Threshold analysis of selected dose‐response data for endocrine active chemicalsNote

ArticleinApmis 109(3):198 - 208 · February 2001with4 Reads
Impact Factor: 2.04 · DOI: 10.1034/j.1600-0463.2001.090303.x
Abstract

Using a biologically relevant mathematical model, the Michaelis-Menten equation, we examined published data from endocrine active chemicals for evidence of no-threshold dose-response curves. Data were fit to a modified Michaelis-Menten equation which accounted for total background response. Subsequently, the data sets were analyzed using non-linear regression in order to estimate the four parameters of interest (non-hormone controlled background (Bnh), maximum response (Rmax), endogenous hormone level (D0), and the dose at which a half-maximal response was observed (ED50)) and to determine the fit to the fully modified Michaelis-Menten equation. Subsequently, response data were adjusted to account for Bnh and then normalized to Rmax, while dose data were adjusted to account for D0 and then normalized to the ED50. This data set was combined into a single, composite data set and fit to the fully modified Michaelis-Menten equation. We examined 31 data sets (24 endpoints) from studies on 9 different chemical/hormone treatments. Twenty-six of the data sets fit the modified Michaelis-Menten equation with high multiple correlation coefficients (r>0.90). The normalized data demonstrated a good fit to the modified Michaelis-Menten equation. These results indicate that a variety of biological responses fit the modified Michaelis-Menten equation, which does not have a threshold dose term.

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    No preview · Article · Jan 2001
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    Full-text · Article · Dec 2001 · Biology of Reproduction
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  • [Show abstract] [Hide abstract] ABSTRACT: Clenbuterol (CLEN), diethylstilbestrol (DES) and trenbolone (TbOH) are xenobiotic growth promoters and have been illegally applied as repartitioning agents in meat-producing animals. In the EU, proper detection methods and control systems are established for many hormones and anabolic substances, and therefore there is a need to improve the accuracy and repeatability of quantification. In order to define a common basis for accurate measurement and quantification, standard materials were produced as lyophilised bovine urine: ‘negative’ drug free controls, and ‘positive’ standards (CLEN ∼3 μg kg−1; DES ∼2 μg kg−1, TbOH ∼3 μg kg−1). Two milligram of CLEN or DES were applied orally in milk replacer to the calves. Two gram of TbOH-acetate was given as an implant. ‘Positive’ urine was sampled frequently between 12 and 24 h after application. ‘Negative’ urine was sampled from untreated calves only fed with milk replacer. Urines were pooled separately and diluted with ‘negative’ urine to a suitable level of final concentrations. Exactly 5.0 g portions of each standard were weighed into 750 brown glass containers and lyophilised in one batch. Container filling, dry matter and residual water content were determined and optimized. A 3-year stability and homogeneity study at various storage temperatures (−20 °C, +4 °C and +20 °C and +37 °C) indicate that all standards are stable and homogeneous. Reconstituted urines were quantified via ELISA and findings after reconstitution remained constant in ‘positive’ (+) standards and ‘negative’ (−) controls: CLEN+ (2.75±0.62 μg kg−1; n=133), CLEN− (0.023 μg kg−1 equivalents; n=130), DES+ (1.75±0.52 μg kg−1; n=133), DES− (0.034 μg kg−1 equivalents; n=133), TbOH+ (2.78±0.47 μg kg−1; n=133), TbOH− (0.022 μg kg−1 equivalents; n=133). Additionally, concentrations and identities of the drugs were confirmed by two independent accredited laboratories via HPLC–ELISA, LC–MS and GC–MS.
    Full-text · Article · Apr 2003 · Analytica Chimica Acta
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