PubMed Health⌕ Search

Biomedical subjects

A C Walker

Publications and source records attributed to A C Walker.

At least 37 records · Page 2Linked to original sources
PubMed1984

Contrast media reactions: experimental evidence against the allergy theory.

In order to clarify whether adverse reactions to radiographic contrast media may be allergic in nature, it is essential to show that contrast media can act as haptens and induce antibody formation in vivo. Experiments in Hooded Lister rats have been carried out using techniques specifically designed to produce favourable conditions for the production of antibodies. No evidence of any antibody formation was found in any of the experiments. These experiments suggest that radiographic contrast media in their native form do not act as haptens, contrary to previously published data, and it therefore seems unlikely that the majority of adverse reactions to radiographic contrast media are allergic in nature.

Animals↗
PubMed1983

Epidemic of acute poststreptococcal glomerulonephritis in aboriginal communities.

During 1980, an epidemic of acute glomerulonephritis occurred in a number of Aboriginal communities in the Northern Territory, affecting several hundred people. Clinical and laboratory studies of children admitted to the paediatric wards of Darwin Hospital, and of a larger number of individuals in the communities, have established that the nephritis was preceded by a streptococcal infection, and a recognised nephritogenic streptococcal strain was isolated from a number of children. The clinical course of the disease was mild in the majority of those affected and none had had serious complications. Serological results indicate the importance of measuring antideoxyribonuclease B levels as well as antistreptolysin O titres when seeking evidence of antecedent streptococcal infection.

Acute Disease↗
PubMed1983

Playing 'possum: a microwear experiment.

A recent experiment to evaluate the reliability of dental microwear as an indicator of diet seems to show that differences in diets fed to laboratory animals are not reflected by their tooth wear. We feel that these results are misleading, and reflect not so much the limits of microwear analysis per se, but rather result from the problems of design and execution inherent in any experimental simulation of natural feeding behavior. The experiment in question was flawed in several respects, and we think that changes in the methods and assumptions of the study are necessary before the experimental approach can yield meaningful results with which to test hypotheses about microwear analyses.

Animals↗
PubMed1979

Craniofacial growth in olive baboons (Papio cynocephalus anubis): browridge formation.

The examination of a developmental series of skulls of olive baboons has led to the generation of a model for browridge formation. Evidence in support of the model comes from surface configuration of browridge bone as studied by scanning electron microscopy, histology of the bone by the analysis of thin sections and by measurements that reflect both depositional cycles in browridge bone and the biomechanics of mastication. The model relates sudden changes in the biomechanical system caused by the tooth eruption cycle to cyclical changes in the formation of browridge bone. We suggest that browridge bone is added during periods of peak stress caused by the positive response of the masticatory muscles to relatively sudden reduction of their biomechanical efficiency.

Animals↗
PubMed1976

New hominids from East Turkana, Kenya.

Thirty-five new fossil hominid specimens are described. They were recovered from the Plio-Pleistocene sediments to the east of Lake Turkana (formerly Lake Rudolf). They include cranial and mandibular parts, teeth, and postcranial bones of upper and lower limbs. Parts of a single skeleton are also described. All of the specimens are described in anatomical detail and selected measurements are given. Some of the specimens are illustrated. It is proposed that they should be attributed to the family Hominidae, with genus and species undetermined until detailed comparative studies have been undertaken.

Animals↗
PubMed1976

Transcription-translation and translation-messenger RNA decay coupling: separate mechanisms for different messengers.

Antibiotics were used to inhibit protein synthesis at specific steps in the biosynthetic pathway. In this way, it was possible to study the coupling of protein synthesis to the accumulation of biologically active mRNA in T4-infected Escherichia coli. Functional mRNA for the phage enzymes deoxynucleotide kinase (EC 2.7.4.4; ATP: nucleoside monophosphate phosphotransferase or nucleosidemonophosphate kinase) and alpha-glucosyltransferase (EC 2.4.1.5; 1, 4-alpha-D-glucan: 1, 6-alpha-D-glucan 6-alpha-glucosyltransferase or dextrin dextranase) accumulated during inhibition of protein synthesis irrespective of the step in the synthesis of protein that was blocked. Under these conditions, however, the rate of mRNA synthesis for both enzymes was significantly inhibited. In contrast, the rate of degradation of these mRNAs was markedly dependent on the step in protein synthesis that was inhibited. That is, the site for mRNase action was different for each message. The most important step in protein synthesis required for the stability of deoxynucleotide kinase mRNA is the initiation step. A single ribosome bound to the 5' end of the deoxynucleotide kinase mRNA can stabilize the molecule. On the other hand, the initiation event does not seem to be important for stabilizing the alpha-glucosyltransferase mRNA. Instead, a high ribosome denisty on the alpha-glucosyltransferase messenger is required to achieve significant stability. Therefore, in studying messenger metabolism, it is important to focus on the functional stability of specific mRNAs instead of on total messenger since each mRNA can be metabolized differently.

Anti-Bacterial Agents↗
PubMed1975

New hominids from East Rudolf, Kenya, I.

Important new hominid fossils from the Plio-Pleistocene sediments of Lake Rudolf, Kenya, are described. They include: a partial, adult cranium with most of the calvaria and some of the facial skeleton preserved; a complete right femur; and bones of a single left lower limb--femur and upper and lower parts of the tibia and a small portion of fibula. Detailed comparative descriptions will be published later.

Animals↗
PubMed1975

Indirect suppression of radiation sensitivity of a recA- strain of Escherichia coli K12.

It has been shown previously that the radiation sensitivity of LexA strains of Escherichia coli K-12 can be suppressed by thermosensitive mutations (designated tsl) that are closely linked to the lexA locus. These are thought to be intragenic suppressors that reduce the activity of the diffusible product that gives rise to the LexA- phenotype (Mount et al., 1973). When a recA mutation is crossed into a suppressed tsl- strain, the extreme radiation sensitivity usually conferred by a recA mutation is considerably reduced without any detectable change in genetic recombination deficiency. Suppression of UV sensitivity depends upon the activity of the uvrA+ product. We propose that at least part of the radiation sensitivity of a recA- strain is due to a DNA repair defect that is different from inability to perform genetic exchanges and depends upon the presence of the lexA+ product. We hypothesize that the lexA+ product is a repressor of the synthesis of repair enzymes. In recA+ cells with DNA lesions, repressor is inactivated leading to enzyme induction but this does not occur in recA- cells. tsl mutations inactivate repressor leading to constitute enzyme synthesis and bypassing the need for recA+ product to inactivate the lexA+ product.

Cell Survival↗
PubMed1975

Effect of tsl mutations in decreasing radiation sensitivity of a recA- strain of Escherichia coli K-12.

It has been shown previously that the radiation sensitivity of lexA- strains of Escherichia coli K-12 can be suppressed by thermosensitive mutations (designated tsl) that are closely linked to the lexA locus and are thought to be intragenic suppressors of lexA mutations (Mount et al., 1973). When a recA mutation is crossed into a suppressed tsl- strain, the extreme radiation sensitivity usually conferred by a recA mutation is decreased, but there is no detectable change in genetic recombination deficiency. Increased resistance to UV in the tsl-reA-strains depends upon ability to synthesize active uvrA+ product.

Cell Division↗
PubMed1973

Suppression of lex mutations affecting deoxyribonucleic acid repair in Escherichia coli K-12 by closely linked thermosensitive mutations.

A major class of ultraviolet (UV)-resistant derivatives of lex(-) strains of Escherichia coli K-12 grows normally at 30 C but at 42.5 C fails to produce colonies on complete or minimal agar. At 42.5 C these thermosensitive strains form filaments without septa, due to an apparent defect in cell division. Deoxyribonucleic acid degradation in UV-irradiated cultures of the thermosensitive strains is slow, in contrast to the rapid degradation in UV-irradiated cultures of the parental lex(-) strains. The thermosensitive mutations (tsl) are tightly linked (less than 0.04 min on the E. coli K-12 linkage map) to the site of the lex mutation in the parental strain and could lie within the same gene. The tsl(+)/tsl(-) heterozygotes grow at 42.5 C and are UV resistant when grown at 30 or 42.5 C. The tsl mutations are, therefore, recessive in contrast to lex mutations, which are dominant. It appears likely that the tsl mutations alter the diffusible product that gives rise to the Lex(-) mutant phenotype. This product appears to be necessary for deoxyribonucleic acid repair and cell division.

Bacterial Proteins↗