6.5 : Adrenergic Agonists: Direct-Acting Agents

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.