Alfaadrenerga

Alfaadrenerga, or alpha-adrenergic receptors, are a subset of adrenergic receptors that respond to catecholamines such as norepinephrine and epinephrine. They play a key role in regulating vascular tone and various organ functions as part of the sympathetic nervous system. The two main subtypes are alpha-1 and alpha-2 receptors, each with distinct signaling pathways and physiological effects.

Alpha-1 receptors are primarily located on vascular smooth muscle and certain other smooth muscles. They couple to Gq proteins, activating phospholipase C and increasing intracellular calcium. The resulting effect is vasoconstriction, which raises systemic vascular resistance and blood pressure. Alpha-1 receptors also contribute to pupil dilation and contraction of the bladder neck and urethral sphincter.

Alpha-2 receptors are found presynaptically on sympathetic nerve terminals as autoreceptors, and also in various tissues including some vascular beds and the central nervous system. They couple to Gi proteins, reducing cyclic AMP levels and inhibiting neurotransmitter release. Central alpha-2 activation diminishes sympathetic outflow, promoting effects such as lower blood pressure and sedation, while peripheral alpha-2 actions can influence vascular tone and insulin release.

Pharmacology centers on agonists and antagonists of these receptors. Alpha-1 agonists (e.g., phenylephrine) cause vasoconstriction and are used as decongestants or vasopressors, whereas alpha-1 antagonists (e.g., doxazosin) can treat hypertension and benign prostatic hyperplasia. Alpha-2 agonists (e.g., clonidine, dexmedetomidine) reduce sympathetic activity and have sedative or analgesic applications; alpha-2 antagonists are less commonly used. Understanding alfaadrenerga receptors helps explain many cardiovascular and autonomic responses to drugs and physiological stress.