American Board of Surgery Qualifying Exam (ABS QE) Practice Test

Which agents primarily induce smooth muscle relaxation through vasodilation?

1. Alpha agonists

2. Beta agonists

3. Vasopressin

4. Phosphodiesterase inhibitors

The correct answer is: Beta agonists

Beta agonists primarily induce smooth muscle relaxation through vasodilation. These agents work by stimulating beta-adrenergic receptors, which leads to the relaxation of vascular smooth muscle. This process generally results in vasodilation, decreasing vascular resistance and improving blood flow. Beta agonists have specific receptor selectivity; for instance, beta-2 adrenergic receptors are particularly significant in mediating smooth muscle relaxation in vascular beds. When activated, these receptors increase intracellular cyclic AMP (cAMP) levels, which induces a cascade that ultimately leads to relaxation of smooth muscle fibers. Phosphodiesterase inhibitors can also lead to vasodilation, but their mechanism is different. They prevent the breakdown of cAMP and cyclic GMP (cGMP), enhancing their vasodilatory effects. However, their primary action is indirect, whereas beta agonists directly activate the receptors that promote relaxation. In contrast, alpha agonists generally cause vasoconstriction, not relaxation, by stimulating alpha-adrenergic receptors. Vasopressin primarily has vasopressor effects and is involved in water retention in the kidneys rather than causing relaxation of smooth muscle in a vasodilatory context.