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

Which action is primarily mediated by beta-2 receptors?

1. Increased myocardial oxygen consumption

2. Relaxation of smooth muscle in bronchioles

3. Increased systemic vascular resistance

4. Contraction of vascular smooth muscles

The correct answer is: Relaxation of smooth muscle in bronchioles

Beta-2 receptors are primarily responsible for the relaxation of smooth muscle, particularly within the bronchial passages. When these receptors are activated, they lead to bronchodilation, which is crucial in conditions such as asthma and chronic obstructive pulmonary disease (COPD). This action facilitates easier airflow in and out of the lungs by reducing the resistance in the airways, making it a pivotal function of the beta-2 adrenergic system. In contrast, activities like increased myocardial oxygen consumption, increased systemic vascular resistance, and contraction of vascular smooth muscles are associated with alpha-1 receptors or beta-1 receptors, rather than beta-2 receptors. Beta-1 receptors are mainly involved with heart rate and myocardial contractility, contributing to higher oxygen demand, while alpha-1 receptors mediate vasoconstriction, increasing systemic vascular resistance. Thus, the action primarily mediated by beta-2 receptors is indeed relaxation of smooth muscle in bronchioles.