Drugs that mimic the action of endogenous catecholamines like noradrenaline and adrenaline are called adrenergic agonists or sympathomimetics. Based on their mechanism of action, sympathomimetics can be classified as direct-, indirect-, or mixed-acting sympathomimetics. Direct-acting adrenergic agonists activate adrenoceptors without affecting presynaptic neurons, making them independent of neuronal catecholamine-depleting agents like reserpine and guanethidine.
These agents can be classified based on their selectivity to specific adrenoceptor types. Non-selective agents, like oxymetazoline, have diverse effects. Selective direct-acting agonists target specific adrenoceptors, avoiding unwanted side effects. Examples include α1-selective agents (e.g., phenylephrine) that enhance cardiovascular function, α2-selective agents that inhibit it, ꞵ1-selective agents (e.g., dobutamine) that increase heart rate and cardiac output, and ꞵ2-selective agents (e.g., salbutamol, terbutaline) that act as bronchodilators and uterine relaxants. Additionally, ꞵ3-selective agents like mirabegron are used to treat urinary incontinence by acting on the detrusor muscle of the bladder.
