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Biomedical subjects

D A Walker

Publications and source records attributed to D A Walker.

At least 55 records · Page 3Linked to original sources
PubMed1994

Multidrug resistance gene (MDR 1) expression in neuro-axial tumours of children and young adults.

Drug resistance in many cancers outside the CNS has been associated with over-expression of the multidrug resistance gene (MDR1), which codes for the transmembrane efflux pump P-glycoprotein (Pgp). To determine whether tumours of the neuroaxis over-express MDR1 and to identify the site of Pgp expression we examined 50 tumour specimens from 46 children and young adults using immunocytochemistry. Pgp was not expressed by any neoplastic cells, but was detected in the endothelium of tumour blood vessels in 35 of the 50 samples (70%). 11/35 (31%) were Pgp positive in the majority of vessels, 11/35 (31%) in a proportion, but < 50% of vessels, and 13/35 (37%) in one or two vessels. Pgp was also detected in surrounding normal brain capillaries. MDR1 may play a role in the chemoresistance of neuro-axial tumours either by its expression in the normal blood-brain barrier or by forming a blood-tumour barrier. The proportion of vessels expressing Pgp may determine the degree of resistance.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1992

Changes in fluorescence quenching brought about by feeding dithiothreitol to illuminated leaves.

When CO(2) is abruptly removed from the atmosphere surrounding an illuminated leaf, the primary electron-accepting plastoquinone of photosystem II (Q(A)) (as measured by photochemical quenching, q(p)) is rapidly reduced and then, after some seconds, becomes more oxidized. The reoxidation of Q(A) (-) is accompanied by an increase in DeltapH (as measured by nonphotochemical quenching, q(N)) with kinetics consistent with a causal relationship. The fact that, in such circumstances, Q(A) can become more oxidized in the absence of CO(2) than in its presence indicates a diminished rate of reduction of Q(A), consequent upon impaired photosystem II efficacy. Dithiothreitol (DTT) feeding, which does not affect quantum yield or the maximum rate of photosynthesis, inhibits the reoxidation of Q(A) (-) but not the increase in the proton gradient. When leaves are reilluminated in high light following a dark interval of several minutes, DTT also abolishes the separation in time between the first maximum in q(P) and the first maximum in the rate of O(2) evolution. It also diminishes subsequent oscillations. These results are held to demonstrate DeltapH control of photosystem II and are consistent with DTT inhibition of the xanthophyll cycle and hydrogen peroxide reduction. They support the concept that oxygen and hydrogen peroxide are involved, as Hill oxidants, in a pH-related manner, during oscillatory behavior.

Journal Article↗
PubMed1992

Comparison of polymerase chain reaction amplification of two mycobacterial DNA sequences, IS6110 and the 65kDa antigen gene, in the diagnosis of tuberculosis.

BACKGROUND: Knowledge of the sequences of mycobacterial genes and the availability of DNA amplification techniques have raised the possibility that identification of mycobacterial DNA may offer a rapid and specific diagnostic test for tuberculosis. The correlation between the presence of Mycobacterium tuberculosis DNA and clinical tuberculosis, however, is not known. This study compared the results of polymerase chain reaction amplification of two M tuberculosis DNA sequences, IS6110 and the gene encoding the 65kDa heat shock protein (65kDa Ag), from sputum, bronchoscopy washings, and bronchoalveolar lavage fluid and related these findings to the presence of active and past tuberculosis. METHODS: Highly specific primers were used for amplification of IS6110 and 65kDa Ag DNA. Analysis was performed on one or more samples from 87 patients. RESULTS: IS6110 DNA was identified in samples from all six patients with active tuberculosis, from 15 to 18 patients with past tuberculosis, from five of nine contacts of patients with tuberculosis, and from nine of 54 patients with lung disease unrelated to tuberculosis. The 65kDa Ag DNA was identified in samples from all patients with active and past tuberculosis, from contacts of patients with tuberculosis, and from 14 of 42 patients with non-tuberculous lung diseases. CONCLUSION: These data suggest that the presence of IS6110 DNA correlates more closely with a tuberculosis related diagnosis than that of 65kDa Ag DNA and that both DNAs are found in most subjects with past tuberculosis or contacts of patients with tuberculosis. This may limit the clinical usefulness of these tests.

Base Sequence↗
PubMed1989

Coregulation of electron transport and Benson-Calvin cycle activity in isolated spinach chloroplasts: studies on glycerate 3-phosphate reduction.

Glycerate 3-phosphate-dependent O2 evolution was measured in intact chloroplasts in the absence of CO2. At all concentrations of added glycerate 3-phosphate oxygen evolution ceased before stoichiometric amounts of oxygen were evolved. The inhibition of glycerate 3-phosphate-dependent-O2 evolution increased with increasing concentrations of substrate added. A similar response was observed in chloroplasts treated with KCN which inhibits ribulose-1,5-bisphosphate carboxylase-oxygenase. Oxygen uptake via the oxygenase activity of this enzyme is therefore not the cause of the discrepancy in stoichiometry of oxygen release in this system. The addition of NaHCO3 to chloroplasts in which oxygen evolution was inhibited by glycerate 3-phosphate caused an immediate sustained rate of oxygen evolution in the absence of KCN but not with KCN present. Simultaneous measurements of chlorophyll a fluorescence showed that qQ remained oxidized, although net O2 evolution had ceased. As O2 evolution decreased, qE and delta pH increased. Upon the addition of the NaHCO3, QA became more oxidized while delta pH and qE were decreased, suggesting that the inhibition of electron transport at high glycerate 3-phosphate concentrations was mediated by photosynthetic control via delta pH. However, the levels of ATP, ADP, ribulose 1,5-bisphosphate, and Pi concentrations and ATP/ADP ratio. The stromal glycerate 3-phosphate content declined upon illumination until O2 evolution ceased. At this time a constant stromal glycerate 3-phosphate concentration of 8-10 mM was maintained while net import of glycerate 3-phosphate into the stroma had virtually ceased. The stromal triosephosphate content remained at a constant low level throughout but the glycerate 3-phosphate level increased slightly after addition of NaHCO3. The data provided by the measurements of thylakoid reactions and stromal metabolites suggest that photosynthetic electron transport is tightly coupled to the requirements of the stroma for ATP and NADPH. Glycerate 3-phosphate reduction requires much less ATP than the operation of the complete Benson-Calvin cycle since the stoichiometry of ATP and NADPH utilization is reduced to 1:1. We conclude that thylakoid electron flow is not sufficiently flexible to maintain NADPH and ATP production in the ratio of 1:1. This situation will favor overenergization of the thylakoid membrane, increased leakiness of protons, increased electron drainage to O2, and result in progressive inhibition of noncyclic electron flow.

Antimycin A↗
PubMed1989

Enhanced cis-platinum ototoxicity in children with brain tumours who have received simultaneous or prior cranial irradiation.

We report on four children who received cis-platinum simultaneously with, or in one case 10 months after, cranial irradiation and experienced exaggerated ototoxicity affecting all audible frequencies. The hearing loss was severe, affecting the critical areas for speech perception, and necessitated the provision of bilateral hearing aids. The audiograms of these patients are shown and compared to those of four children who had received cis-platinum as part of their treatment for neuroblastoma but without cranial irradiation. The precipitation of the exaggerated hearing loss with the administration of cis-platinum in one patient 10 months after finishing cranial irradiation suggests that care should be taken in the timing of cis-platinum administration in relation to concurrent or previous cranial irradiation.

Adolescent↗
PubMed1988

Alterations in nasal respiration and nasal airway size following superior repositioning of the maxilla.

Twenty patients who underwent superior repositioning of the maxilla via Le Fort I down fracture had their respiratory mode and nasal cross-sectional area determined prior to and 6 months following surgery. Inductive plethysmography and nasal air flow techniques were used in the determination of these parameters. Prior to surgery, five patients were nasal breathers, five were predominantly nasal breathers, six were oral-nasal breathers, and four were predominantly oral breathers. Nine patients had inadequate nasal airways. Six months following surgery, 14 patients were nasal breathers and six were predominantly nasal breathers. Sixteen patients had adequate nasal airways, three had borderline nasal airways and one had an inadequate nasal airway postsurgically. These findings suggest that superior repositioning of the maxilla by Le Fort I down fracture does not adversely affect nasal respiration. Nasal function actually improved in 17 of the 20 subjects studied.

Adolescent↗
PubMed1988

Regulation of ribulose-1,5-bisphosphate carboxylase activity by the activase system in lysed spinach chloroplasts.

Ribulose-1,5-bisphosphate (RuBP) carboxylase in lysed spinach (Spinacia oleracea L. cv virtuosa) chloroplasts that had been partly inactivated at low CO(2) and Mg(2+) by incubating in darkness with 4 millimolar partially purified RuBP was reactivated by light. If purified RuBP was used to inhibit dark activation of the enzyme, reactivation by light was not observed unless fructose-1,6-bisphosphate, ATP, or ADP plus inorganic phosphate were also added. Presumably, ADP plus inorganic phosphate acted as an ATP-generating system with a requirement for the generation of DeltapH across the thylakoid membrane. When the RuBP obtained from Sigma Chemical Co. was used, light did not reactivate the enzyme. There was no direct correlation between DeltapH and activation. Therefore, thylakoids are required in the ribulose-1,5-bisphosphate carboxylase activase system largely to synthesize ATP. Inactivation of RuBP carboxylase in isolated chloroplasts or in the lysed chloroplast system was not promoted simply by a transition from light to dark conditions but was caused by low CO(2) and Mg(2+).

Journal Article↗
PubMed1988

Relationship between Steady-State Fluorescence Yield and Photosynthetic Efficiency in Spinach Leaf Tissue.

The relationship between steady-state photosynthetic efficiency, as moles CO(2) per mole of incident visible photons under 2% O(2), and chlorophyll fluorescence quenching has been investigated in intact leaf tissue of Spinacia oleracia. Fluorescence yield was measured using a pulse amplitude modulation technique that permitted rapid and sensitive resolution and quantitation of photochemical and nonphotochemical quenching coefficients. A highly linear relationship was observed between photosynthetic efficiency and the ratio of photochemical:nonphotochemical quenching coefficients for values of the latter less than 1.6. This relationship applied whether irradiance or CO(2) concentration was varied. The observed relationships between photochemical yield and fluorescence yield were compatible with the photosystem II model proposed by Butler and Kitajima (1975 Biochim Biophys Acta 376: 116-125). The results are discussed with respect to the proposed role of nonphotochemical quenching in regulating radiant energy utilization and also the applicability of fluorescence measurements as a means of estimation of the rate of photosynthetic electron transport.

Journal Article↗
PubMed1988

Carbon Dioxide-Induced Oscillations in Fluorescence and Photosynthesis: Role of Thylakoid Membrane Energization in Regulation of Photosystem II Activity.

The response of CO(2) fixation to a sudden increase in ambient CO(2) concentration has been investigated in intact leaf tissue from spinach (Spinacia oleracea) using a dual channel infrared gas analyzer. Simultaneous with these measurements, changes in fluorescence emission associated with a weak, modulated measuring beam were recorded. Application of brief (2-3 seconds) dark intervals enabled estimation of the dark fluorescence level (F(o)) under both steady state and transient conditions. The degree of suppression of F(o) level fluorescence in the light was strongly correlated with nonphotochemical quenching under all conditions. During CO(2)-induced oscillations in photosynthesis under 2% O(2) the changes in nonphotochemical quenching anticipate changes in the rate of uptake of CO(2). At such low levels of O(2) and constant illumination, changes in the relative quantum efficiency of open photosystem II units were estimated as the ratio of the rate of CO(2) uptake and the photochemical quenching coefficient. Under the same conditions the relative quantum efficiency of photosystem II was found to vary inversely with the degree of nonphotochemical quenching. The relationship between changes in the rate of CO(2) uptake: photochemical quenching coefficient and nonphotochemical quenching was altered somewhat when the same experiment was conducted under 20% O(2). The results suggest that electron transport coupled to reduction of O(2) occurs to varying degrees with time during oscillations, especially when ambient O(2) concentrations are high.

Journal Article↗
PubMed1987

Photosynthesis by intact isolated chloroplasts on solid support.

A new approach to measurements of photosynthesis by isolated chloroplasts has been devised. Intact isolated chloroplasts were trapped in the cavities of membrane filters. The thin layers of chloroplasts so obtained were assayed for O(2) evolution and CO(2) assimilation in leaf-chambers. Photosynthetic gas exchange could be demonstrated to take place either in a closed or a flow-through system. The chloroplasts were morphologically intact as shown by light or scanning electron microscopy and displayed stable rates of photosynthesis in the presence of phosphate and alkaline phosphatase. The methods described open the way to in vitro measurement of photosynthesis, by chloroplasts under conditions more closely resembling those in leaves.

Journal Article↗
PubMed1987

Effect of lithium on the metabolic response to parathyroid hormone.

Blood and urine samples were collected at timed intervals for up to 120 min after the start of a 30-min infusion of 30 IU bovine parathyroid hormone (PTH) into 6 normal male subjects. Infusions were performed before and after 7 days' treatment with lithium carbonate. A highly significant increase in the maximum renal tubular reabsorption capacity for calcium (TmCa/GFR) from 2.02 +/- 0.04 to 2.17 +/- 0.07 mmol/l (p less than 0.02) produced a significant rise in plasma calcium. Lithium had no effect on basal fasting PTH or nephrogenous cyclic AMP (cAMP). Changes in nephrogenous cAMP and TmP/GFR in response to PTH were not altered by lithium. The absorption of 47Ca following an oral calcium load was increased in 5 out of 6 subjects also treated for 1 week with lithium. These results suggest that lithium has a direct effect on calcium transport, both at the level of the renal tubule and the gut, which is not mediated by stimulation of parathyroid activity or via modification of PTH-stimulated adenylate cyclase.

Adult↗