Antihypertensive Drugs: Types of β-Blockers

β1 receptors are located in the heart and kidneys, where they increase heart rate, contractility, and renin release to enhance blood pressure during stress. β2 receptors are found in lungs, blood vessels, and skeletal muscles, triggering smooth muscle relaxation that causes bronchodilation and vasodilation, improving oxygen delivery and easing respiration.

Nonselective β-blockers like propranolol block both β1 and β2 receptors, reducing blood pressure but potentially worsening bronchoconstriction in patients with respiratory disorders. Selective β-blockers such as metoprolol and atenolol target only β1 receptors, effectively lowering blood pressure while avoiding airway constriction and reducing unwanted respiratory side effects.

Cardioselective β-blockers like metoprolol and atenolol specifically target β1 receptors in the heart, reducing blood pressure without inhibiting β2-mediated bronchodilation. This selectivity prevents increased airway resistance and bronchoconstriction, making them safer for patients with respiratory conditions like asthma or COPD who require blood pressure management.

Propranolol inhibits β1 receptor activity in the heart and kidneys, reducing cardiac output and renin release. This decreases blood volume and peripheral resistance, effectively lowering blood pressure. However, its nonselective nature also blocks β2 receptors, which can exacerbate respiratory conditions in susceptible patients.

Propranolol has high lipophilicity, allowing it to cross the blood-brain barrier and cause CNS-related side effects such as fatigue or depression. Atenolol, a selective β-blocker with low lipophilicity, has limited blood-brain barrier entry, minimizing CNS side effects. This makes atenolol a better choice for patients sensitive to neurological complications.

β-blockers competitively block β-receptor sites by binding where norepinephrine and epinephrine normally attach. Under stress, catecholamines typically activate these receptors to increase blood pressure and heart function. By occupying these sites, β-blockers prevent catecholamine activation, reducing cardiac output and renin release to lower blood pressure.

β2 receptors in lungs normally promote smooth muscle relaxation and bronchodilation when activated. Nonselective β-blockers like propranolol inhibit this β2-mediated bronchodilation, increasing airway resistance and worsening bronchoconstriction. This is particularly dangerous for patients with asthma or COPD, making selective β-blockers the safer therapeutic option.