Pathophysiology
Clinical meaning
Beta-blockers are classified by their receptor selectivity, intrinsic sympathomimetic activity (ISA), lipophilicity, and additional properties. The NP must understand these pharmacological differences to select the optimal agent for each clinical scenario. Beta-1 receptors are predominantly cardiac: they increase heart rate (chronotropy), conduction velocity (dromotropy), contractility (inotropy), and renin release from the juxtaglomerular cells. Beta-2 receptors are found in bronchial smooth muscle (relaxation), vascular smooth muscle (vasodilation), hepatocytes (glycogenolysis), and pancreatic beta cells (insulin release). Beta-3 receptors mediate lipolysis and thermogenesis. Cardioselective beta-blockers (metoprolol, atenolol, bisoprolol, nebivolol) preferentially block beta-1 receptors at therapeutic doses, minimizing beta-2 mediated bronchospasm and hypoglycemia masking. However, cardioselectivity is dose-dependent and lost at high doses. Non-selective beta-blockers (propranolol, nadolol, carvedilol, labetalol) block both beta-1 and beta-2 receptors. Special properties that guide selection: carvedilol and labetalol have additional alpha-1 blockade (vasodilation โ useful in HFrEF and hypertensive emergencies); nebivolol stimulates endothelial nitric oxide synthase (eNOS) producing vasodilation; propranolol is highly lipophilic (CNS penetration โ useful for migraine, essential tremor, performance anxiety; but causes more CNS side effects); atenolol and nadolol are hydrophilic (renally cleared โ less CNS...