Catecholamines and Hepatic Drug Metabolism

The thesis embodies work designed to investigate the acute effects of catecholamines on the hepatic metabolism of foreign compounds. Most of the studies were performed on the isolated perfused liver of the rat. The historical development of the techniques used in the perfusion of the liver is reviewed. The literature concerning the properties of the hepatic mixed function system responsible for the metabolism of foreign compounds has been reviewed. Previous work on the effects of catecholamines on drug metabolism is also discussed. The operative procedure and the technique for the perfusion of the liver were developed to enable rapid transfer of the liver to the perfusion chamber without a period of anoxia, which has characterised many previous attempts at perfusion of the liver. The liver was perfused with a semisynthetic medium. The viability of the liver was assessed by a variety of tests of biochemical and physiological function and by histological examination. The technique developed allowed the liver to be successfully perfused for periods of up to 6 h, whilst maintaining a functional integrity. The metabolism of the type I and the type II substrates, hexobarbitone and aniline respectively, were investigated. Hexobarbitone is removed by a first order reaction at a rate comparable to that previously reported for the liver in vivo. Aniline metabolism, which has not previously been investigated in the perfused liver, was studied in some detail. Aniline is removed biphasically from the perfusion medium. Half of the aniline removed is converted to an acid-labile conjugate, possibly aniline-N-glucuronide, and a further 25% is converted to p-aminophenol and its conjugates. 3H-aniline added to the perfused liver could be quantitatively accounted for after 3 h perfusion. In agreement with previous findings, it was found that p-aminophenol reacts with haemoglobin, thus making it impossible to detect free p-aminophenol in the perfusion medium. The differences between the pattern of aniline metabolism by the perfused liver and by the whole animal, where aniline is excreted mainly as p-aminophenol and its conjugates, are discussed. The kinetics of aniline removal have been analysed by curve stripping and model fitting. The mathematics of these procedures are described separately in Appendix II. In the perfused liver aniline is distributed throughout a two-compartment system. The first compartment probably represents the total aqueous phase and aniline is removed from this compartment only. The identity of the second compartment is less certain. Several possibilities are discussed and in view of evidence obtained with SKF 525-A, an inhibitor of drug metabolism, it is suggested that the second compartment may represent the binding of aniline to a non-metabolic site of cytochrome P-450. Both adrenaline and noradrenaline inhibit the metabolism of hexobarbitone by the perfused liver but neither catecholamine has any effect on the metabolism of aniline. Catecholamines do not inhibit the metabolism of either aniline or hexobarbitone by the microsomal fraction or by liver slices. It was thus concluded that catecholamines are not inhibiting hexobarbitone removal directly. The possibility that their effect is mediated by cyclic AMP, a compound reported to inhibit hexobarbitone metabolism by the perfused liver, was investigated. Papaverine, an inhibitor Of phosphodiesterase, does not potentiate the inhibitory effects of catecholamines on hexobarbitone metabolism, nor does it cause them to have any inhibitory effect on the metabolism of aniline. Cyclic AMP was shown to inhibit drug metabolism by liver slices, whereas catecholamines are without effect. The probability that cyclic AMP is not important in mediating the effects of catecholamines on drug metabolism is discussed in some detail. Both catecholamines increase the portal pressure in the -perfused liver by over 100%. When Ca2+ is omitted from the perfusion medium it was found that this pressor effect is almost totally inhibited. Catecholamines also no longer in- hibit the metabolism of hexobarbitone. The omission of Ca2+ leaves the metabolic effects of catecholamines relatively unaffected. It was thus suggested that catecholamines might be inhibiting hexobarbitone metabolism through an effect on the hepatic vasculature. In a constant flow system, as used in this study, one way in which such an effect of catecholamines might manifest itself is as a redistribution of perfusate through the liver. It was found that considerable controversy exists as to whether or not catecholamines can cause a redistribution of blood flow within the liver. The available evidence is assessed separately in Appendix II. The effects of adrenaline on the intrahepatic distribution of perfusate have been investigated by X-radiography, and by perfusing the liver with Indian ink followed by histology. It was concluded from these studies that catecholamines can cause a redistribution of blood flow within the liver, away from the periphery of the lobes towards more central regions. (Abstract shortened by ProQuest.)

Increased Expression of Histone Proteins during Estrogen-Mediated Cell Proliferation

There is concern about the potential risk posed by compounds with estrogen-like activity present in the environment. As previous studies have shown that combined exposure to such compounds results in dose additivity, it should be possible to assess estrogen exposure with suitable biomarkers of effect

Defective spermatogenesis: Martin et al. respond

This is an Open Access article - Copyright @ National Institute of Environmental Health Science.BACKGROUND: Male reproductive tract abnormalities such as hypospadias and cryptorchidism, and testicular cancer have been proposed to comprise a common syndrome together with impaired spermatogenesis with a common etiology resulting from the disruption of gonadal development during fetal life, the testicular dysgenesis syndrome (TDS). The hypothesis that in utero exposure to estrogenic agents could induce these disorders was first proposed in 1993. The only quantitative summary estimate of the association between prenatal exposure to estrogenic agents and testicular cancer was published over 10 years ago, and other systematic reviews of the association between estrogenic compounds, other than the potent pharmaceutical estrogen diethylstilbestrol (DES), and TDS end points have remained inconclusive. OBJECTIVES: We conducted a quantitative meta-analysis of the association between the end points related to TDS and prenatal exposure to estrogenic agents. Inclusion in this analysis was based on mechanistic criteria, and the plausibility of an estrogen receptor (ER)-α–mediated mode of action was specifically explored. RESULTS: We included in this meta-analysis eight studies investigating the etiology of hypospadias and/or cryptorchidism that had not been identified in previous systematic reviews. Four additional studies of pharmaceutical estrogens yielded a statistically significant updated summary estimate for testicular cancer. CONCLUSIONS: The doubling of the risk ratios for all three end points investigated after DES exposure is consistent with a shared etiology and the TDS hypothesis but does not constitute evidence of an estrogenic mode of action. Results of the subset analyses point to the existence of unidentified sources of heterogeneity between studies or within the study population

Characterising vaping products in the UK: An analysis of Tobacco Products Directive notification data

Aims: To analyse content and emission data submitted by manufacturers for nicotine‐containing vaping products in the UK in accordance with the European Union Tobacco Products Directive.Design: Analysis of ingredient and emission data reported for all e‐liquid‐containing e‐cigarettes, cartridges or refill containers notified to the Medicines and Healthcare Regulatory Agency (MHRA) from November 2016 to October 2017.Setting: United KingdomCases: A total of 40,785 e‐liquid containing products.Measurements: The average number of ingredients per product, nicotine concentrations, frequency of occurrence ingredients and frequency and levels of chemical emissions.Findings: Reports were not standardised in relation to units of measurement or constituent nomenclature. Products listed an average of 17 ingredients and 3.3% were reported not to contain nicotine. 59% of products contained less than 12 mg nicotine per mL, and less than 1% were reported to have nicotine concentrations above the legal limit of 20 mg/mL. Over 1500 ingredients were reported, and other than nicotine the most commonly reported non-flavour ingredients were propylene glycol (97% of products) and glycerol (71%). The most common flavour ingredients were ethyl butyrate (42%), vanillin (35%) and ethyl maltol (33%). The most frequently reported chemical emissions were nicotine (65%), formaldehyde (48%) and acetaldehyde (40%). The reporting of the concentration of emissions was not standardised; emissions were reported in a format allowing analysis of median estimated concentration for between 13% and 100% of products for each reported emission. Most of the frequently reported emissions, other than nicotine, were present in median estimated concentrations below 1 μg/L of inspired air, and with the exception of nicotine, acrolein and diacetyl, at median levels below European Chemicals Agency Long Term Exposure and United States (US) Department of Labor Occupational Safety and Health Administration (OSHA) limits, where these were available

Application of the TTC concept to unknown substances found in analysis of foods

Unknown substances, not previously observed, are frequently detected in foods by quality control laboratories. In many cases, the assessment of these 'new' substances requires additional chemical analysis for their identification prior to assessing risk. This identification procedure can be time-consuming, expensive and in some instances difficult. Furthermore, in many cases, no toxicological information will be available for the substance. Therefore, there is a need to develop pragmatic tools for the assessment of the potential toxicity of substances with unknown identity to avoid delays in their risk assessment. Hence, the 'ILSI Europe expert group on the application of the threshold of toxicological concern (TTC) to unexpected peaks found in food' was established to explore whether the TTC concept may enable a more pragmatic risk assessment of unknown substances that were not previously detected in food. A step-wise approach is introduced that uses expert judgement on the source of the food, information on the analytical techniques, the dietary consumption of food sources containing the unknown substance and quantitative information of the unknown substance to assess the safety to the consumer using the TTC. By following this step-wise approach, it may be possible to apply a TTC threshold of 90. µg/day for an unknown substance in food. © 2011 Elsevier Ltd

The Key Events Dose-Response Framework: A Cross-Disciplinary Mode-of-Action Based Approach to Examining Dose-Response and Thresholds

The ILSI Research Foundation convened a cross-disciplinary working group to examine current approaches for assessing dose-response and identifying safe levels of intake or exposure for four categories of bioactive agents—food allergens, nutrients, pathogenic microorganisms, and environmental chemicals. This effort generated a common analytical framework—the Key Events Dose-Response Framework (KEDRF)—for systematically examining key events that occur between the initial dose of a bioactive agent and the effect of concern. Individual key events are considered with regard to factors that influence the dose-response relationship and factors that underlie variability in that relationship. This approach illuminates the connection between the processes occurring at the level of fundamental biology and the outcomes observed at the individual and population levels. Thus, it promotes an evidence-based approach for using mechanistic data to reduce reliance on default assumptions, to quantify variability, and to better characterize biological thresholds. This paper provides an overview of the KEDRF and introduces a series of four companion papers that illustrate initial application of the approach to a range of bioactive agents

The role of hazard- and risk-based approaches in ensuring food safety

AbstractBackgroundFood legislation in the European Union and elsewhere includes both hazard- and risk-based approaches for ensuring safety. In hazard-based approaches, simply the presence of a potentially harmful agent at a detectable level in food is used as a basis for legislation and/or risk management action. Risk-based approaches allow consideration of exposure in assessing whether there may be unacceptable risks to health.Scope and approachThe advantages and disadvantages of hazard- and risk-based approaches for ensuring the safety of food chemicals, allergens, ingredients and microorganisms were explored at an ILSI Europe workshop.Key findings and conclusionsIt was concluded that both types of approach have their place, depending on the context. However, problems can arise when both types of approach are used in regulation by separate agencies that address different aspects of the same agent/substance present in food. This separation of decision-making can result in hazard-based restrictions on marketing and use, whereas risk-based assessments for those exposed show there is reasonable certainty no harm will result. This in turn can lead to contradictory, confusing and ultimately unnecessary actions. Use of hazard-based approaches for foods also means that comparisons with benefits for nutrition and food security cannot be undertaken. This has the potential to lead to bias in the overall conclusions of regulators and risk managers, who may not have been presented with the benefits of particular foods. The value of risk-based approaches is becoming increasingly recognised

The role of hazard- and risk-based approaches in ensuring food safety

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Chemical carcinogenicity revisited 2: Current knowledge of carcinogenesis shows that categorization as a carcinogen or non-carcinogen is not scientifically credible

Abstract Developments in the understanding of the etiology of cancer have undermined the 1970s concept that chemicals are either "carcinogens" or "non-carcinogens". The capacity to induce cancer should not be classified in an inflexible binary manner as present (carcinogen) or absent (non-carcinogen). Chemicals may induce cancer by three categories of mode of action: direct interaction with DNA or DNA replication including DNA repair and epigenetics; receptor-mediated induction of cell division; and non-specific induction of cell division. The long-term rodent bioassay is neither appropriate nor efficient to evaluate carcinogenic potential for humans and to inform risk management decisions. It is of questionable predicitiveness, expensive, time consuming, and uses hundreds of animals. Although it has been embedded in practice for over 50 years, it has only been used to evaluate less than 5% of chemicals that are in use. Furthermore, it is not reproducible because of the probabilisitic nature of the process it is evaluating combined with dose limiting toxicity, dose selection, and study design. The modes of action that lead to the induction of tumors are already considered under other hazardous property categories in classification (Mutagenicity/Genotoxicity and Target Organ Toxicity); a separate category for Carcinogenicity is not required and provides no additional public health protection

Improving selection of markers in nutrition research: evaluation of the criteria proposed by the ILSI Europe Marker Validation Initiative

The conduct of high-quality nutrition research requires the selection of appropriate markers as outcomes, for example as indicators of food or nutrient intake, nutritional status, health status or disease risk. Such selection requires detailed knowledge of the markers, and consideration of the factors that may influence their measurement, other than the effects of nutritional change. A framework to guide selection of markers within nutrition research studies would be a valuable tool for researchers. A multidisciplinary Expert Group set out to test criteria designed to aid the evaluation of candidate markers for their usefulness in nutrition research and subsequently to develop a scoring system for markers. The proposed criteria were tested using thirteen markers selected from a broad range of nutrition research fields. The result of this testing was a modified list of criteria and a template for evaluating a potential marker against the criteria. Subsequently, a semi-quantitative system for scoring a marker and an associated template were developed. This system will enable the evaluation and comparison of different candidate markers within the same field of nutrition research in order to identify their relative usefulness. The ranking criteria of proven, strong, medium or low are likely to vary according to research setting, research field and the type of tool used to assess the marker and therefore the considerations for scoring need to be determined in a setting-, field- and tool-specific manner. A database of such markers, their interpretation and range of possible values would be valuable to nutrition researchers