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D Walker

Publications and source records attributed to D Walker.

At least 91 records · Page 5Linked to original sources
PubMed1998

The first international standard for serum amyloid A protein (SAA). Evaluation in an international collaborative study.

An ampouled preparation of acute phase serum rich in serum amyloid A protein (SAA) was evaluated in seven laboratories in six countries for its suitability to serve as the international standard for immunoassay of SAA. A variety of different immunoassays were used. On the basis of the results reported here and with the authorization of the Expert Committee on Biological Standardization of the World Health Organization (WHO) this preparation (coded 92/680) was established as the first international standard of SAA.

Enzyme-Linked Immunosorbent Assay↗
PubMed1998

Expression of pyruvate dehydrogenase isoforms during the aerobic/anaerobic transition in the development of the parasitic nematode Ascaris suum: altered stoichiometry of phosphorylation/inactivation.

The pyruvate dehydrogenase complex (PDC) plays a key role in the anaerobic metabolism of the parasitic nematode Ascaris suum. Two isoforms of the alpha-subunit of pyruvate dehydrogenase (E1) have been identified: alpha I is most abundant in anaerobic adult muscle and alpha II in aerobic larvae. Both isoforms have been expressed as alpha 2 beta 2 tetramers with a muscle-specific beta-subunit, purified to apparent homogeneity, reconstituted with E1-deficient adult A. suum muscle PDC, and assayed for PDC and E1 kinase activity. Recombinant alpha II is a poor substrate for the adult E1 kinase, but its stoichiometry of phosphorylation/inactivation is similar to that reported for the human E1. Initially, inactivation parallels the incorporation of about 1 mol 32P/mol E1 and at maximal phosphorylation about 2.4 32P/mol E1 is incorporated. In contrast, recombinant alpha I (r alpha I) is phosphorylated rapidly, and substantially more phosphorylation accompanies inactivation. To examine this altered pattern of phosphorylation, the two phosphorylation sites in each E1 alpha subunit of the r alpha I (site 1 and site 2) were changed either individually or together from Ser to Ala by site-directed mutagenesis. Site 1 was phosphorylated more rapidly than site 2, but the phosphorylation of either site resulted in inactivation, and the phosphorylation of only a single E1 alpha subunit of the tetramer was necessary for inactivation. However, both E1 alpha subunits of the tetramer were phosphorylated, based on the incorporation of about 3.5 mol 32P/mol E1 at maximal phosphorylation and the altered mobility of most of the E1 alpha subunits during SDS-PAGE. These observations suggest that the regulation of both E1 isoforms is modified to maintain PDC activity during the transition to anaerobiosis.

Aerobiosis↗
PubMed1998

Pleistocene phylogeographic effects on avian populations and the speciation process.

Pleistocene biogeographic events have traditionally been ascribed a major role in promoting speciations and in sculpting the present-day diversity and distributions of vertebrate taxa. However, this paradigm has recently come under challenge from a review of interspecific mtDNA genetic distances in birds: most sister-species separations dated to the Pliocene. Here we summarize the literature on intraspecific mtDNA phylogeographic patterns in birds and reinterpret the molecular evidence bearing on Pleistocene influences. At least 37 of the 63 avian species surveyed (59%) are sundered into recognizable phylogeographic units, and 28 of these separations (76%) trace to the Pleistocene. Furthermore, use of phylogroup separation times within species as minimum estimates of 'speciation durations' also indicates that many protracted speciations, considered individually, probably extended through time from Pliocene origins to Pleistocene completions. When avian speciation is viewed properly as an extended temporal process rather than as a point event, Pleistocene conditions appear to have played an active role both in initiating major phylogeographic separations within species, and in completing speciations that had been inaugurated earlier. Whether the Pleistocene was exceptional in these regards compared with other geological times remains to be determined.

Animals↗
PubMed1998

Substrate upregulation of the human small intestinal peptide transporter, hPepT1.

1. Molecular mechanisms underlying physiological adaptation to increased levels of dietary peptides have been elucidated by studying the response to the substrate glycyl-L-glutamine (Gly-Gln) of the proton-coupled peptide transporter, hPepT1, in the Caco-2 human intestinal cell line. Vmax for apical uptake of [14C]glycyl-[14C]sarcosine was increased 1.64 (+/- 0.34)-fold after incubation of Caco-2 cells for 3 days in a peptide-rich medium (4 mM Gly-Gln replacing 4 mM Gln). 2. A full-length Caco-2 hPepT1 cDNA clone was identical to human small intestinal hPepT1 with the exception of a single amino acid substitution Ile-662 to Val. Transcript sizes, on Northern blots of Caco-2 poly(A)+ RNA probed with a 630 bp 5' hPepT1 cDNA probe, correspond to the reported band pattern seen with human small intestinal RNA. The dipeptide-induced increase in substrate transport was accompanied by a parallel increase of 1.92 (+/- 0.30)-fold (n = 9) in hPepT1 mRNA. This was in part due to an increase in hPepT1 mRNA half-life from 8.9 +/- 1.1 to 12.5 +/- 1.6 h (n = 3), but the increase in half-life does not account fully for the observed increase in mRNA levels, suggesting that there was also a dipeptide-mediated increase in hPepT1 transcription. 3. Anti-hPepT1-specific antipeptide antibodies localized hPepT1 exclusively to the apical membrane of human small intestinal enterocytes and Caco-2 cells. Gly-Gln supplementation of media resulted in a 1.72 (+/- 0.26)-fold (n = 5) increase in staining intensity of Caco-2 cells. 4. We conclude that Caco-2 cells provide an appropriate model for the study of adaptation of intestinal hPepT1, at the molecular level, to increased levels of dietary peptide. The magnitude of functional increase in apical peptide transport activity in response to Gly-Gln can be fully accounted for by the increased levels of hPepT1 protein and mRNA, the latter mediated by both enhanced hPepT1 mRNA stability and increased transcription. The signalling pathway between increased dietary peptide and hPepT1 upregulation, therefore, involves direct action on the enterocyte, independent of hormonal and/or neural control.

Adenocarcinoma↗
PubMed1998

A beta 4 isoform-specific interaction site in the carboxyl-terminal region of the voltage-dependent Ca2+ channel alpha 1A subunit.

The voltage-gated calcium channel beta subunit is a cytoplasmic protein that stimulates activity of the channel-forming subunit, alpha 1, in several ways. Complementary binding sites on alpha 1 and beta have been identified that are highly conserved among isoforms of the two subunits, but this interaction alone does not account for all of the functional effects of the beta subunit. We describe here the characterization in vitro of a second interaction, involving the carboxyl-terminal cytoplasmic domain of alpha 1A and showing specificity for the beta 4 (and to a lesser extent beta 2a) isoform. A deletion and chimera approach showed that the carboxyl-terminal region of beta 4, poorly conserved between beta isoforms, contains the interaction site and plays a role in the regulation of channel inactivation kinetics. This is the first demonstration of a molecular basis for the specificity of functional effects seen for different combinations of these two channel components.

Animals↗
PubMed1998

Subunit interaction sites in voltage-dependent Ca2+ channels: role in channel function.

Voltage-dependent Ca2+ channels are heteromeric complexes found in the plasma membrane of virtually all cell types and show a high level of electrophysiological and pharmacological diversity. Associated with the pore-forming alpha 1 subunit are the membrane anchored, largely extracellular alpha2-delta, the cytoplasmic beta and sometimes a transmembrane gamma subunit; these subunits dramatically influence the properties and surface expression of these channels. Effects vary depending on subunit isoforms, suggesting that functional diversity of native channels reflects heterogeneity of combinations. Interaction sites between subunits have been identified and advances have been made in our understanding of the molecular basis of functional effects of the auxiliary subunits, their capacity to be regulated by G proteins, and their interaction with related cellular systems.

Amino Acid Sequence↗
PubMed1998

Triptorelin acetate administration in early pregnancy: case reports and review of the literature.

When using a long protocol with cycle day 23 gonadotrophin-releasing hormone agonists (GnRH-a) administration, an elevated estradiol level or a missed period 10-14 days after initiating pituitary downregulation should alert the physician to the possibility of a pregnancy. We report 4 pregnancies occurring during pituitary downregulation with Triptorelin acetate in 366 in-vitro fertilization (IVF) cycles resulting in 3 deliveries of 4 normal neonates at term and 1 first trimester abortion. This supports published data reporting a 1% spontaneous pregnancy incidence in women undergoing pituitary desensitization GnRH-a during the luteal phase prior to planned IVF treatment, a 15.9% abortion rate and a 1.7% malformation rate. Our cases together with other published data suggest that pregnancy outcome is not adversely affected by GnRH-a administration during the luteal phase of the conception cycle. However, long term follow-up of these babies is still lacking and the number of reported cases is too small adequately to rule out the possibility of any detrimental effect related GnRH-a administration in pregnancy.

Abortion, Spontaneous↗
PubMed1998

Intranuclear trafficking and gene targeting by members of the steroid/nuclear receptor superfamily.

Upon binding to regulatory elements in mammalian chromosomes, steroid receptors induce specific transitions in the nucleoprotein structure of the template. These transitions reflect, in part, the reorganization of chromatin structure to permit interaction of secondary factors with target sequences in promoter regulatory regions. Steroid receptors represent a class of transcriptional activators that are able to interact with repressed nucleoprotein templates and recruit necessary activities for chromatin remodeling. The ligand-induced movement of nuclear receptors from inactive states, either in the cytoplasm or in the nucleus, to productive interactions with chromatin is complex and likely reflects the interaction with multiple protein complexes and subcellular structures. Regulation of gene expression by nuclear receptors is thus mediated through the subcellular distribution of inactive receptors, the redistribution of activated receptor complexes to appropriate nuclear domains, the reorganization of chromatin structures for interaction with soluble components of the nucleoplasm, and direct protein-protein contacts between receptors and the basal transcription apparatus.

Animals↗
PubMed1998

Species differences in size discrimination in the paracellular pathway reflected by oral bioavailability of poly(ethylene glycol) and D-peptides.

Animal models are frequently used to aid prediction of intestinal absorption in humans. However, there is little comparative quantitative information on species differences in paracellular permeation, which is an important route for oral absorption of small to medium-sized hydrophilic drug molecules. This study addresses this issue by comparing the molecular mass (MM) dependency in oral bioavailability between rat and dog of poly(ethylene glycol) (PEG), a polydispersed model mixture commonly used to characterize paracellular absorption, and of a series of eight D-peptides (based on D-phenylalanine). Fasted rats and dogs received PEG (400/900) and the D-peptides (MM 236-406 Da), orally and intravenously, with total 24-48 h urine collection to estimate oral bioavailability. After HPLC separation, the individual PEG oligomers and D-peptides were determined using radiometric detection, for radiolabeled material, and LC-MS, for unlabeled (PEG) material. All compounds were predominantly excreted unchanged following intravenous administration. After oral administration, the predominant peak in the radiochromatogram was unchanged material, indicating stability of the compounds in the gastrointestinal tract. A clear molecular mass dependency in oral bioavailability was seen with both series, but with absorption much greater in dog than rat. Thus, for PEG in rat, bioavailability decreased sharply from 79 to around 2% with increasing MM between 282 and 591 Da, and then tapered to around 1. 5% up to 1295 Da. Whereas in dog, bioavailability remained around 100% for oligomers up to 600 Da and then decreased quite sharply with increasing MM, tending to plateau around 13% beyond 900 Da. Likewise, for the d-peptides in rat, bioavailability decreased from 30 to 1% with increasing MM between 236 and 406 Da, whereas in dog it was 100%, declining to 16% over the same molecular range. This species difference appears to be due to a larger pore size and greater frequency of pores in the paracellular pathway of dog compared to rat. Furthermore, on the basis of comparison with literature data for PEG and selected drugs, rat would appear to be a better predictor than dog of absorption of hydrophilic compounds in human.

Absorption↗
PubMed1998

Phase I study of temozolomide in paediatric patients with advanced cancer. United Kingdom Children's Cancer Study Group.

A phase I study of temozolomide administered orally once a day, on 5 consecutive days, between 500 and 1200 mg m(-2) per 28-day cycle was performed. Children were stratified according to prior craniospinal irradiation or nitrosourea therapy. Sixteen of 20 patients who had not received prior craniospinal irradiation or nitrosourea therapy were evaluable. Myelosuppression was dose limiting, with Common Toxicity Criteria (CTC) grade 4 thrombocytopenia occurring in one of six patients receiving 1000 mg m(-2) per cycle, and two of four patients treated at 1200 mg m(-2) per cycle. Therefore, the maximum-tolerated dose (MTD) was 1000 mg m(-2) per cycle. The MTD was not defined for children with prior craniospinal irradiation because of poor recruitment. Plasma pharmacokinetic analyses showed temozolomide to be rapidly absorbed and eliminated, with linear increases in peak plasma concentrations and systemic exposure with increasing dose. Responses (CR and PR) were seen in two out of five patients with high-grade astrocytomas, and one patient had stable disease. One of ten patients with diffuse intrinsic brain stem glioma achieved a long-term partial response, and a further two patients had stable disease. Therefore, the dose recommended for phase II studies in patients who have not received prior craniospinal irradiation or nitrosoureas is 1000 mg m(-2) per cycle. Further evaluation in diffuse intrinsic brain stem gliomas and other high-grade astrocytomas is warranted.

Adolescent↗
PubMed1998

Efficacy of CI-1020, an endothelin-A receptor antagonist, in hypoxic pulmonary hypertension.

We previously showed that CI-1020, an endothelin (ET)-A-selective receptor antagonist, dose-dependently blocked acute hypoxic pulmonary hypertension (PH) in rats. In this study we show that CI-1020 can reverse existing PH and prevent progression of right ventricular hypertrophy (RVH) in rats exposed to chronic hypoxia. Male Sprague-Dawley rats were exposed to 20 days of hypoxia (10% O2) with CI-1020 treatment (20 or 40 mg/kg/day) starting on day 10. On day 20 of hypoxia, the rats were instrumented under anesthesia with a pulmonary artery cannula and allowed to recover to consciousness before measurement of mean pulmonary arterial pressure (MPAP). Blood samples were then collected for plasma ET-1 measurements, the rats killed, and their hearts dissected, dried, and weighed. RV/LV + septum ratio (g/g) was used as an index of RVH (RVHi). Normoxic rats and rats exposed to hypoxia for only 10 days were also evaluated as controls. Normoxic rats had MPAPs of 13 +/- 1 mm Hg, plasma ET-1 levels of 2.1 +/- 0.1 pg/ml, and an average RVHi of 0.29 +/- 0.03. Rats exposed to 10 or 20 days of hypoxia had MPAPs of 33 +/- 2 and 44 +/- 0 mm Hg, plasma ET-1 levels of 4.2 +/- 0.8 and 4.6 +/- 0.8 pg/ml, and average RVHis of 0.47 +/- 0.05 and 0.52 +/- 0.03, respectively. In comparison, rats treated with CI-1020 had MPAPs that were 37% (20 mg/kg/day) and 44% (40 mg/kg/day) lower than untreated 20-day hypoxic rats. Furthermore, rats dosed with 40 mg/kg/day of CI-1020 had MPAPs that were significantly lower (24%) than control 10-day hypoxic rats, indicating a significant reversal of PH. Along with this reversal in PH, their average RVHi was 23% lower (p < 0.05) relative to untreated 20-day hypoxic rats.

Animals↗
PubMed1998

Rickettsia honei sp. nov., the aetiological agent of Flinders Island spotted fever in Australia.

The name Rickettsia honei, strain RBT, has been proposed for a unique spotted fever group (SFG) agent which is pathogenic for humans. This agent has previously been compared to the other SFG agents and was shown to be distinct in protein structure by SDS-PAGE and by immunoblotting. Genetic comparisons of the 16S rRNA, rompA, gltA and the 17 kDa antigen genes with the other SFG rickettsiae confirmed the phylogenetic distance between R. honei and the previously described species. Genetically, Rickettsia honei is more closely related to the Thai tick typhus (TT-118) rickettsia than to any other member of the SFG. Indeed, it is proposed that TT-118 is a strain of R. honei which was previously isolated in Thailand. These results elucidate the presence of a unique SFG rickettsial species in Australasia.

Antigens, Bacterial↗
PubMed1998

Improving musculoskeletal clinical skills teaching. A regionwide audit and intervention study.

OBJECTIVE: To identify factors that influence medical students' perceptions of the quality of a clinical skills course; to apply these factors to the course at one hospital; to measure the effect of this change. DESIGN: Cross sectional questionnaire survey; application of identified factors; repeat questionnaire survey. SETTING: Three teaching hospitals and five district general hospitals in north east England. SUBJECTS: Third year medical students attending locomotor clinical skills courses in two consecutive years. MAIN OUTCOME MEASURES: Score awarded by students in five categories; numbers of patients seen by each student; comparisons with other clinical skills weeks. RESULTS: Response rates were 71 of 150 and 89 of 161. Factors associated with a high awarded score were: organisation of the course by a rheumatologist (p < 0.01); teaching from a rheumatologist (p < 0.01); higher number of patients seen (r = 0.76). Mean number of patients seen varied widely, from 7 per student at one hospital to 20.4 at another. Teaching hospitals scored poorly. In the second year, after making changes at one teaching hospital the mean total score improved (p < 0.01), and students saw more patients (p < 0.01). The ranking of this hospital rose from 6 to 1. The additional cost of the modified course was 640 pounds per student. CONCLUSIONS: The standard of teaching of locomotor clinical skills varies widely and can be improved by application of factors identified in this survey, although additional costs are incurred.

Clinical Competence↗
PubMed1998

A three-decade perspective on community and public psychiatry training in Oregon.

The public psychiatry training program at Oregon Health Sciences University, established in 1973, educates psychiatric residents to work in community mental health centers and state hospitals. The authors present a brief history of this program, which spans three decades, and describe recent developments in its operation, with special attention to financing, administrative structure, and educational elements. Several program graduates have chosen careers in public-sector work. The program is founded on the principle that just as dollars should follow patients in health care systems, so should residents in training follow patients. Administrative and fiscal arrangements must be flexible to support this mobility.

Career Choice↗