Antihypertensive Drugs

Beta-blockers competitively antagonize catecholamines at beta-adrenoceptors. The acute antihypertensive effect is reduction in heart rate and myocardial contractility, lowering cardiac output. With chronic use, additional mechanisms contribute: suppression of renin release from juxtaglomerular cells (reducing angiotensin II and aldosterone), and with lipophilic agents, reduced central sympathetic outflow through central nervous system beta-receptor blockade.

The receptor distribution determines the clinical consequences of blockade. Beta-1 receptors are located in the heart and kidney. Beta-2 receptors are in bronchial smooth muscle, vascular smooth muscle, pancreatic beta cells, and skeletal muscle. Cardioselective agents — metoprolol, bisoprolol, atenolol, nebivolol — preferentially block beta-1 receptors at therapeutic doses. Selectivity is relative, not absolute, and diminishes at higher doses. Non-selective agents — propranolol, nadolol, carvedilol — block both beta-1 and beta-2, producing bronchoconstriction, peripheral vasoconstriction, and greater metabolic effects.

Beta-blockers are generally not recommended as first-line monotherapy for uncomplicated hypertension. The ASCOT-BPLA trial demonstrated inferior stroke prevention with atenolol-based therapy compared to amlodipine plus perindopril despite equivalent blood pressure reduction. They are also less effective in older patients and Black patients, who more often have low-renin hypertension where renin-suppressing mechanisms provide less antihypertensive benefit.

Beta-blockers are the preferred or necessary choice when a specific comorbidity is present: heart failure with reduced ejection fraction (carvedilol, metoprolol succinate, bisoprolol — each with proven mortality benefit as guideline-directed medical therapy); post-myocardial infarction; stable angina or acute coronary syndrome; rate control in atrial fibrillation; acute aortic dissection (target heart rate below 60 beats per minute); and hypertension concurrent with migraine prophylaxis or essential tremor.

Metoprolol succinate (extended-release) is the preferred formulation for heart failure with reduced ejection fraction and hypertension — the MERIT-HF trial demonstrated 34 percent reduction in all-cause mortality. Bisoprolol has high cardioselectivity and dual elimination (hepatic and renal), making it suitable when either organ is impaired. Atenolol is renally eliminated and was found inferior to losartan in the LIFE trial for cardiovascular event reduction and left ventricular hypertrophy regression — its use in uncomplicated hypertension without other indications is generally not preferred. Nebivolol combines beta-1 blockade with endothelial nitric oxide release, producing vasodilation and the most favorable metabolic profile of the class — preferred when a beta-blocker is needed in metabolic syndrome or diabetes. Propranolol is non-selective and highly lipophilic, with specific niches in essential tremor, migraine prophylaxis, hyperthyroidism, and hypertrophic obstructive cardiomyopathy.

Beta-2 blockade in pancreatic beta cells impairs insulin secretion and in skeletal muscle impairs glycogenolysis. Most hypoglycemia warning symptoms (tremor, tachycardia, palpitations) are adrenergically mediated and may be masked — diaphoresis is the exception, being cholinergically mediated and preserved. Cardioselective agents cause less metabolic impairment than non-selective agents; nebivolol has the most favorable glucose and lipid profile.

Non-selective agents raise triglycerides and lower high-density lipoprotein through reduced lipoprotein lipase activity. Carvedilol and nebivolol have neutral to favorable lipid profiles. Bronchoconstriction from beta-2 blockade is most pronounced with non-selective agents — cardioselective agents are relatively safer in mild to moderate reactive airway disease but should not be considered safe in severe asthma. Erectile dysfunction, fatigue, exercise intolerance, and — with lipophilic agents — sleep disturbance and vivid dreams are additional class effects. Abrupt withdrawal may precipitate rebound hypertension, tachycardia, or angina in patients with coronary artery disease — taper over one to two weeks when discontinuing.

Clonidine stimulates presynaptic alpha-2 adrenoceptors in the brainstem nucleus tractus solitarius, reducing central sympathetic outflow. The net effects are reduced norepinephrine release from sympathetic terminals, decreased heart rate and cardiac output, reduced total peripheral resistance, and suppressed renin release. The transdermal patch provides more stable plasma levels than twice-daily oral dosing and is useful in patients with adherence difficulties.

Clinical applications include resistant hypertension as an add-on agent, opioid withdrawal (where it reduces adrenergically mediated symptoms including tachycardia, diaphoresis, and agitation), attention deficit hyperactivity disorder in extended-release formulation, and menopausal hot flashes off-label. The most common adverse effects are dry mouth, sedation, bradycardia, and constipation. Contact dermatitis occurs in up to 20 percent of patients using the transdermal patch.

Methyldopa is a prodrug converted in the central nervous system to alpha-methyl-norepinephrine, a potent alpha-2 agonist that reduces central sympathetic outflow by the same mechanism as clonidine. Its primary clinical indication is hypertension in pregnancy. Methyldopa has the most extensively studied safety record of any antihypertensive in pregnancy, with decades of data across all trimesters. It crosses the placenta but has not been associated with fetal harm at therapeutic doses. It is a first-line option alongside labetalol and long-acting nifedipine.

Outside of pregnancy, the adverse effect profile substantially limits its use. Sedation and fatigue are often prohibitive. A positive direct Coombs test occurs in up to 20 percent of patients on long-term therapy; clinically significant hemolytic anemia is uncommon. Drug-induced lupus and hepatotoxicity are rare. Where a centrally acting agent is needed in non-pregnant patients, clonidine is generally preferred.