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

Which of the following statements about calcium and smooth muscle relaxation is true?

1. Calcium decreases smooth muscle relaxation

2. Calcium is not involved in smooth muscle relaxation

3. Calcium increases smooth muscle relaxation

4. Calcium has a neutral effect on smooth muscle

The correct answer is: Calcium increases smooth muscle relaxation

In the context of smooth muscle physiology, calcium plays a critical role in the contraction and relaxation of smooth muscle cells. When calcium ions increase within the cytoplasm of smooth muscle cells, they bind to calmodulin, which then activates myosin light chain kinase (MLCK). This enzyme phosphorylates myosin light chains, promoting contraction. However, for relaxation to occur, the calcium levels must decrease. Thus, while elevated calcium is necessary for contraction, it is actually the reduction of calcium that allows for smooth muscle relaxation. Agents that promote smooth muscle relaxation typically do so by decreasing intracellular calcium or by enhancing the activity of other molecules that cause relaxation in the presence of lower calcium levels. Therefore, the correct understanding is that calcium itself does not increase smooth muscle relaxation; rather, it is its decrease that leads to relaxation. This distinction is key to understanding the action of various pharmacological agents and physiological mechanisms at play in vascular and gastrointestinal smooth muscle function.