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H K Ward

Publications and source records attributed to H K Ward.

9 recordsLinked to original sources
PubMed1989

Glutaminase inhibition and the release of neurotransmitter glutamate from synaptosomes.

Cerebrocortical synaptosomes were incubated with glutamine together with 6-diazo-5-oxo-L-norleucine (DON) and NH4Cl (which are known to inhibit phosphate-stimulated glutaminase) in order to assess the effect of such inhibition on the pool sizes and extent of evoked release of endogenous amino acids, particularly glutamate. DON (5 mM) inhibited glutaminase by 73-89% and NH4Cl (1-4 mM) inhibited the enzyme by 45-53% under the conditions employed. NH4Cl (4 mM) incubated with synaptosomes for 30 min reduced pool sizes of aspartate and glutamate by 28% and inhibited release of glutamate by 55% compared to control release. DON caused a decrease in both the pool size of glutamate (22%) and the extent of veratrine-evoked release of amino acids (21%).

Ammonium Chloride↗
PubMed1985

Effect of cortico-striate pathway lesion on the activities of enzymes involved in synthesis and metabolism of amino acid neurotransmitters in the striatum.

The activities of several enzymes involved in the metabolism of aspartate and glutamate were measured in striatal (nucleus caudatus and putamen) homogenates 2-3, 6-7, and 35-40 days following frontoparietal and frontal cortical ablation. The activity of glutamine synthetase (GS) was substantially increased (46-48%) on the operated side 6-7 days following the lesion whereas smaller changes were observed at 2-3 and 35-40 days after lesion. In contrast, decreased levels of glutaminase and malate dehydrogenase (MDH) were observed by 6-7 days while no significant change was found at either 2-3 or 35-40 after the lesion. The activities of glutamate dehydrogenase (GDH) and glutamate decarboxylase (GAD) were elevated after 35-40 days whereas no changes in the levels of either GDH or aspartate aminotransferase (ASAT) were found at 2-3 or 6-7 days after the fronto-parietal decortication. When only the frontal cortex was removed quantitatively similar changes were observed in striatal GS and glutaminase activity. The content of glutamate and glutamine in the denervated striatum followed qualitatively the changes in glutaminase and GS. The results indicate that the degeneration of cortico-striatal terminals causes a profound glial reaction in the striatum, and both glutaminase and MDH are present in relatively high concentrations in the corticostriatal terminals.

Amino Acids↗
PubMed1984

Kinetic properties of glutaminase from cerebral cortex.

The rates of phosphate activated glutaminase activity in finely homogenised cerebral cortex and synaptosomes were measured. Activity was 25-50% higher at pH 7.0 than at pH 8.0. Glutamate inhibited activity with a Ki of 2-3 mM while aspartate had little effect. Calcium (1 mM) activated the enzyme but magnesium was without action. The pH profiles of the effects of these modulators of glutaminase activity in these finely ground preparations showed that all agents were more effective at pH 7.0 than at pH 8.0.

Animals↗
PubMed1983

Glutamine and glucose as precursors of transmitter amino acids: ex vivo studies.

Radiolabelled glutamine and glucose were infused into lateral ventricles of rats in order to label transmitter amino acid pools in vivo. Brain regions close to the lateral ventricle (hippocampus, corpus striatum, hypothalamus) were labelled more effectively than more distant structures such as cerebral cortex or cerebellum. All regions were labelled to much the same extent over 30-150 min by [U-14C]glucose, [U-14C]glutamine, or [3H]glutamine administered alone or together in double-label experiments when allowance was made for any differences in precursor specific radioactivities. Slices of cerebral cortex or hippocampus from brains labelled in vivo were incubated and stimulated in vitro with veratrine (75 microM); tetrodotoxin (1 microM) was present in the control medium. Single-label experiments showed that [U-14C]glutamine was more effective than [U-14C]glucose for labelling releasable glutamate and GABA. Double-label experiments showed that [3H]glutamine and [U-14C]glucose given together in vivo labelled glutamate and GABA releasable in vitro to a similar extent. Both types of experiment emphasise the large contribution made by glutamine in vivo to pools of transmitter glutamate and GABA.

Amino Acids↗
PubMed1976

On glutaminase activity in mammalian synaptosomes.

A large proportion (30%) of tissue glutaminase activity was found localized in synaptosome fractions as well as purified mitochondrial fractions, where it is also enriched on a protein basis (2-fold). Sulphate was more effective (2-8-fold) than phosphate or chloride in activating the enzyme at concentrations above 10-12 mM, but equivalent to phosphate at lower concentrations (1-3 mM). At CSF levels (0.5 mM), glutamine is readily used by synaptosomes as a substrate in the presence of 10 mM glucose. It constitutes at least 50% to the carbon of aspartate, glutamate and GABA. Pool sizes of these amino acids are maintained in the presence of glutamine and glucose at CSF levels, but not by glucose alone. Ammonium ion at 1 mM substantially (30%) inhibits glutaminase utilisation by synaptosomes. It is suggested that CSF glutamine is a major substrate for the nerve ending in situ.

Animals↗
PubMed1975

Biochemical and morphological studies of the short and long term survival of isolated nerve-endings.

The respiratory rate, potassium, amino acid, ATP and phosphocreatine content of cortical synaptosomes were measured over periods from one hour to 5 days. Incubation medium or tissue culture medium at 37 degrees C was employed. In suspensions, synaptosomes show diminished levels of all these parameters after 3 or 4 h, but synaptosome beds showed no loss of potassium, ATP or phosphocreatine at this time. Morphologically the synaptosomes showed good preservation of structure during the first 24 h in tissue culture. Respiratory response to electrical pulses was detectable even after 24 h in culture when basal respiration was low.

Adenosine Triphosphate↗