Hormones possible mechanism

Brouwer, A., Morse, D.C., and Lans, M.C. et al. (1998). Interactions of persistent enviromnen-tal organohalogens with the thyroid hormone system Mechanisms and possible conse-qnences for animal and hnman health. Toxicology and Industrial Health 14, 59-84. 

The possible mechanism kidney-induced hypertension is discussed in Section 2.4.2, Mechanisms of Toxicity. Lead appears to affect vitamin D metabolism in renal tubule cells, such that circulating levels of the vitamin D hormone, 1,25-dihydroxyvitamin D, are reduced. This effect is discussed later in this section under Other Systemic Effects. 

Li X et al Reproductive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in female rats Ovulation, hormonal regulation and possible mechanisms. Toxicol Appl Pharmacol 133 321, 1995... 

The coupling reaction by which the aromatic group from one residue of mono- or diiodotyrosine is joined in ether linkage with a second residue is also catalyzed readily by peroxidases. One dehydroalanine residue is formed for each molecule of hormone released.108 A possible mechanism involves formation of an electron-deficient radical, which can undergo (3 elimination to produce a dehydroalanine residue and an aromatic radical. The latter could couple with a second radical to form triiodothyronine or thyroxine. However, as depicted in Eq. 25-6, the radical coupling may occur prior to chain cleavage. While P elimination (pathway... 

Possible mechanisms of hormone action, (a) The hormone (H) theoretically activates an enzyme (E) directly as an allosteric effector. (b) Alternatively, a separate binding protein for the hormone, called a receptor (R), may then activate an enzyme, (c) Another possibility interposes an acceptor protein (A) between the receptor and the enzyme. Each interaction is reversible. 

Increased clearance of steroid hormones due to induction of hepatic biotransformation enzymes following chemical exposure often has been cited as a possible mechanism by which toxicants could lower circulating testosterone or 17/3-estradiol levels. While enhanced clearance of sex steroids has been demonstrated following chemical exposure and induction of hepatic biotransformation enzymes, elegant feedback control mechanisms tend to ensure that more hormone is produced and homeostasis is maintained (Figure 17.2). Enhanced clearance of sex steroids can contribute to endocrine disruption if the toxicity also results in impaired hormone synthesis (i.e., gonadal toxicity or interference with the feedback control of hormone synthesis). 2,3,7,8-Tetrachlorodibenzodioxin appears to lower circulating sex steroid levels via this dual effect. 

The sexual dimorphism of cuticular hydrocarbons is completed during the first three days after imaginal eclosion. During the same period, important physiological events take place, female oocyte maturation and vitellogenesis. A number of mutations have been described which affect ovarian development or endocrine control. These mutants were used to elucidate a possible hormone control mechanism used to regulate hydrocarbon biosynthesis. 

However, the levels of Gsa, Gia-1, Gia-2, Gia-3, Goa, and Gp were also shown to be unaltered in myocardium from SHRs, and adenylyl cyclase activity stimulated by PGEi, glucagon, and isoproterenol was reduced in SHRs, whereas FSK-stimulated enzyme activity was greater in SHRs as compared to WKY (McLellan et al. 1993). On the other hand, a diminished stimulation of adenylyl cyclase by stimulatory hormones, guanine nucleotides, FSK, and NaF in aorta and heart sarcolemma from SHRs (Anand-Srivastava 1992), renal hypertensive rats (Anand-Srivastava 1988) 1K1C HRs (Ge et al. 1999, 2006), and DOCA-salt HRs (Anand-Srivastava et al. 1993) has been demonstrated The reduction in the hormone receptor binding sites may be one of the possible mechanisms responsible for such an impaired response of hormones (Limas and Limas 1978 Woodcock et al. 

Brouwer A, Morse DC, Lans MC, SchuurAG, MurkAJ, Klasson-Wehler E, Bergman A, Visser TJ (1998) Interactions of persistent environmental organohalogens with the thyroid hormone system Mechanisms and possible consequences for animal and human health. Toxicol Ind Health, 14 59-84. 

High-density lipoprotein functions as a shuttle that moves cholesterol throughout the body. HDL binds and esterifies cholesterol released from the peripheral tissues and then transfers cholesteryl esters to the liver or to tissues that use cholesterol to synthesize steroid hormones. A specific receptor mediates the docking of the HDL to these tissues. The exact nature of the protective effect of HDL levels is not known however, a possible mechanism is discussed in Section... 

Although this literature has been reviewed extensively16,36,47 54-56 65 it is still not understood precisely how the fish thyroid system responds to any given xenobiotic and what the biological consequences might be. In light of this we have focused on possible mechanisms of fish thyroid disruption. We have commenced with brief overviews of the properties of thyroid hormones (TH) and the thyroid cascade and have then considered the problems inherent in evaluating fish thyroidal responses. 

Gro 1C-/actor signaling. Human growth hormone binds to a cell-surface membrane protein that is not a receptor tyrosine kinase. The intracellular domain of the receptor can bind other proteins inside the cell. Furthermore, studies indicate that the receptor is monomeric in the absence of hormone but dimerizes on hormone binding. Propose a possible mechanism for growth-hormone signaling. 

Autoimmunity. Autoimmune diseases are disorders of immune regulation in which several different factors (e.g. viral, genetic, hormonal, environmental) may each play a role. Autoimmune diseases may belong to any of the four Coombs and Cell classifications of hypersensitivity and include the production of autoantibodies, destructive inflammatory cell infiltrates in various organs, and deposition of immune complexes in vascular beds. Chemically induced autoimmunity may result from any of several possible mechanisms. These include the alteration or release of autoantigens, or the cross-reaction of the chemical with autoantigens, or alternatively a direct effect on the immune system via lymphocytes or macrophages (60). 

McClure, M.T. and 1. Stupans, (1995). Hormonal perturbation as a possible mechanism for the alteration of cytochrome P450 by cyclophosphamide. Biochem. Pharmacol. 49, 1827-1836. 

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