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

At low doses, what is the primary effect of epinephrine?

1. Vasodilation

2. Vasoconstriction

3. Improvement of cardiac contractility

4. Increased peripheral resistance

The correct answer is: Vasodilation

The primary effect of epinephrine at low doses is vasodilation, particularly in vascular beds such as muscle and the heart. At these doses, the stimulation of beta-2 adrenergic receptors predominates. These receptors, when activated, cause smooth muscle relaxation in blood vessels, leading to vasodilation. This effect is crucial as it allows for increased blood flow to skeletal muscles and essential organs, particularly during times of stress or physical activity. While epinephrine does have other effects, such as improving cardiac contractility through beta-1 adrenergic receptor activation and vasoconstriction at higher doses through alpha-1 receptor activation, these are not the primary actions seen at low doses. Understanding the specific receptor activity at different concentrations of epinephrine is essential in grasping its varied physiological effects. By focusing on the balance between beta and alpha receptor activation, it becomes clearer why vasodilation is the dominant effect at low levels of this hormone.