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Biomedical subjects

G Chang

Publications and source records attributed to G Chang.

At least 91 records · Page 5Linked to original sources
PubMed1988

Use of an established human hepatoma cell line with endogenous bioactivation for gene mutation studies.

Genetic toxicology assays that rely on S9 microsomal mixes are subject to artifacts related to the generation of mutagenic metabolites by acidic pHs, variation in individual isolations of microsomes and the failure of subcellular fractions to faithfully produce metabolites generated in intact cells. We have developed a gene mutation assay utilizing the human hepatoma cell line HepG2, which has been shown to metabolize a broad spectrum of promutagens. Optimal conditions for assaying the induction of 6-thioguanine-resistant mutants in this cell line include: 1) growth of colonies for three weeks on lethally irradiated feeder layers of 10(6) thioguanine-resistant HepG2 cells (average plating efficiency = 60-80%); 2) a thioguanine concentration in selection dishes of 10(-4) M with a maximum seeding density of 2.5 x 10(5) cells per 100 mm culture dish; and 3) a minimum expression time of 6 days. In addition to ultraviolet light C (254 nm), a cytochrome P450 (cyclophosphamide)-dependent and a cytochrome P448 (aflatoxin B1)-dependent promutagen were shown to induce cytotoxicity and mutations in this test system. The present studies, therefore, suggest that the HepG2 cell line may be useful for a variety of assays in genetic toxicology.

Aflatoxins↗
PubMed1988

Uptake and urinary excretion of aluminum among welders.

The urinary excretion of aluminum was measured in 23 welders before and after an exposure-free interval of 16 to 37 days. In addition, the concentration of aluminum in the air was measured at the work site of 16 aluminum welders on the same workday as the first urine sample was taken. The concentration of aluminum in the urine depended on both the level and duration of exposure. The postshift urinary concentration of aluminum prior to an exposure-free interval was mainly related to the current air concentration, whereas the urinary concentration of aluminum determined after the exposure-free interval was related to total exposure duration (years). Among welders exposed for less than 1 year, the half-time for urinary concentration was about 9 days whereas welders exposed for more than 10 years had half-times calculated to be 6 months or longer. The results indicate that aluminum is retained and stored in at least two functional compartments of the body and is eliminated from these compartments at different rates.

Adult↗
PubMed1988

Linear free energy relationship studies of enzyme active site binding: thymidylate synthase.

The requirements for active-site binding of thymidylate synthase from three sources, Lactobacillus casei, murine leukemia L1210, and human lymphoblast (Molt/4F), were investigated by analyzing the binding of a series of 5-(p-substituted phenyl)-2'-deoxyuridylates (N1-substituted 5-aryl-2, 4-dioxopyrimidines) to the enzyme. Multiple regression analysis revealed that an increase in electron density of the heterocyclic ring and hydrophobic substituents enhance affinity. Correlations of biological results with spectral data indicated that higher electron densities at the oxygen atoms are responsible for increase in binding. These results support the presence of both a cationic binding site and a hydrophobic region. In addition, the results revealed an unusual reversal of electronic requirements for binding and catalysis. The formation of the binary complex is enhanced by electron-donating substituents, while the initial catalytic reaction, formation of the covalent ternary complex, is promoted and stabilized by electron-withdrawing substituents.

Animals↗
PubMed1988

Patient characteristics in unplanned discharge from day hospital.

In order to identify the distinguishing characteristics of patients with unplanned discharges from day hospital, the author reviewed 96% of all 1987 admissions. Unplanned discharges included precipitous in-patient hospitalization, discharge before 30-day program completion, and discharge against medical advice. Forty-three percent of reviewed admissions ended by unplanned discharge. Psychiatric patients with recent and/or remote substance abuse, and patients with multiple day-hospital admissions were especially vulnerable to unplanned discharge. Use of a backup bed during admission and being referred from the general-hospital emergency room or parent mental-health facility were associated with high rates of unplanned discharge. Patients with multiple admissions were more likely than those with a single admission to have personality disorders and to be female and white. Using logistic regression analysis, the author found that when patients had several characteristics increasing their risk for unplanned discharge, the odds of leaving before program completion were considerable.

Community Mental Health Centers↗
PubMed1988

Effects of methylxanthines on cell-cycle redistribution and sensitization to killing by low-dose-rate radiation.

The effects of two methylxanthines, caffeine and isobutylmethylxanthine (IBMX), on the response to radiation given at two low dose rates were studied in a human hepatoma cell line, HepG2. These dose rates are in the range to which tumor cells are exposed in radiolabeled immunoglobulin therapy of primary hepatoma. At the higher dose rate (0.3 Gy/hr for 24 hr), the radiation completely inhibited growth, and cells accumulated in the G2 phase of the cell cycle. Caffeine (1 mM), given either continuously or during the last 4 hours of the radiation exposure, counteracted the growth inhibition and G2 accumulation and enhanced cell killing. The 0.5-mM dose of IBMX had little effect on cells. For a lower dose rate and longer exposure period (0.06 Gy/hr for 3 days), the radiation partially inhibited growth and the accumulation of cells in G2 was small. Continuous, but not short, exposure to caffeine enhanced killing at this dose rate and counteracted the G2 accumulation. At the lower dose rate, IBMX enhanced the growth inhibition, G2 accumulation, and killing above that with radiation alone.

1-Methyl-3-isobutylxanthine↗
PubMed1987

Theoretical and experimental analysis of cellular adhesion to polymer surfaces.

An exact discrete numerical solution to the Grabowski model for predicting cell adhesion to polymer surfaces is discussed. The solution technique allows the possibility of taking into account cell-cell interactions within the flow situation and the multistep process involved in thrombus formation. The proposed solution also allows modification of the wall reaction rate model into a two species reaction rate which distinguishes between the kinetics of contact adhesion and irreversible adhesion. The solution allows determination of effective diffusivity (De) and surface reaction rate (k) constants. Use of the model to examine available experimental data results in the following conclusions: (1) static or dynamic cell adhesion cannot be considered to be diffusion limited; (2) for flow conditions De is a monotonically increasing function of shear rate; (3) under static, i.e., zero flow conditions, De appears to be markedly larger than for flow conditions.

Cell Adhesion↗
PubMed1985

Effects of pyrimidine antagonists on sialic acid regeneration in HL-60 cells.

Because alterations in cell membrane sialoglycoconjugates can affect the behavior of neoplastic cells, we investigated the effects of in vitro treatment with antimetabolites used in cancer therapy on the expression of membrane sialic acid in cultured HL-60 leukemic cells. In these studies, cells were incubated with Vibrio cholerae neuraminidase to remove surface sialic acid. Reappearance of membrane sialic acid during drug treatment was followed (a) by measuring changes in radioactive surface labeling of viable cells with sodium metaperiodate-sodium[3H]-borohydride, (b) by measuring the decline in accessible surface galactosyl receptor sites which occurred coincident with membrane sialic acid replacement, and (c) by measuring the incorporation of [3H]glucosamine into membrane-associated neuraminidase-labile sialic acid. We were especially interested in learning whether drugs that affect intracellular pools of cytidine triphosphate (CTP), an important nucleotide intermediate in sialylation reactions, could inhibit regeneration of membrane sialic acid. 3-Deazauridine, a competitive inhibitor of CTP synthetase, depleted CTP pools and curtailed surface membrane resialylation with little or no effect on synthesis of de novo sialic acid from precursor sugars. The addition of cytidine restored CTP pools and sialic acid regeneration. Acivicin, a glutamine antagonist, also depleted CTP pools and curtailed surface membrane resialylation. In addition, it retarded de novo synthesis of sialic acid. The addition of cytidine restored intracellular CTP pools and sialic acid regeneration. However, both cytidine and guanosine were required to restore sialic acid synthesis from precursor sugars. 1-beta-D-Arabinofuranosylcytosine, a competitive inhibitor of sialic acid synthetase and of sialyltransferase, inhibited both de novo sialic acid synthesis and membrane resialylation. Only the latter effect was reversed by the addition of exogenous cytidine. Hydroxyurea, an agent shown previously to inhibit glycoconjugate production in hamster fibroblasts, curtailed membrane resialylation and de novo synthesis of sialic acid without depleting CTP pools. Doxorubicin, at levels that caused marked arrest of cell proliferation, had no effect on sialic acid synthesis or expression on the membrane surface. These data suggest that antimetabolites, apart from their cytotoxic effects or effects on cellular growth, may directly inhibit the expression of membrane sialic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

3-Deazauridine↗
PubMed1984

The effects of cholecystokinin-octapeptide and pentagastrin on electrical and motor activities of canine colonic circular muscle.

The effects of cholecystokinin-octapeptide (CCK-OP) and pentagastrin on electrical and motor activities of circular muscle of the canine colon were studied with the sucrose gap technique. Additional organ bath experiments were performed to further characterize the motor response to the peptides and to elucidate their site of action. The electrical activity consisted of slow waves having an initial potential followed by a plateau potential, at a regular frequency of 4.5 cycles/min. Both peptides prolonged the duration and increased the amplitude of the plateau phase of the slow waves. Concomitantly, the slow wave frequency was reduced. In addition, CCK-OP increased spiking activity. Both spiking activity and the prolonged plateau potential generated contractile activity, prolonged phasic contraction occurring with slow waves with a prolonged plateau. In organ bath experiments, both CCK-OP and pentagastrin increased the basal tone of the muscle strips and prolonged the duration of the phasic contractions. The prolongation of the duration of the contractions was not antagonized by tetrodotoxin (TTX) and atropine. CCK-OP but not pentagastrin increased the force of contractions, this action was not affected by atropine but was reduced in the presence of TTX, suggesting that the increase in force may be partially mediated by noncholinergic excitatory nerves. The increase in basal tension by the peptides was enhanced in the presence of TTX indicating that myenteric inhibitory neurones were tonically active under our experimental conditions. The results provide the electrophysiological basis for CCK-OP and pentagastrin induced changes in colonic motility.

Animals↗
PubMed1984

Electrophysiological basis of excitation of canine colonic circular muscle by cholinergic agents and substance P.

The circular muscle layer of the canine colon exhibits omnipresent rhythmic periodic waves of depolarization (slow waves), acting as pacemaker activity. The electrophysiological and motor responses of this layer to the muscarinic agonists acetylcholine and carbachol, and to the excitatory peptide substance P, were studied using the sucrose-gap technique. In addition, changes in the contractile activity were examined in organ bath experiments. The slow waves consisted of an initial potential followed by a plateau potential. All substances depolarized the membrane and increased dramatically the duration of the plateau potential resulting in a decrease of the slow wave frequency. In addition, the amplitude of the plateau potential was often increased significantly. Carbachol and substance P readily evoked spiking activity whereas acetylcholine did not. Both spiking activity and the plateau potential generated contractile activity. The prolongation of the slow wave duration caused a profound alteration of the pattern of contractions. Long-lasting tachyphylaxis to the effect of substance P, but not to acetylcholine or carbachol, occurred. The electrophysiological and motor effects of the drugs were due to a direct action on the smooth muscle cell membrane. This study provides an electrophysiological basis for prolonged circular muscle contractions of the colon, and it emphasizes the pacemaker activity of gastrointestinal smooth muscle as an important site of drug action.

Acetylcholine↗
PubMed1983

S-100-mediated inhibition of brain protein phosphorylation.

The effects of the glial-specific, calcium-binding, S-100 protein on brain membrane and supernatant protein phosphorylation were assessed. S-100 concentrations as low as 5 micrograms/ml caused a marked inhibition of the phosphorylation of a soluble brain protein having a molecular weight of 73,000 daltons (73K). This protein was designated the S-100 protein-modulated phosphoprotein (SMP). Half-maximal inhibition of the phosphorylation of SMP by S-100 was obtained at concentrations of 12 micrograms/ml (0.57 microM). The inhibition of SMP phosphorylation by S-100 was calcium-dependent, with a calculated calcium Ka of 2.0 +/- 0.3 microM. SMP phosphorylation was also inhibited by calmodulin, but only partially and with a much lower potency. The inhibition of SMP phosphorylation by S-100 was not inhibited by fluphenazine, whereas the effect of calmodulin was. SMP was found in many brain areas, with the highest levels seen in the corpus callosum. Various peripheral tissues, such as kidney; liver; and pineal, pituitary, and adrenal glands, did not contain detectable SMP levels. At higher S-100 concentrations, greater than 10 micrograms/ml, the phosphorylation of several other soluble proteins was markedly inhibited. These proteins have molecular weights of 56K, 50K, and 47K. The phosphorylation of these proteins was enhanced by calmodulin. These data suggest that the S-100 protein may function to modulate the phosphorylation of brain proteins in a manner analogous to (although in a reciprocal fashion) that of calmodulin.

Animals↗
PubMed1976

Mechanism of malic enzyme from pigeon liver. Magnetic resonance and kinetic studies of the role of Mn2+.

As determined by EPR, malic enzyme from pigeon liver binds Mn2+ with a half-site stoichiometry of two tight binding sites (KD=6 to 10 mum) per enzyme tetramer and at two to four weak binding sites (KD=0.43 to 1.34 mM). The activation of malic enzyme by Mn2+ at high levels of L-malate shows biphasic kinetics yielding two activator constants for Mn2+. The dissociation constants of Mn2+ for both classes of sites are of the same order as the kinetically determined activator constants of Mn2+, indicating active site binding at both classes of binding sites. The binding of Mn2+ to the tight sites enhances the paramagnetic effect of Mn2+ on 1/T1 of water protons by a factor (epsilon) of 17, while binding at the weak sites yields a smaller epsilon of 11. The coenzymes TPN and TPNH have no effects on epsilon, while the carboxylic acid substrates L-malate and pyruvate and the inhibitors D-malate and oxalate significantly decrease epsilon. TPNH causes a 38-fold tightening of binding of the substrate L-malate to the enzyme-Mn2+ complex, consistent with the previously described highly ordered kinetic scheme, but only a 2-fold tightening of binding of the competitive inhibitor D-malate. The dissociation constant of L-malate from the quaternary E-Mn2+-TPNH-L-malate complex (32 muM) agrees with the Km of L-malate (25 muM), indicating active site binding. The dissociation constants of pyruvate from the ternary E-Mn2+-pyruvate complex (12 mM) and from the quaternary E-Mn2+-TPN-pyruvate complex (20 mM) are similar to the Km of pyruvate (5 mM), also indicating active site binding and a less highly ordered kinetic scheme for the reactions of pyruvate than for those of L-malate. Analysis of the frequency dependence of 1/T1 of water protons indicates that two fast exchanging water ligands remain coordinated to Mn2+ in the binary E-Mn2+ complex. The binding of the substrates L-malate and pyruvate and of the transition state analog oxalate to the E-Mn2+ complex decrease the number of fast exchanging water ligands on Mn2+ by approximately 1, but the binding of D-malate has no significant effect on this parameter, indicating the occlusion or replacement of a water ligand of the enzyme-bound Mn2+ by a properly oriented substituent on C-2 of the substrate. Occlusion rather than replacement of a water ligand by pyruvate is established by studies of 1/T1 of 13COO- and 13CO-enriched pyruvate which indicate second sphere Mn2+ to pyruvate distances of 4.6 A (COO-) and 4.8 A (CO) in the ternary enzyme-Mn2+-pyruvate complex. Formation of the quaternary complex with TPN increases these distances by 0.8 A, indicating the participation of a second sphere enzyme-Mn2+-(H2O)-pyruvate complex in catalysis. Thus, malic enzyme, like five other enzymes which utilize metals to polarize carbonyl groups, forms a second sphere complex with its substrate.

Animals↗
PubMedDate not supplied

Emergency physicians' ratings of alcoholism treaters.

The purpose of this study is to identify treaters whom emergency physicians perceive to offer effective treatment of alcoholism. A random sample of 2,500 emergency physicians received a questionnaire comparing attitudes toward Alcoholics Anonymous (AA) and professional alcoholism treaters. Physician agreement on the efficacy of alcoholism treaters was greatest for AA (87%), moderate for mental health professionals (including psychiatrists and psychologists, 55%) and least for physicians and surgeons (excluding psychiatrists, 23%; chi-square = 1,024, p = .000000005, df = 2). Physician education about other alcoholism treaters may be necessary if all types of treatment are to be considered for the emergency room patient.

Adolescent↗
PubMedDate not supplied

Psychological distress among women with recurrent spontaneous abortion.

Fifty-seven of 100 women with recurrent abortion completed questionnaires designed to assess demographic and reproductive variables, depression, anxiety, social desirability, self-esteem, locus of control, and marital adjustment. The results indicated that 32% of the women could be classified as depressed. The subjects also reported higher than average levels of acute and chronic anxiety. Fifty-three percent of the subjects reported below average marital adjustment. Post hoc analyses indicated that the women who had a previous elective abortion had higher levels of anxiety, lower marital adjustment, and different attributions regarding their pregnancy losses than the women who had not had an elective abortion. Having a living child was not a protective buffer against psychological distress.

Abortion, Habitual↗