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We examined the effects of thrombosis on vascular tone in vivo, using small mesenteric arteries of rats (about 300 microns i.d.) and a video recording system attached to a microscope. To induce thrombosis we damaged the vessel wall over a short segment by compression and exposed the media to the bloodstream. A thrombus mainly consisting of platelets gradually enlarged at the damaged site and caused 50-100% narrowing of the lumen. The diameter at the site downstream from the thrombus did not change as long as the blood flow was not interrupted. Instead, vasoconstrictor response to serotonin (3.0 and 30 micrograms/ml) or norepinephrine (0.1, 0.3, and 1.0 microgram/ml) given topically was significantly inhibited at the site downstream from the thrombus compared with that at the upstream site (p less than 0.01). Nonthrombotic mechanical narrowing of the vascular lumen did not affect vascular constriction by these agents. After damage to the endothelium by intra-arterial infusion of a detergent (0.3% CHAPS), the inhibition of the constrictor response by the thrombus was no longer observed. Once the thrombus occluded the vascular lumen and the blood flow was interrupted, the diameter at the downstream site declined markedly, from 291 +/- 22 to 77 +/- 21 microns (mean +/- SEM, n = 7, p less than 0.01) even without endothelial damage. Mechanical nonthrombotic occlusion of the vascular lumen induced only a small diameter reduction to 241 +/- 16 microns. We conclude that a partially occlusive thrombus may release a material that inhibits vascular constriction by serotonin or norepinephrine through an endothelium-dependent mechanism and that an occlusive thrombus induces constriction of the downstream vascular bed even with endothelium.

Effects of thrombosis on vascular tone in rat mesenteric arteries with endothelium in vivo.