Beta-blockers competitively antagonize catecholamines at beta-adrenoceptors.³ The acute antihypertensive effect is a reduction in heart rate and myocardial contractility, lowering cardiac output. Over time, additional mechanisms contribute: suppression of renin release from juxtaglomerular cells, with downstream reductions in angiotensin II and aldosterone; and, with lipophilic agents, reduced central sympathetic outflow through CNS beta-receptor blockade. Total peripheral resistance initially rises due to unopposed alpha activity but tends to normalize with continued use.

Beta-1 receptors are located in the heart, kidney, and adipose tissue. Beta-2 receptors are in bronchial smooth muscle, vascular smooth muscle, pancreatic beta cells, skeletal muscle, uterus, and eye. Beta-3 receptors are found in adipose tissue, bladder, and the heart in a minor capacity.

Cardioselective agents, including metoprolol, atenolol, bisoprolol, and nebivolol, preferentially block beta-1 receptors at therapeutic doses. Selectivity is relative, not absolute, and diminishes at higher doses. These agents generally produce less bronchoconstriction, less peripheral vasoconstriction, and less impairment of insulin secretion than non-selective agents. They are preferred when a beta-blocker is required in patients with reactive airway disease, diabetes, or peripheral arterial disease.

Non-selective agents, including propranolol, nadolol, and timolol, block both beta-1 and beta-2 receptors. The clinical consequences include bronchoconstriction, masking of hypoglycemic symptoms, and peripheral vasoconstriction.

Agents with intrinsic sympathomimetic activity (ISA), such as pindolol and acebutolol, are partial agonists at beta receptors. They blunt resting bradycardia but reduce resting heart rate less effectively than full antagonists. ISA agents are generally not preferred in heart failure or post-MI, and their use in contemporary hypertension management is limited.

Figure 1  ·  Reference Table Beta-antagonists: generic and US trade names

Beta-adrenoceptor antagonists: generic names and principal US trade names. Cardioselective agents preferentially block beta-1 receptors at therapeutic doses, reducing the bronchoconstriction and metabolic adverse effects associated with beta-2 blockade.

Beta-blockers are generally not recommended as first-line monotherapy for uncomplicated hypertension without a specific compelling indication. The ASCOT-BPLA trial⁸ compared atenolol-based therapy to amlodipine plus perindopril and found inferior stroke prevention with atenolol despite equivalent BP reduction.

ACC/AHA and ESH guidelines do not list beta-blockers as preferred first-line agents for hypertension in the absence of a compelling indication.^1,2 They are also generally less effective in older patients and in Black patients, who more commonly have low-renin hypertension where renin-suppressing mechanisms provide less antihypertensive benefit.

Beta-2 blockade in pancreatic beta cells may impair insulin secretion. Beta-2 blockade in skeletal muscle may impair glycogenolysis. Most hypoglycemia symptoms depend on adrenergic pathways and may be masked; diaphoresis is an exception, as it is mediated by cholinergic pathways and is generally preserved. Beta-blockers do not directly cause hypoglycemia, but they may mask warning symptoms and slow recovery. Cardioselective agents have less effect on glucose metabolism than non-selective agents; nebivolol has the most favorable metabolic profile in this class.

Non-selective and non-vasodilatory agents may raise triglycerides and lower HDL through reduced lipoprotein lipase activity. Carvedilol and nebivolol have neutral to favorable lipid profiles. Erectile dysfunction has been reported more frequently with beta-blockers than with most other antihypertensive classes, an effect attributed to reduced peripheral blood flow and possible CNS effects. It appears least common with nebivolol.

Fatigue and exercise intolerance reflect the reduced heart rate response to exertion and are more pronounced with non-selective and hydrophilic agents. CNS effects, including sleep disturbance and vivid dreams, occur more commonly with lipophilic agents such as propranolol and metoprolol, and less commonly with hydrophilic agents such as atenolol and bisoprolol.

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 fully safe in severe asthma or bronchospastic COPD. Abrupt withdrawal may precipitate rebound hypertension, tachycardia, or angina, particularly in patients with coronary artery disease. Gradual tapering over one to two weeks is advisable when discontinuing.

Selective alpha-1 adrenoceptor blockers competitively antagonize norepinephrine and epinephrine at postsynaptic alpha-1 receptors in vascular smooth muscle and other tissues.^1,2 Alpha-1 blockade produces arteriolar and venodilation, reducing total peripheral resistance and preload without direct cardiac effects.

First-dose hypotension is the most clinically relevant adverse effect. It results from rapid venodilation in a vascular system accustomed to elevated sympathetic tone, and occurs most commonly 30–90 minutes after the first dose, particularly in volume-depleted patients. Starting at the lowest available dose and administering the first dose at bedtime reduces this risk.

Hydralazine directly relaxes arteriolar smooth muscle. The mechanism is not fully characterized but may involve activation of soluble guanylate cyclase, opening of ATP-sensitive potassium channels, and antioxidant effects.² The net hemodynamic result is reduced arteriolar resistance. Reflex sympathetic activation produces tachycardia and increased cardiac output, and secondary renin-angiotensin-aldosterone system (RAAS) activation causes sodium and water retention. These reflex responses generally require co-administration of a beta-blocker and a diuretic when hydralazine is used chronically.

Hepatic acetylation via the NAT2 enzyme substantially influences pharmacokinetics. Slow acetylators tend to have higher plasma levels and a greater risk of drug-induced lupus. Fast acetylators may require higher doses for adequate BP control. Oral dosing ranges from 25–100 mg two to four times daily. IV bolus dosing of 10–20 mg given slowly has an onset of approximately 10–20 minutes.

Clinical indications include acute severe hypertension in pregnancy (IV, second-line after labetalol and nifedipine), HFrEF in patients who cannot tolerate RAAS inhibitors (the A-HeFT trial¹¹ reported benefit with hydralazine plus isosorbide dinitrate in Black patients with HFrEF on standard therapy), and as an oral add-on agent in resistant hypertension when other options have been exhausted.

Drug-induced lupus erythematosus has been reported in approximately 5–10% of patients on long-term therapy. Risk appears higher in slow acetylators and at doses above 200 mg/day. Presentation typically includes arthralgias, myalgias, rash, serositis, and positive ANA with anti-histone antibodies; renal involvement is uncommon. Discontinuation of hydralazine generally leads to resolution over weeks to months. Peripheral neuropathy attributable to pyridoxine deficiency has been reported with long-term high-dose use and may be reduced with vitamin B6 supplementation.

Minoxidil is a prodrug converted to minoxidil sulfate, which activates ATP-sensitive potassium channels (K-ATP channels) in vascular smooth muscle. Channel opening causes membrane hyperpolarization, preventing voltage-gated calcium channel activation and producing profound arteriolar vasodilation.² It is among the more potent oral antihypertensive agents available.

This potency is accompanied by substantial reflex responses. Marked tachycardia from sympathetic activation and severe sodium and water retention are expected; weight gains of 2–10 kg have been reported. The degree of sodium retention is among the most pronounced associated with any antihypertensive agent. Minoxidil should generally be co-administered with both a beta-blocker and a loop diuretic. Thiazide diuretics are usually insufficient to manage the fluid retention that minoxidil produces.

Minoxidil is reserved for severe, treatment-resistant hypertension unresponsive to multiple agents. Dosing ranges from 2.5–100 mg daily in one to two divided doses. Hypertrichosis — hair growth in non-scalp areas including the face, arms, and back — occurs in most patients and is dose-dependent. This effect led to the development of topical minoxidil for androgenetic alopecia. Pericardial effusion has been reported with prolonged use, particularly in the setting of renal failure.

Sodium nitroprusside releases nitric oxide spontaneously. Nitric oxide activates soluble guanylate cyclase in vascular smooth muscle, producing balanced arteriolar and venodilation. It is administered as an IV continuous infusion at 0.3–10 mcg/kg/min, with onset within seconds and offset within 1–2 minutes of discontinuation. The solution is light-sensitive and requires opaque covering during infusion.