CVS Practice Test

The second messenger primarily responsible for vasoconstriction in systemic vessels is calcium. In smooth muscle cells within these vessels, the influx of calcium ions leads to contraction of the muscle fibers. This contraction narrows the diameter of the blood vessels, resulting in vasoconstriction. The increase in intracellular calcium can be stimulated by various signaling pathways, including those activated by hormones such as norepinephrine.

Other options, while they play roles in various cellular processes, do not primarily induce vasoconstriction. For instance, cAMP is typically associated with vasodilation and smooth muscle relaxation. Diacylglycerol contributes to the activation of protein kinase C and is often involved in regulating other signaling pathways but does not directly cause vasoconstriction on its own. cGMP is connected with vasodilation processes as well, particularly through relaxation of smooth muscle cells. Therefore, the role of calcium as a second messenger in inducing vasoconstriction stands out in this context.