Pharmacodynamics

Pharmacodynamics Mechanisms of drug action: Drugs may act by: * A) Receptor mechanism: * Most drugs produce their action by binding to specific receptors located on the cell membrane (as adrenoceptors and cholinoceptors) or inside the cell (as steroid receptors). * This is the most important mechanism of drug action. * Drugs produce their effects by interacting with these receptors. * These drugs may be: a) Agonists: * Drugs have the affinity to bind with their specific receptors and Mimic (produce an action similar to the action of) the endogenous regulatory substance are called agonists. * Example: * salbutamol which is used in the treatment of bronchial asthma acts by binding to beta adrenergic receptors located on the smooth muscle of the lung producing bronchodilatation. b) Antagonists: * Drugs that bind to the receptors without producing effects but block the function of endogenous agonists. * Example: * prazosin which is used in the treatment of hypertension acts by blocking the action of endogenous catecholamines on alpha adrenergic receptors by occupation of these receptors * B) Non-receptor mechanisms: 1- Chemical interaction as the antacids which neutralize gastric acidity and the chelating agents which bind different metal cations. 2- Physical alterations of cellular environment: * i-Alteration of osmosis: e.g., I.V. mannitol to reduce cerebral edema and oral magnesium sulphate as cathartic. * ii-Lubrication: e.g., mineral oil administered by mouth to facilitate passage of stool. * iii-Adsorption: e.g., activated charcoal given orally to reduce the absorption of harmful toxins in case of poisoning. Also, Kaolin and pectin can adsorb toxins that cause diarrhea. 3- Enzymatic alteration * Drugs altering enzyme activity and this alter processes catalyzed by these enzymes. * Examples: Cholinesterase inhibitors Monoamine oxidase inhibitors 4-