Correct option is C
Explanation:
We are evaluating the following statements about endothelial cells and their secretion of vasoactive substances:
A. Prostacyclin produced by the endothelial cells promotes vasoconstriction.
This statement is incorrect.
Prostacyclin (PGI2) is produced by endothelial cells and promotes vasodilation, not vasoconstriction. It helps relax vascular smooth muscle and dilates blood vessels. Therefore, A is false.
B. Inhibitors of the cyclooxygenase increase the production of prostacyclin.
This statement is incorrect.
Cyclooxygenase (COX) is the enzyme responsible for synthesizing prostacyclin and other prostanoids. Inhibitors of COX, like aspirin, reduce prostacyclin production. These inhibitors decrease prostacyclin production and increase thromboxane production, which promotes vasoconstriction. Therefore, B is false.
C. When endothelial cells are stimulated by acetylcholine or serotonin, nitric oxide (NO) is released that causes relaxation of vascular smooth muscle.
This statement is correct.
Acetylcholine and serotonin are well-known stimulators of endothelial cells. They trigger the production of nitric oxide (NO), which then relaxes vascular smooth muscle, leading to vasodilation. This is a key mechanism in regulating blood flow and pressure. Hence, C is true.
D. NO is short-lived and inactivated by hemoglobin.
This statement is correct.
Nitric oxide (NO) is a short-lived molecule that has a very short half-life in the bloodstream. It is inactivated by hemoglobin, which binds NO, limiting its vasodilatory effect. This ensures that NO acts locally and does not cause widespread dilation. Hence, D is true.
Conclusion:
Statements C and D are correct.
C correctly describes the role of NO in vascular smooth muscle relaxation.
D correctly explains the short-lived nature of NO and its inactivation by hemoglobin.
