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The Neuromuscular Junction
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
Antagonists of a given neurotransmitter, also referred to sometimes as blockers, are substances that bind to the receptor of the neurotransmitter but reduce or block its action. Antagonists could be competitive, noncompetitive, or uncompetitive. As its name implies, a competitive antagonist competes with the neurotransmitter or agonist for the receptor sites. The binding of the competitive antagonist to the receptor site could be reversible (or surmountable), or it could be irreversible (or insurmountable). In the case of a reversible competitive antagonist, the bond to the receptor site is chemically reversible, so that the blocking action depends on the concentration of the antagonist and is reduced by a higher concentration of the neurotransmitter. On the other hand, increasing the concentration of the neurotransmitter does not reduce the blocking effect of an irreversible competitive antagonist that has bound to the site because the bond of the antagonist to the receptor site is chemically irreversible.
Pharmaceuticals: Some General Aspects
Published in Peter Grunwald, Pharmaceutical Biocatalysis, 2019
Deviations of a patient’s response to an administered drug from the normal behavior in presence of a second one may be a result of pharmocodynamic or/and pharmacokinetic drug-drug interactions (DDI; Srinivas et al., 2017). Pharmacodynamic interactions are coined by the pharmacological response changed through agonism or antagonism. Agonists are pure ones when they bind to the same (main) locus of the receptor with an effect similar to the main drug but partial ones when binding to a secondary locus—such an agonist provokes the same effect but with a lower intensity. However, an antagonist produces an opposite effect by binding to the receptor’s main locus in a competitive or an uncompetitive way. Pharmacodynamic DDIs include the occurrence of unwanted signal transduction mechanisms as side effect. In contrast, pharmacokinetic interactions include those where the availability of a drug is altered by induction or inhibition of metabolic enzymes or transporters involved in drug absorption, distribution, metabolism or excretion leading to a behavior of the drugs being different to that when administered individually (Kohler et al., 2000). Metabolism-based DDIs resulting in induction or inhibition of CYP 450 enzymes are among the most dangerous ones as estimated, e.g., by Pirmohamed and Park (2003) in their paper about Cytochrome P450 enzyme polymorphisms and adverse drug reactions.
Steroids: Arthritis, Fertility, Heart Attacks, And Home Run Records
Published in Richard J. Sundberg, The Chemical Century, 2017
Cholesterol is an important constituent of biological membranes, but at elevated levels, it contributes to arterial plaque and is also the major constituent of gall stones. Deoxycholic acid and cholic acid are liver metabolites of cholesterol and are called bile acids. Estrone and testosterone are the primary female and male sex hormones, respectively. The sex hormones are involved in many aspects of the reproductive cycle, including onset of puberty, fertility, and pregnancy. Progesterone is involved in the female reproductive cycle and inhibits ovulation during pregnancy. The steroids function through binding to various receptors called androgen, estrogen, and progesterone, etc. receptors. These receptors in turn function to control activation of genes. Some of the systems appear to be controlled on the basis of a balance between particular steroids. From a pharmacological point of view, it is of interest to have both agonist and antagonists that can either activate or inhibit a specific receptor and its function. Structures such as cortisone are called corticosteroids and are produced by the adrenal glands. They are characterized by an oxygen substituent at C-11 and the hydroxyacetyl group at C-17. The corticosteroids are involved in the hypothalamus–pituitary–adrenal axis and are critical messenger in the endocrine system. The corticosteroids influence metabolism, cell differentiation and function of the immune system. Aldosterone is an important mediator of blood pressure and salt and water retention (see Section 14.1).
Plant pharmacology: Insights into in-planta kinetic and dynamic processes of xenobiotics
Published in Critical Reviews in Environmental Science and Technology, 2022
Tomer Malchi, Sara Eyal, Henryk Czosnek, Moshe Shenker, Benny Chefetz
Drugs generally do not create a new effect, but rather, activate or inhibit a specific receptor-related activity. Receptors are sites to which specific ligands bind, thereby altering the cell's biochemical activity. An agonist binds to a receptor and activates a sequence of events that leads to a response. An antagonist binds to a receptor to inhibit the action, without initiating any effect itself. Drugs conferring a short duration of receptor activation generally interact via weaker bonds (ionic, hydrogen or van der Waals), whereas long-duration or irreversible drug–receptor interactions may form stronger bonds, such as covalent bonds. A drug’s ability to affect a specific receptor is related to its affinity and efficacy, which are parameters determined by its chemical structure. Affinity is defined as the probability of a drug occupying its target receptor upon interaction. Efficacy is defined as the extent to which a drug activates a receptor, resulting in a cellular response also known as drug effect (Buxton & Benet, 2013; Page & Maddison, 2008; Raj & Raveendran, 2019).
Human exposure to synthetic endocrine disrupting chemicals (S-EDCs) is generally negligible as compared to natural compounds with higher or comparable endocrine activity. How to evaluate the risk of the S-EDCs?
Published in Journal of Toxicology and Environmental Health, Part A, 2020
Herman Autrup, Frank A. Barile, Sir Colin Berry, Bas J. Blaauboer, Alan Boobis, Herrmann Bolt, Christopher J. Borgert, Wolfgang Dekant, Daniel Dietrich, Jose L. Domingo, Gio Batta Gori, Helmut Greim, Jan Hengstler, Sam Kacew, Hans Marquardt, Olavi Pelkonen, Kai Savolainen, Pat Heslop-Harrison, Nico P. Vermeulen
Receptors are cellular components that bind molecules of diverse chemical structures. Known as ligands, these molecules activate or inhibit the receptor function and thereby elicit a physiological response. Ligands that activate a response are agonists; those that block the response are antagonists. Potency of the EDs depends on the strength of interaction of their ligand molecules with a specific receptor or enzyme.