Local measurements of thrombus induced vasoconstriction in vivo.

No data is available describing the local effect of thrombus formation on the vessel's diameter in vivo. As platelet or thrombus induced vasoconstriction seems to be of clinical importance in various diseases, it is desirable to get insight into the interrelation of thrombus formation and local vasoactivity. With a contactless photochemical process, endothelial cells can be damaged and thrombogenesis may be initiated in vivo. After first platelets stick to endothelial cells, a strong vasoconstriction is observed in arterioles. In venules, no vascular reaction can be observed. Constriction is most pronounced at the site of thrombus formation and decreases at growing distance. The thrombus is squeezed by the vascular constriction and thus mechanically held in place and squeezed out. A slight relaxation follows 30-80 min later, but the vessel remains constricted during the following three hours. Thrombus induced vasoconstrictions can be reduced by papaverine, EP 092 (a thromboxane A2 antagonist) and phentolamine in dose-dependent manner in vivo. Papaverine was most effective followed by EP 092, Phentolamine was less effective, showing the minor importance of alpha-receptor stimulation in this model.