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R Howden

Publications and source records attributed to R Howden.

At least 19 recordsLinked to original sources
PubMed2001

The reproducibility of tolerance to lower-body negative pressure and its quantification.

The reproducibility of tolerance to lower-body negative pressure (LBNP) has not been assessed sufficiently. Furthermore, there has been confusion concerning the most appropriate index by which LBNP tolerance can be quantified. The purpose of this study was to assess the degree of reproducibility in presyncopal-symptom-limited LBNP (LBNPtol), using an LBNP chamber. Twenty physically active subjects [median age (range) 21 (18-27) years] underwent three successive LBNPtol tests with 72-120 h between each test. LBNPtol was quantified using the LBNP tolerance index (LTI; delta mmHg.min), cumulative stress index (CSI; mmHg.min), duration of negative pressure (DNP) and maximum magnitude of negative pressure (MNP). Heart rate (fc), systolic (SBP) and diastolic (DBP) blood pressures from the three repeated tests were compared during a control period. The changes from control to maximum response (fc, SBP, DBP) during LBNP were also compared, and percentage changes in estimated blood volume were measured. There were no statistical differences between any of these comparisons (P > 0.05). LTI and CSI were greater in the third test when compared to the first two tests (P < 0.05). The values for DNP and MNP were not statistically different between tests (P > 0.05). Measures of LTI and CSI showed an acceptable level of reproducibility for the first two repeated tests. However, there was an increase in LBNPtol on the third successive exposure to LBNP. These findings have shown that it is possible to achieve reproducible measures of tolerance to LBNP when using a custom-built chamber. This only applies to a test-retest procedure. Furthermore, these data also suggest that DNP and MNP do not adequately reflect the differences shown in LBNP tolerance when using LTI and CSI as measures.

Adult↗
PubMed1999

Phytochelatin synthase genes from Arabidopsis and the yeast Schizosaccharomyces pombe.

Phytochelatins (PCs), a family of heavy metal-inducible peptides important in the detoxification of heavy metals, have been identified in plants and some microorganisms, including Schizosaccharomyces pombe, but not in animals. PCs are synthesized enzymatically from glutathione (GSH) by PC synthase in the presence of heavy metal ions. In Arabidopsis, the CAD1 gene, identified by using Cd-sensitive, PC-deficient cad1 mutants, has been proposed to encode PC synthase. Using a positional cloning strategy, we have isolated the CAD1 gene. Database searches identified a homologous gene in S. pombe, and a mutant with a targeted deletion of this gene was also Cd sensitive and PC deficient. Extracts of Escherichia coli cells expressing a CAD1 cDNA or the S. pombe gene catalyzing GSH-dependent, heavy metal-activated synthesis of PCs in vitro demonstrated that both genes encode PC synthase activity. Both enzymes were activated by a range of metal ions. In contrast, reverse transcription-polymerase chain reaction experiments showed that expression of the CAD1 mRNA is not influenced by the presence of Cd. A comparison of the two predicted amino acid sequences revealed a highly conserved N-terminal region, which is presumed to be the catalytic domain, and a variable C-terminal region containing multiple Cys residues, which is proposed to be involved in activation of the enzyme by metal ions. Interestingly, a similar gene was identified in the nematode, Caenorhabditis elegans, suggesting that PCs may also be expressed in some animal species.

Amino Acid Sequence↗
PubMed1998

The glutathione-deficient, cadmium-sensitive mutant, cad2-1, of Arabidopsis thaliana is deficient in gamma-glutamylcysteine synthetase.

This paper reports that the glutathione (GSH)-deficient mutant, cad2-1, of Arabidopsis is deficient in the first enzyme in the pathway of GSH biosynthesis, gamma-glutamylcysteine synthetase (GCS). The mutant accumulates a substrate of GCS, cysteine, and is deficient in the product, gamma-glutamylcysteine. In vitro enzyme assays showed that the cad2-1 mutant has 40% of wild-type levels of GCS activity but is unchanged in the activity of the second enzyme in the pathway, GSH synthetase. The CAD2 locus maps to chromosome 4 and is tightly linked to a gene, GSHA, identified by a previously isolated cDNA. A genomic clone of GSHA complements both the phenotypic and biochemical deficiencies of the cad2-1 mutant. The nucleotide sequence of the gene has been determined and, in the mutant, this gene contains a 6 bp deletion within an exon. These data demonstrate that the CAD2 gene encodes GCS. The cad2-1 mutation is close to the conserved cysteine which is believed to bind the substrate glutamate and the specific inhibitor L-buthionine-[S,R] sulfoximine (BSO). Both root growth and GCS activity of the cad2-1 mutant was less sensitive than the wild-type to inhibition by BSO, indicating that the mutation may alter the affinity of the inhibitor binding site.

Alcohol Oxidoreductases↗
PubMed1998

Selection of T-DNA-tagged male and female gametophytic mutants by segregation distortion in Arabidopsis.

As a strategy for the identification of T-DNA-tagged gametophytic mutants, we have used T-DNA insertional mutagenesis based on screening for distorted segregation ratios by antibiotic selection. Screening of approximately 1000 transgenic Arabidopsis families led to the isolation of eight lines showing reproducible segregation ratios of approximately 1:1, suggesting that these lines are putative gametophytic mutants caused by T-DNA insertion at a single locus. Genetic analysis of T-DNA transmission through reciprocal backcrosses with wild type showed severe reductions in genetic transmission of the T-DNA through the male and/or female gametes. Direct evidence for mutant phenotypes in these lines was investigated by DAPI staining of mature pollen grains and by the analysis of seed set and embryo sac morphology in cleared ovules. One line, termed limpet pollen, showed a novel pollen phenotype in that the generative cell failed to migrate inward after pollen mitosis I, such that the generative or sperm cells remained against the pollen wall. Two other lines, andarta and tistrya, were defective in female transmission and showed an early arrest of embryo sac development with the viable megaspore not initiating the nuclear division cycles. These data demonstrate the efficacy of a segregation ratio distortion strategy for the identification of T-DNA-tagged gametophytic mutants in Arabidopsis.

Arabidopsis↗
PubMed1998

The Arabidopsis thaliana gametophytic mutation gemini pollen1 disrupts microspore polarity, division asymmetry and pollen cell fate.

Pollen development and male gametogenesis are critically dependent upon cell polarization leading to a highly asymmetric cell division termed pollen mitosis I. A mutational approach was adopted in Arabidopsis thaliana to identify genes involved these processes. Four independent gemini pollen mutants were isolated which produce divided or twin-celled pollen. The gemini pollen1 mutant was characterized in detail and shown to act gametophytically resulting in reduced transmission through both sexes. gemini pollen1 showed an incompletely penetrant phenotype resulting in equal, unequal and partial divisions at pollen mitosis I. The division planes in gemini pollen1 were shown to be aligned with the polar axis (as in wild type) and evidence was obtained for incomplete nuclear migration, which could account for altered division symmetry. gemini pollen1 also showed division phenotypes consistent with spatial uncoupling of karyokinesis and cytokinesis suggesting that GEMINI POLLEN1 may be required for the localization of phragmoplast activity. Cell fate studies showed that in both equal and unequal divisions a vegetative cell marker gene was activated in both daughter cells. Daughter cells with a range of intermediate or hybrid vegetative/generative cell fates suggests that cell fate is quantitatively related to cell size. The potential mode of action of GEMINI POLLEN1 and its effects on cell fate are discussed in relation to proposed models of microspore polarity and cell fate determination.

Arabidopsis↗
PubMed1995

Cadmium-sensitive, cad1 mutants of Arabidopsis thaliana are phytochelatin deficient.

An allelic series of cad1, cadmium-sensitive mutants of Arabidopsis thaliana, was isolated. These mutants were sensitive to cadmium to different extents and were deficient in their ability to form cadmium-peptide complexes as detected by gel-filtration chromatography. Each mutant was deficient in its ability to accumulate phytochelatins (PCs) as detected by high-performance liquid chromatography and the amount of PCs accumulated by each mutant correlated with its degree of sensitivity to cadmium. The mutants had wild-type levels of glutathione, the substrate for PC biosynthesis, and in vitro assays demonstrated that each of the mutants was deficient in PC synthase activity. These results demonstrate conclusively the importance of PCs for cadmium tolerance in plants.

Alleles↗
PubMed1995

A cadmium-sensitive, glutathione-deficient mutant of Arabidopsis thaliana.

The roots of the cadmium-sensitive mutant of Arabidopsis thaliana, cad1-1, become brown in the presence of cadmium. A new cadmium-sensitive mutant affected at a second locus, cad2, has been identified using this phenotype. Genetic analysis has grown that the sensitive phenotype is recessive to the wild type and segregates as a single Mendelian locus. Assays of cadmium accumulation by intact plants indicated that the mutant is deficient in its ability to sequester cadmium. Undifferentiated callus tissue was also cadmium sensitive, suggesting that the mutant phenotype is expressed at the cellular level. The level of cadmium-binding complexes formed in vivo was decreased compared with the wild type and accumulation of phytochelatins was about 10% of that in the wild type. The level of glutathione, the substrate for phytochelatin biosynthesis, in tissues of the mutant was decreased to about 15 to 30% of that in the wild type. Thus, the deficiency in phytochelatin biosynthesis can be explained by a deficiency in glutathione.

Arabidopsis↗
PubMed1992

Cadmium-Sensitive Mutants of Arabidopsis thaliana.

A screening procedure for identifying Cd-sensitive mutants of Arabidopsis thaliana is described. With this procedure, two Cd-sensitive mutants were isolated. These represent independent mutations in the same locus, referred to as CAD1. Genetic analysis has shown that the sensitive phenotype is recessive to the wild type and segregates as a single Mendelian locus. Crosses of the mutant to marker strains showed that the mutation is closely linked to the tt3 locus on chromosome 5. In addition to Cd, the mutants are also significantly more sensitive to mercuric ions and only slightly more sensitive to Cu and Zn, while being no more sensitive than the wild type to Mn, thus indicating a degree of specificity in the mechanism affected by the mutation. Undifferentiated callus tissue is also Cd sensitive, suggesting that the mutant phenotype is expressed at the cellular level. Both wild-type and mutant plants showed increased sensitivity to Cd in the presence of buthionine sulfoximine, an inhibitor of the biosynthesis of the cadmium-binding (gamma-glutamylcysteine)(n)-glycine peptides, suggesting that the mutant is still able to synthesize these peptides. However, the effects of a cad1 mutation and buthionine sulfoximine together on cadmium sensitivity are essentially nonadditive, indicating that they may affect different aspects of the same detoxification mechanism. Assays of Cd uptake by intact plants indicate that the mutant is deficient in its ability to sequester Cd.

Journal Article↗
PubMed1990

Bacterial reservoirs in cystic fibrosis.

To establish whether colonisation of the upper respiratory tract or bacterial contamination of inhaler devices or solutions predisposes to colonisation of the lower respiratory tract in patients with cystic fibrosis, bacterial isolates from groups of children who were positive (n = 13) or negative (n = 18) for Pseudomonas aeruginosa were studied. Cultures of swabs from inhaler devices, toothbrushes, and upper airways were compared with cough swabs or sputum cultures. No pathogens were obtained from inhaler equipment administering unit dose medications. Upper airway carriage of Staphylococcus aureus and Haemophilus influenzae was identified in both groups but correlated poorly with sputum isolates. P. aeruginosa was found only in the upper respiratory tract of children with established colonisation of the lower airways. No P aeruginosa isolates were obtained from the upper airways of the group with negative sputum, including one patient who became colonised by P aeruginosa during the study. Our results did not support the suggestion that colonisation of the upper respiratory tract by P aeruginosa predisposes to colonisation of the lower airways. Failure to isolate pathogenic organisms consistently from the upper airways in patients with positive sputum argues against a local epithelial factor predisposing to bacterial colonisation.

Adolescent↗
PubMed1983

In vitro colonisation of cerebrospinal fluid shunts.

A reliable method of colonizing cerebrospinal fluid shunts has been developed in vitro. A simulated cerebrospinal fluid is described in which the test organism (Staphylococcus epidermidis) multiplied. All experiments were conducted in a CO2 enriched atmosphere.

Carbon Dioxide↗
PubMed1983

Possible prevention and eradication of cerebrospinal fluid shunt infection with povidone iodine in vitro.

The results of preliminary in vitro studies of the efficacy of povidone iodine (P.V.I.) in preventing and eradicating infection in cerebrospinal fluid shunts are reported. After preliminary flushing of the system with P.V.I., it was not found possible to colonize valves. To eradicate infection from a previously colonised shunt it was found necessary to inject P.V.I. three times at 24 hourly intervals at a point above the level of colonisation.

Bacterial Infections↗
PubMed1976

Bacteriological survey of acute appendicitis in children.

A bacteriological study of 110 emergency appendicectomies is reported. In two-thirds of these the appendix was inflamed or gangrenous, and in 45 cases positive cultures were obtained from swabs taken at operation. Bacteroides were found frequently in these swabs and also in those taken from wound infections. Although this study is too small to draw any definite conclusions, it is felt that bacteroides should be considered an important pathogen in appendicitis and should be taken into account in the few ill patients where antibiotic treatment is contemplated. It was also noted that swabs taken from the surface of the appendix itself were more often positive than those from the peritoneal cavity, and this difference apperars to be significant.

Adolescent↗
PubMed1976

Use of anaerobic culture for the improved isolation of Streptococcus pneumoniae.

The following report compares the results of aerobic and anaerobic cultivation for the primary isolation of Streptococcus pneumoniae (the pneumococcus) from the respiratory tract of children. Of 414 specimens of respiratory tract secretions cultured, 65 (15-7%) yielded pneumococci; 31 (47-7%) grew both aerobically and anaerobically, but 34 (52-3%) strains were isolated only from the anaerobic culture. Pneumococci cultured anaerobically with added carbon dioxide characteristically produce large mucoid colonies which are more easily distinguished than the 'normal' colonies commonly seen in the mixed flora isolated from respiratory sites. This advantage justifies the inclusion of anaerobic culture when attempting to isolate Str. pneumoniae from clinical material.

Aerobiosis↗