, plasma amino acid patterns, and the rate of entry of valine into the brain. Rats were fed (8 h/day for 7-10 days) diets containing 6, 18, or 50% casein before receiving one meal of a diet containing 0, 6, 18, or 50% casein. After 4-7 h, they were anesthetized and infused intravenously with [14C]valine for 5 min before plasma and brain samples were taken for determination of radioactivity and content of individual amino acids. As protein content of the meal was increased from 0 to 50% casein, plasma and brain concentrations of valine and most other large neutral amino acids (LNAA) increased severalfold; also the ratio of [14C]valine in brain to that in plasma decreased by greater than 50%, and the rate of valine entry into the brain increased 3.5-fold. The increase in valine flux slowed as plasma levels of LNAA, competitors for valine transport, increased. The results were far more dependent on protein content of the final meal than on that of the adaptation diet; thus changes in protein intake, as reflected in altered plasma amino acid patterns, markedly altered valine entry into the brain.

Amino Acids↗
PubMed1987

Changes in the activation of red blood cell transketolase of alcoholic patients during treatment.

Erythrocyte transketolase activation by thiamin diphosphate has been studied in alcoholic patients on admission and after treatment, which included vitamin therapy. The high proportion of the patients who showed an abnormal activation of transketolase, not only by 0.3 mM thiamin diphosphate but also further activation by increasing the thiamin diphosphate to 3 mM, was reduced considerably after treatment. A few patients, however, still showed continuing abnormalities even after treatment. Further study of the enzyme activation in vitro confirmed the presence of an enzyme variant, abnormal both in the ease with which the thiamin diphosphate could be removed and in requiring a high concentration of thiamin diphosphate for activation. Since the modified transketolase activation test appears not only to monitor the effectiveness of thiamin therapy but also independently to warn of persisting enzyme abnormalities, it could prove to be of general use in alcohol detoxification units.

Adult↗
PubMed1986

Transport of thiamin across the blood-brain barrier of the rat in the absence of aerobic metabolism.

By employing an anaerobic brain perfusion technique the dependency of thiamin transport across the blood-brain barrier upon high-energy phosphate production has been studied. Analysis of the data revealed essentially identical influx kinetics as those previously reported in vivo, the influx being considerably greater than that for the non-transported small molecule mannitol. These results provide for the first time direct evidence that, unlike at other cell surfaces, the influx of thiamin at the blood-brain barrier is independent of energy-yielding metabolism.

2,4-Dinitrophenol↗
PubMed1986

The effect of dexamethasone on vascular permeability of experimental brain tumours.

The vessels of experimental gliomas show an abnormally high permeability to small polar molecules, such as mannitol. To establish whether this change in vessel permeability is modified by treatment with the corticosteroid dexamethasone, the kinetics of [14C]mannitol transfer into rat astrocytomas were estimated in both steroid-and saline-treated, tumour-bearing animals. This was achieved by injecting [14C]-mannitol i.v., using a specially devised technique, so as to maintain a constant concentration of tracer in the blood plasma. In separate experiments steady levels of the tracer were maintained in the circulation from 1 to 30 min. Mean plasma and tumour radioactivity were measured, and the apparent transfer constant of mannitol across the vascular endothelium and the size of the extravascular extracellular mannitol space in the tumours were calculated. Despite a significant clinical improvement in the treated animals and adequate circulating levels of dexamethasone at the time of the permeability studies, no difference in either the apparent transfer constant for the movement of mannitol into the tumours or the fractional extracellular mannitol space was detected between these animals and the controls. With steroid treatment both tumour-bearing and non-tumour bearing animals lost weight, and in the latter there was no consistent change in routine biochemical or haematological parameters. It was concluded that under these conditions it is unlikely that clinical improvement with dexamethasone therapy was due to a non-specific reduction in tumour vessel permeability to polar substances.

Animals↗
PubMed1986

Reduced stability of rat brain transketolase after conversion to the apo form.

The stability of rat brain transketolase, whether measured at 37 or 0 degree C, was reduced after conversion to the apo form by removal of thiamine diphosphate, as shown by a decline in the activity recovered when assayed in the presence of thiamine diphosphate. Both the shape of the breakdown curve and the failure to recover the full activity, even after incubation with thiamine diphosphate, showed that the breakdown of the apotransketolase was complex. The initial rate of breakdown of the apoenzyme was sharply pH dependent, being minimal at 37 degrees C at a pH value of 7.6, close to that likely to exist in vivo. The rate rose sharply with deviation of the pH in either direction. The stability of the enzyme on storage at 0 degree C showed a similar pattern of pH dependence, provided that allowance is made for temperature effects on dissociation constants. These findings provide further support for the hypothesis that differences in brain transketolase may play a part in the etiology of Wernicke-Korsakoff's syndrome.

Animals↗
PubMed1985

Maintenance of the integrity of the blood-brain barrier in the rat during an in situ saline-based perfusion.

Integrity of the blood-brain barrier to the small polar tracer mannitol was maintained for up to 30 min during an in situ perfusion of the brain with a saline-based solution containing the metabolic inhibitor 2,4-dinitrophenol. The patency of the capillary bed after perfusion was demonstrated by injecting a solution of Indian ink and gelatin, and ultrastructural examination showed the microvasculature to be well preserved. These findings suggest that the blood-brain barrier can be studied under conditions that are independent of normal cerebral function and metabolism.

2,4-Dinitrophenol↗
PubMed1985

Comparison of the effects of some thiamine analogues upon thiamine transport across the blood-brain barrier of the rat.

The flux of thiamine from the blood into the brain has been measured by a specially devised procedure in which a steady raised level of the vitamin, with or without radioactive labelling, was achieved rapidly and maintained steadily in the circulating blood plasma. This was done by a single rapid I.V. injection followed by a continuous injection given at a rate adjusted according to a pre-determined programme, so as to replace the injected material at the rate at which it had been found to leave the circulation in preliminary experiments. A series of four chemical analogues of thiamine were given to see how each affected the flux of thiamine into the brain and the findings are compared with those for a fifth analogue studied in previous work. Pyrithiamine, thiamine disulphide and acetylthiamine, like amprolium, inhibited thiamine transport across the blood-brain barrier. Kinetic analysis shows that they compete mainly for the saturable component of thiamine flux across the blood-brain barrier, with only a slight inhibition of the non-saturable component, most clearly seen with pyrithiamine. Oxythiamine, despite its close chemical similarity to thiamine did not have any significant effect upon the flux of the vitamin into the brain. These findings help to explain the efficacy of pyrithiamine administration, especially in conjunction with a thiamine-deficient diet, in rapidly producing central neurological signs of deficiency.

Amprolium↗
PubMed1985

Transketolase variant enzymes and brain damage.

Human red blood cell transketolase has been resolved into two components by gel filtration. One component has its thiamine diphosphate coenzyme firmly bound whilst the other variant of the enzyme is a smaller molecule which is inactive without added thiamine diphosphate, for which it has a reduced affinity. It is concluded that the failure to detect an increase in activation in the commonly used clinical test of red cell transketolase activation by raising the thiamine diphosphate concentration above about 0.3 mmol/l is likely to be due to masking of the effect of activation of the low affinity variant in haemolysates from normal red blood cells by the inhibitory effect of excess thiamine diphosphate upon the activity of the high affinity form of the enzyme with which it is mixed. Increased activation by higher thiamine diphosphate concentrations is sometimes seen in haemolysates from the blood of chronic alcoholics, as well as in the low molecular weight fraction separated from normal haemolysates. It is considered likely that there are at least two variants of the enzyme and that the low molecular weight variant represents a damaged form of the enzyme normally present in small amounts but formed in larger proportions in vivo in abnormal conditions like chronic alcoholism and thiamine deficiency as well as by enzyme breakdown in vitro. In the light of these conclusions some recently proposed hypotheses regarding the role of transketolase in the genesis of brain damage in thiamine deficiency are reconsidered and a modified mechanism is proposed consistent with these and other recent findings.

Adult↗
PubMed1984

Introduction: what do we know of the mechanisms of alcohol damage in utero?

The newly established role of alcohol as a human teratogen important in the aetiology of mental deficiency poses the problem of how maternal ingestion of alcohol damages the developing central nervous system. Two main approaches to the identification of possible mechanisms are suggested: clinical field studies, especially of relatives of affected children, and the use of experimental animal models. The evidence so far provided by each approach, and the limitations of each, are considered. Four possible mechanisms of alcohol-induced damage, originating at different stages of antenatal development, are outlined. These mechanisms are related to the fetal alcohol syndrome, the high incidence of spontaneous abortion or fetal death, retarded fetal growth, and ill-defined behavioural problems in childhood.

Animals↗
PubMed1984

Range of alcohol-induced damage in the developing central nervous system.

Fourteen previously reported cases of the fetal alcohol syndrome (FAS) showed anomalies of brain structure varying in severity from microscopic disorganization of tissue structure, or abnormalities in neuronal or glial migration only visible microscopically, to complete or partial agenesis of regions such as the corpus callosum or cerebellum and large neuronal heteropias. The difficulty is illustrated of differentiating this type of damage, lacking in specificity and uniformity, from other syndromes of uncertain aetiology, such as De Lange, DiGeorge and Dubowitz, in at least one of which (DiGeorge syndrome) maternal alcoholism has been implicated. Similar brain damage is also seen in other conditions with known causes. In FAS and syndromes with this type of brain damage, most of the non-CNS features which make the conditions clinically recognizable may well be determined by timing or ancillary factors. Alcohol-related antenatal effects should not be identified to restrictively with FAS but should be considered in any condition of unknown aetiology with disorganization of brain structure and mental retardation.

Abnormalities, Multiple↗
PubMed1984

The vasculature of experimental brain tumours. Part 4. The quantification of vascular permeability.

In order to quantify changes in vessel permeability seen previously in experimental astrocytomas produced in rats by an intracerebral injection of cultured neoplastic glial cells, the flux of mannitol across the vascular endothelium from the blood into the normal brain or tumour tissue was measured using a specially devised technique by which a steady level of radioactively labelled mannitol can be achieved rapidly and maintained in the bloodstream. This is done by a continuous injection given at a rate which is adjusted by a predetermined programme so as to replace the tracer at the rate at which it has been found to leave the circulation in previous experiments. In separate experiments on both tumour-bearing and control rats steady levels of the tracer were maintained in the circulation for progressively longer times of up to 30 min. The kinetic parameters of the process gave estimates for the apparent transfer constant of mannitol across the vascular endothelium and of the size of the extravascular extracellular mannitol space in the tumours. The apparent transfer constant for the movement of mannitol across the blood-brain barrier was increased more than a hundred-fold in the region of the tumour compared to the values for the brain of control rats or that of tumour-bearing rats remote from the tumour site. The extracellular extravascular space within the tumour was estimated to be 22%, somewhat larger than accepted normal values.

Animals↗
PubMed1984

Heterogeneity of human erythrocyte transketolase: a preliminary report.

A modified procedure for the preparation of transketolase from erythrocytes by dialysis and column ion-exchange resin chromatography is described. Changes have been made in the resin used, the column dimensions and the elution procedure so as to separate the enzyme with improved resolution, prepare the apoenzyme free of thiamine pyrophosphate and study the kinetics of its activation or reactivation by the coenzyme. On the basis of the elution profile of the enzyme activity from the chromatographic column, two different samples of the transketolase have been isolated, which differ not only in their isoelectric properties, but also in the proportion of the transketolase present in the apoenzyme form. Not only do the apoenzymes isolated from each of the two fractions differ in the way in which they recombine with thiamine pyrophosphate but kinetic analysis of the results shows that each fraction contains at least two variants of transketolase differing in their affinity for thiamine pyrophosphate. Three, probably four, separate variants have been identified which differ in their affinities for thiamine pyrophosphate over a range greater than 10(4). It is concluded that these two fractions of the enzyme must contain different subsets of the eight isoenzymes of transketolase of differing isoelectric points and that some of these isoenzymes must differ also in their affinity for the coenzyme. The implications of these findings for the Blass and Gibson hypothesis about the pathogenesis of the Wernicke-Korsakoff syndrome are considered.

Alcohol Amnestic Disorder↗
PubMed1983

Inhibition of thiamine transport across the blood-brain barrier in the rat by a chemical analogue of the vitamin.

The flux of thiamine from the blood into the brain has been measured using a specially devised technique by which a steady raised level of the vitamin, with or without radioactive labelling, can be achieved rapidly and maintained in the bloodstream. This is done by a continuous injection, given at a rate which is adjusted by a pre-determined programme so as to replace the tracer at the rate at which it has been found to leave the circulation in previous experiments. A further programme was worked out to maintain, in a similar manner by a separate injection, a steady raised level in the bloodstream of a chemical analogue of thiamine, 1-[(4-amino-2-propyl-5-pyrimidinyl)methyl]-2-picolinium chloride HCl (amprolium). In the presence of a high concentration of amprolium the flux of thiamine across the blood-brain barrier was greatly reduced and no longer saturable by raising the blood thiamine concentration up to at least 10 microM. It was concluded that this analogue of thiamine inhibited the saturable component of thiamine transport across the barrier but not the non-saturable component. In a further series of experiments, progressively higher levels of thiamine were maintained in the bloodstream and the influx of the vitamin across the blood-brain barrier was measured. From kinetic analysis of the results, it was clear that the affinity of amprolium for the transport carrier was of a similar magnitude to that of thiamine itself. That the inhibition was competitive was shown by the way in which it could be overcome if the level of thiamine in the blood plasma was raised sufficiently above the normal.

Amprolium↗
PubMed1982

Kinetics of thiamine transport across the blood-brain barrier in the rat.

1. By measurement of the rate of disappearance of injected tracer thiamine from the bloodstream, a programme for the continuous injection of thiamine at a variable rate has been devized by which a steady raised level can be achieved rapidly and maintained in the circulation. By this means the flux of radioactive thiamine across the blood-brain barrier has been measured. 2. In separate experiments progressively higher levels of thiamine were maintained in the bloodstream. Evidence was obtained that the transport of thiamine across the blood-brain barrier is a carrier-mediated process which can be saturated by raised levels of thiamine. 3. The saturation of the transport process was incomplete: kinetic analysis showed that there was a non-saturable component of the transport which was probably due to passive diffusion. 4. The contribution of the non-saturable component was normally small and is probably insufficient to meet the needs of the brain for the vitamin unless the concentration of the vitamin in the blood is raised considerably above normal. 5. This two-component transport process had substantially similar kinetic parameters in different regions of the brain.

Animals↗
PubMed1981

The effect of insulin upon the influx of tryptophan into the brain of the rabbit.

1. The effect of hyperinsulinaemia upon the influx of tryptophan into the brain was determined. A raised level of insulin was maintained in the circulation of rabbits for periods of up to 120 min by means of a continuous, programmed intravenous injection of the hormone, given by an electronically controlled variable-drive syringe. A similar, appropriately programmed, intravenous injection of glucose, given simultaneously with the insulin, maintained the concentration of the blood glucose within normal limits throughout each experiment, so that the results were not vitiated by the development of hypoglycaemia. 2. Raised levels of insulin in the blood affect the supply of tryptophan to the brain in two opposing ways: (a) by increasing the binding of tryptophan to the albumin in the blood, thereby reducing the level of the free tryptophan in the circulation by about a half, which would decrease the influx of tryptophan into the brain; (b) by simultaneously reducing the levels in the blood of six or more of the amino acids which compete with tryptophan for transport carriers into the brain, which would increase the influx of tryptophan. The net result of these two opposing effects is that insulin causes only a slight increase in the influx of tryptophan into the brain. 3. To account in quantitative terms for the effect of insulin upon the influx of tryptophan into the brain it proved necessary to make one assumption. This assumption was that a predictable proportion of the tryptophan which is loosely bound to blood albumin is being stripped off this protein by the transport carrier located on the luminal surface membranes of the endothelial cells during the passage of the blood through the cerebral capillaries. If this assumption is accepted the work reported here explains adequately the effect of insulin on the influx of tryptophan into the brain.

Amino Acids↗