Adrenergic alpha1-Antagonists

Alpha-1 adrenoceptors are G protein-coupled receptors for the catecholamines noradrenaline and adrenaline that signal through the Gq/G11 family: activation stimulates the phosphatidylinositol-calcium second messenger pathway, releasing calcium from intracellular stores and activating protein kinase C, and the receptor acts as a vasopressor in resistance arteries, helping maintain normal arterial blood pressure. Drugs in this class occupy that receptor without activating it, so circulating and neuronally released catecholamines can no longer act through it, and its vasopressor contribution is lost. A drug belongs here because it blocks an alpha-1 receptor; the alpha-1 and alpha-2 antagonist classes are not exclusive, and MED-RT files mirtazapine under both.