Beta-blockers in the Treatment of Hypertension
Overall, the main mechanism behind which beta blockers exert their effect is the reduction of cardiac output by lowering of the heart rate. Beta blockers bind selectively to β-adrenoceptors, acting as competitive antagonists. The three subclasses of beta blockers are non-vasodilating with β1-selectivity, non-vasodilating without β1-selectivity, and vasodilating. Beta blockers with a β1-selectivity tend to target the β1 receptor over the β2 receptor; however, their selectivity is dose-dependent and will diminish at higher doses. Beta blockers without β1-selectivity are classified as non-selective and target both the β1 and β2 adrenergic receptor. β1-adrenergic receptors are located primarily in the heart and kidneys and when stimulated cause cardiac muscle contraction as well as water and salt reabsorption. β2-adrenergic receptors are located primarily in the bronchial musculature and vascular smooth muscle. When stimulated, β2-adrenergic receptors cause bronchial dilation and blood vessel constriction, except in skeletal muscle. In addition, several beta blockers also act on the peripheral vasculature causing vasodilation. Beta blocker induced vasodilation is mediated by the blockade of α1-adrenergic receptors, stimulation of β2-adrenergic receptors, or other mechanisms independent of adrenergic antagonism.
The pharmacokinetics of beta blockers is dependent on their classification. Lipophilic drugs are quickly and completely absorbed from the gastrointestinal tract and metabolized in the gut wall and liver. Lipophilic drugs have a short half-life, typically 1-5 hours, and they easily enter the central nervous system. Whereas hydrophilic drugs are incompletely absorbed from the gastrointestinal tract and are excreted by the kidneys. Hydrophilic drugs have longer half-lives than lipophilic drugs, generally 6-24 hours and scarcely cross the blood brain barrier. Dosages for the treatment of hypertension vary depending on the drug and are typically administered orally, once or twice daily.
The most severe adverse effect of beta blockers is extreme bradycardia and AV node block. Clinicians must be aware that normal cardiac responses to exercise will be diminished and the typical increase in heart rate and cardiac output will be reduced. Since beta blockers reduce blood pressure, an often-seen side effect is orthostatic hypotension. Non-selective beta blockers should be avoided in patients with asthma and reactive airway disease. Beta blockers have also been shown to worsen glucose intolerance and mask the symptoms of hypoglycemia, hence caution should be used in patients with insulin-dependent diabetes.
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