Alfa1agonister

Alfa1agonister, or alpha-1 agonists, are a class of sympathomimetic drugs that selectively activate the alpha-1 adrenergic receptor. The alpha-1 receptor is a G protein–coupled receptor that signals through the Gq pathway, leading to increased intracellular calcium and contraction of smooth muscle. Activation of alpha-1 receptors on vascular smooth muscle produces vasoconstriction, raising systemic vascular resistance and blood pressure. In the eye, stimulation of the iris dilator muscle causes pupil dilation (mydriasis). In the urinary tract, alpha-1 receptor activation increases tone of the bladder neck and prostatic urethra, which can affect urine flow.

Clinical uses vary with the agent and route. Phenylephrine is commonly used as a nasal decongestant and as a constrictor to maintain blood pressure during procedures or to induce mydriasis for ocular exams. Midodrine is an oral prodrug that is converted to desglymidodrine and is used to treat orthostatic hypotension by increasing vascular tone. Methoxamine is an older alpha-1 agonist that is rarely used today. Topical alpha-1 agonists such as oxymetazoline and xylometazoline are employed as nasal decongestants and eye drops.

Adverse effects reflect excessive vasoconstriction and raised blood pressure, including hypertension, reflex tachycardia, palpitations, headache, and potential ischemia with extravasation. Supine hypertension is a concern with certain oral agents like midodrine. Cautions include cardiovascular disease, arrhythmias, hypertension, and interactions with monoamine oxidase inhibitors or other vasopressors.

Subtypes of alpha-1 receptors (A, B, D) have different tissue distributions, contributing to regional differences in response and guiding research on receptor-selective agonists.