Which of the following biologically active substances produced by endothelial cells leads to relaxation of smooth muscle cells and inhibits platelet aggregation?
A. thromboxane
B. P-selectin
C. angiotensin II
D. nitric oxide
E. endothelin
D
Nitric oxide, a very small molecule produced by endothelial cells, was originally known as the endothelial cell-derived relaxation factor. It induces relaxation of vascular smooth muscle cells and also inhibits the aggregation of platelets. Endothelin, thromboxane A2, and angiotensin II cause vascular constriction. P-selectin mediates the rolling of leukocytes and is essential for subsequent adhesion of leukocytes to endothelial cells during inflammation.
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