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Biomedical subjects

B W Stewart

Publications and source records attributed to B W Stewart.

At least 73 records · Page 4Linked to original sources
PubMed1981

Generation and persistence of carcinogen-induced repair intermediates in rat liver DNA in vivo.

Chromatographic separation of native DNA from DNA containing single-stranded regions has been used to determine the relative concentrations of structural intermediates generated during chemically induced DNA repair. Single doses of each of ten compounds were administered to rats. After periods ranging from 90 min to 13 days, hepatic DNA was isolated and analyzed by stepwise elution from benzoylated diethylaminoethyl cellulose with 1.0 M NaCl followed by caffeine solution. The compounds used were benzo(a)pyrene, carbon tetrachloride, diethylnitrosamine, dimethylnitrosamine, ethyl methanesulfonate, galactosamine, N-hydroxy-2-acetylaminofluorene, methyl methanesulfonate, nitrosomorpholine, and beta-propiolactone. Doses of the various agents and/or treatment times were restricted such that hepatic necrosis did not occur. No increase in the amount of caffeine-eluted DNA occurred after administration of carbon tetrachloride or galactosamine. All the remaining chemicals caused a dose-dependent increase in the proportion of hepatic DNA eluted from benzoylated diethylaminoethyl cellulose with caffeine. In most cases, the varying times required to produce maximal increase in the proportion of caffeine-eluted DNA could be related to the rate of metabolism of the carcinogens. A distinction could be made according to whether repair intermediates were detected only within 24 hr of administration (ethyl methanesulfonate, methyl methanesulfonate, and beta-propiolactone) or were present for at least 3 days after treatment (diethylnitrosamine, dimethylnitrosamine, benzo(a)pyrene, N-hydroxy-2-acetylaminofluorene, and nitrosomorpholine). The data, considered with reference to previously ascribed modes of DNA repair for the respective adducts, suggest that base excision repair is immediately operative and rapidly completed in rat liver. However, reactions involved in the completion of nucleotide excision repair may be rate limiting, resulting in persistent structural damage to DNA. Implications of these findings for the use of benzoylated diethylaminoethyl cellulose chromatography as a carcinogen bioassay are considered.

Animals↗
PubMed1980

Reduced yield of pulmonary surfactant: patterns of response following administration of chemicals to rats by inhalation.

Repeated exposure of rats to trichloroethylene, carbon tetrachloride, gasoline vapour or to cigarette smoke is followed by significant reduction in the recovery of pulmonary surfactant, inhibition being evident as early as 5 days after commencement of treatment. The degree of reduction in surfactant recovery was dose-dependent and the kinetics of this reaction indicated the relative toxicities of the treatments. The results are discussed with reference to the use of surfactant recovery as an indicator of non-specific injury to respiratory tissue.

Animals↗
PubMed1980

Occupational cancers--a perspective.

Occupationally-caused tumours probably account for only a minor fraction of the total cancer burden in the community. However, circumstances dictate that such neoplasms provide singular opportunities firstly for further elucidation of the biology of chemically-induced cancer in humans, and secondly for prevention of additional cancer through the identification of hazardous work situations.

Carcinogens↗
PubMed1979

Structural defects in rat liver deoxyribonucleic acid. Endogenous single-strained regions in comparison with damage induced in vivo by a carcinogen.

Rat liver DNA may be separated into two fractions by stepwise elution from benzoylated-DEAE-cellulose with NaCl and caffeine solutions respectively. Other studies using bacterical and yeast DNA suggested that the first fraction contains native DNA, whereas the second may exhibit some degree of single-stranded character. In the present experiments, chromatography of DNA was monitored by labelling in vivo with [methyl-3H]thymidine in rats previously subjected to partial hepatectomy. In animals killed up to 1 h after thymidine injection, radioactivity eluted in the second fraction was inversely related to the incorporation time, being greatest when animals were killed 10 min after radioisotope injection. However, for most experiments, animals were allowed to survive 2-4 weeks after surgery before use, analysis being made on non-dividing DNA. Under these conditions, the proportion of caffeine-eluted DNA was decreased by subjecting the preparation to shear, before chromatography. A procedure that resulted in 12% of the recovered radioactivity being eluted with caffeine was adopted for experiments involving comparisons of the two DNA fractions. Under these conditions, cross-contamination could be detected by rechromatography, but this did not preclude distinction being made between the two fractions in terms of DNA structure. NaCl-eluted DNA did not bind to nitrocellulose filters. Caffeine-eluted DNA was retained by the filters and released by washing with 3mM-Tris/HCl,pH9.4. The fractions did not differ in terms of isopycnic centrifugation in CsCl. The NaCl-eluted fraction migrated as a single band in polyacrylamide gels, and this pattern was not modified by prior digestion with Neurospora crassa endonuclease. In contrast, caffeine-eluted DNA contained a minor component having a wide molecular-weight distribution and was subject to limited digestion by the endonuclease. The kinetics of denaturation of NaCi-eluted DNA in the presence of formaldehyde, in common with unfractionated DNA, were consistent with double-stranded structure. The same analysis of caffeine-eluted DNA revealed structural abnormality equivalent to two defects per 10000 base-pairs. The data are consistent with the minor fraction of rat liver DNA, separated by using benzoylated-DEAE-cellulose, containing regions of local denaturation. We previously showed that administration of the hepatocarcinogen dimethylnitrosamine is associated with an increase in the proportion of caffeine-eluted DNA. In terms of most analysis, differences between DNA fraction from nitrosamine-treated rats were similar to differences exhibited by preparations from control animals. However, structural analysis using denaturation kinetics indicated defects in both the NaCl- and caffeine-eluted DNA isolated from nitrosamine-treated rats. The two fractions differed from each other in that caffeine-eluted DNA exhibited a degree of structural damage far greater than that detected in any preparation from control animals...

Animals↗
PubMed1979

Correlation of biochemical and morphological changes induced by chemical injury to the lung.

Comparison has been made of injury to the rat pulmonary alveolar parenchyma evoked by intravenous injection of N-nitrosomethylurethane, intratracheal instillation of 3-methylcholanthrene or repeated inhalation for up to 15 days of carbon tetrachloride, trichloroethylene or gasoline vapour. Biochemical analyses, including assessment of rates of RNA and DNA synthesis and secretion of pulmonary surfactant, were correlated with morphological changes determined by electron microscopy. Single doses of N-nitrosomethylurethane or 3-methylcholanthrene inhibited incorporation of [14C] orotate into lung RNA 1--3 days after treatment. Daily exposure for 30 min to carbon tetrachloride or trichloroethylene vapour caused less marked reduction in orotate incorporation. Ultrastructural examination revealed that 3-methylcholanthrene toxicity was characterised by cytoplasmic change including disruption of surfactant lamellaie of Type 2 pneumocytes and variable degenerative changes Type 1 pneumocytes. Eight to ten days after treatment, the morphological evidence of hypertrophy/hyperplasia and transformation of Type 2 pneumocytes correlated well with biochemical evidence of stimulated incorporation of [3H]thymidine. Inhalation of carbon tetrachloride or trichloroethylene vapour produced milder responses including occasional degenerative changes in Type 1 pneumocytes, reduced numbers of surfactant lamellae in Type 2 pneumocytes and no change in [3H]thymidine incorporation. In contrast to the gradation of injury produced by the various chemicals, all procedures caused a marked and reproducible reduction in secretion of pulmonary surfactant as determined by endobronchial lavage. Following solvent inhalation, reduced recovery of surfactant was detected within 5 days of repeated exposure and thereafter no further change in this depressed level resulted from continued exposure for a further 10 days. The data are discussed in terms of a generalised pattern of response by pulmonary alveolar tissue to chemical injury and the apparent sensitivity of surfactant secretion as an indicator of damage to the lung.

Animals↗
PubMed1979

Repair and de novo replication of rat lung DNA following instillation of 3-methylcholanthrene.

Structural analysis of rat lung DNA was made by stepwise elution of the nucleic acid from benzoylated--DEAE-cellulose with 1.0 M NaCl and 30% formamide solutions respectively, the DNA content of collected fractions being determined by absorbance. The proportion of the minor fraction, which was eluted with formamide and thought to contain DNA having single-stranded regions, was increased by intratracheal instillation of 3-methylcholanthrene prior to death of the animal. Maximum initial increase was observed 14 h after treatment. Analysed up to 10 days after treatment, there was a biphasic response, the early maxima being dose dependent and the second apparently independent of the amount of hydrocarbon administered. The data implicate DNA repair processes and cellular proliferation stimulated by 3-methylcholanthrene as being involved in recovery of the lung from this potentially carcinogenic insult.

Animals↗
PubMed1979

Pulmonary responses to atmospheric pollutants. I: an ultrastructural study of fibrosing alveolitis evoked by petrol vapour.

Rats exposed to an atmosphere contaminated with petrol vapour at a concentration of 100 parts per million for up to 12 weeks exhibit a high incidence of electron microscopic changes in the lung parenchyma characterized by interstitial fibrosis with associated alveolar collapse. Initial changes appearing after 6 weeks include degeneration of endothelium and interstitial fibroblasts followed by hypertrophy of Type 2 pneumocytes. Subsequent degeneration of surfactant organelles of the hypertrophied Type 2 pneumocytes correlates with the appearance of focal alveolar collapse and associated interstitial fibrosis. Because of the rapidity with which lesions are induced in the rat lung, this experimental technique provides an economical and reproducible model for an integrated study of the sequential morphological and biochemical events preceding pulmonary fibrosis which might well lead to a better understanding of the enigmatic human syndrome of fibrosing alveolitis.

Animals↗
PubMed1979

Pulmonary responses to atmospheric pollutants. II. Effect of petrol vapour inhalation on secretion of pulmonary surfactant.

Inhalation of air contaminated with petrol vapour has been shown to produce reduced surfactant levels in the lungs of rats. Pulmonary surfactant was obtained by endobronchial lavage followed by salt extraction and freeze drying to obtain the dry, hydrophobic product. During 45 days of continuous exposure, the lowest yield of surfactant was obtained after 15 days of treatment. During the following 30 days of treatment, the surfactant yield reached a relatively constant level, approximately half the mean value for control animals. Chromatographic analysis indicated no qualitative alteration in the phospholipid components of surfactant with increasing times of exposure to the irritant. It has been possible to correlate biochemical evidence of toxic lung injury with signs of cellular damage obtained from ultrastructural studies.

Animals↗
PubMed1978

Fibrosing alveolitis (pulmonary interstitial fibrosis) evoked by experiemntal inhalation of Gasoline vapours.

Rats exposed to an atmosphere contaminated with gasoline vapour for 6--12 weeks exhibit progressive focal interstitial fibrosis of the lung, which is associated with irregular alveolar collapse. It is suggested that this experimental model of diffuse fibrosing lung disease constitutes a useful tool for the study of the dynamics of pulmonary reactivity to atmospheric pollutants and which may facilitate an understanding of the pathogenesis of fibrosing alveolitis in man.

Air Pollutants↗
PubMed1978

Environmental chemical carcinogenesis.

The methodology of revealing and studying chemical carcinogens in the environment is based on epidemiology, animal testing, and short-term laboratory studies. The techniques and limitations of these respective investigations are described. From evidence of carcinogenicity, extrapolation must be made to assess whether a particular substance is a cause of cancer in humans. This inference depends upon the type of evidence for carcinogenicity. In particular, parameters limiting extrapolation of all laboratory assays include species' specificity and dose response. Control of the environmental distribution of punative carcinogens poses difficulty, both in the selection of substances, and in legislative design. Certain personal habits still constitute the major established carcinogenic hazards for the community-at-large.

Animals↗
PubMed1978

Dimethylnitrosamine-induced structural damage to DNA results from repair of O6-methylguanine rather than repair of 7-methylguanine.

DNA isolated from the livers of rats treated with a non-necrotizing dose of [14C]dimethylnitrosamine was fractionated by chromatography of benzoylated-DEAE-cellulose. The preparations of native DNA and DNA containing single stranded regions were then hydrolysed and amounts of methylated guanine determined. Four hours after dimethylnitrosamine treatment there was no difference in the levels of 7-methylguanine and O6-methylguanine in the 2 DNA fractions. By 24 h, although there was no difference in the amount of 7-methylguanine between the DNA fractions, there was a 10-fold difference in the level of O6-methylguanine. The elimination of O6-methylguanine from the fraction of DNA containing single stranded regions is discussed in terms of differing repair processes initiated by 7-methylguanine and O6-methylguanine.

Animals↗
PubMed1978

Hydroxyurea-sensitive thymidine incorporation in rat liver and kidney following administration of dimethylnitrosamine.

In rats treated with dimethylnitrosamine (DMN), the sensitivity of [3H] thymidine incorporation to inhibition by hydroxyurea was determined in the liver and kidney. A 70% inhibition was produced in control animals with doses of hydroxyurea from 50 to 250 mg/kg body wt. Twelve hours after administration of DMN, hepatic thymidine incorporation was unaffected by 50 mg/kg hydroxyurea, progressive reduction in incorporation occurred as the dose of inhibitor was increased. A similar result was obtained in kidney only by dietary conditioning of rats to ensure a high renal metabolism of the DMN. In contrast, at 72 h after DMN treatment thymidine incorporation in both liver and kidney was rapidly inhibited by administration of 50 mg/kg hydroxyurea. The data, assessed together with various parameters of DMN-induced repair and replicative DNA synthesis, suggest the use of hydroxyurea as a means to study DNA repair in intact animals.

Animals↗
PubMed1978

Alterations in thermal stability of rat liver chromatin and DNA induced in vivo by dimethylnitrosamine and diethylnitrosamine.

A study was made of the effects of administration to rats of dimethylnitrosamine and diethylnitrosamine on the transition temperature (Tm) of sheared chromatin and DNA isolated from the liver. The analysis was made by thermal chromatography on hydroxylapatite with the use of DNA prelabeled with [3H]thymidine and following the elution pattern during the operation of a continuous temperature gradient. With a nonnecrogenic dose of dimethylnitrosamine (10 mg/kg), the alterations in chromatin were maximal at 24 hr and disappeared by 3 days. Greatest differences in elution profiles of chromatin after dimethylnitrosamine treatment were observed in the region above 80 degrees. Administration of the carcinogen caused a lowering of the "melting" curve in this region, the displacement from control position being proportional to the dose. The maximum dose (60 mg/kg) displaced the complete chromatin melting curve up to 5 degrees to the lower side. DNA isolated from this chromatin melted 3 degrees less than that from control rats. However, administration of lower doses of dimethylnitrosamine did not affect the melting profile of DNA. The administration of diethylnitrosamine caused a similar type of change. However, the modification was also seen at 50-60 degrees.

Animals↗
PubMed1977

Short-term carcinogen-induced changes in composition of rat liver chromatin protein.

Changes in the composition of rat liver chromatin protein were detected by electrophoretic analysis 4 h after a single injection of 10-30 mg/kg body weight dimethylnitrosamine. Non-histone bands were reduced in staining intensity and increases in all F2 histone subfractions were observed. The staining intensity of F1 and F3 histone was unchanged throughout the experiment. By 24 h after administration of a non-necrotizing dose of dimethylnitrosamine, the proportion of chromatin proteins, when compared to controls, was marked by an increase in non-histones and decreased amounts of F2 histone subfractions. Slight differences between treated and control were evident at 48 h, suggesting that at least this interval is required for recovery from in vivo methylation.

Animals↗
PubMed1977

Length of in vivo exposure to a carcinogenic dose of dimethylnitrosamine necessary for subsequent expression of morphological transformation by rat kidney cells in vitro.

Rat kidney cells isolated in vitro at intervals from one to 24 hours after the administration of a single, carcinogenic dose of dimethylnitrosamine (DMN) were characterized by the expression of morphological transformation and a prolonged life span. In cultures isolated four hours or more following the dose of DMN, morphological transformation was expressed at subculture five, but this effect was progressively delayed with earlier times of isolation. Relative to control kidney cell cultures, transformed cells exhibited enhanced proliferative properties evident from assays for plating efficiency and DNA synthesis. The capacity for increased cloning efficiency by the test cultures was acquired several subcultures after the expression of morphological transformation, while ability for colony formation in semi-solid media evolved later still. The results suggest that target cells permanently altered by the carcinogen are present one hour after the administration of DMN, but the effect is more significant by four hours.

Animals↗