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

What effect do alpha-2 receptors primarily mediate?

1. Vasodilation in skeletal muscle

2. Relaxation of bronchioles

3. Vasoconstriction in venous smooth muscle

4. Increased cardiac output

The correct answer is: Vasoconstriction in venous smooth muscle

Alpha-2 receptors primarily mediate vasoconstriction in venous smooth muscle. These receptors are part of the sympathetic nervous system and are located on presynaptic nerve terminals as well as on vascular smooth muscle cells. When activated, alpha-2 receptors inhibit the release of norepinephrine, which leads to a decrease in sympathetic outflow and a subsequent effect on vascular tone. The activation of alpha-2 receptors in the venous smooth muscle results in vasoconstriction, which can increase venous return to the heart and ultimately affect cardiac output. This mechanism is important in regulating blood pressure and vascular resistance. The other options refer to various physiological responses mediated by different receptor types or mechanisms. For instance, vasodilation in skeletal muscle is primarily mediated by beta-2 adrenergic receptors, while relaxation of bronchioles is predominantly a function of beta-2 receptors as well. Increased cardiac output is more directly influenced by beta-1 adrenergic receptor activation in the heart rather than alpha-2 receptor activity. Therefore, the correct answer is firmly grounded in the specific physiological role of alpha-2 receptors in vasoconstriction within the venous system.