Background and purpose: Activation of plasma protein C (PC) zymogen by thrombin-thrombomodulin at the endothelial surface is an important endogenous antithrombotic mechanism. It is unknown whether activated protein C (APC) is generated in vivo in the cerebrovasculature, because there is only limited thrombomodulin expression in human brain vascular endothelium. Therefore, we tested the hypothesis that carotid occlusion produces brain-specific PC activation.
Methods: Blood samples were simultaneously collected from the ipsilateral internal jugular vein and radial artery before and during carotid cross-clamping and on "de-occlusion" in 8 awake patients undergoing routine carotid endarterectomy. Plasma PC zymogen and circulating APC levels were measured using enzyme immunocapture assay and expressed as percent of pooled plasma controls.
Results: Internal jugular vein APC levels increased 28% exclusively during carotid occlusion and then decreased 32% with de-occlusion (F=8.1, P<0.005). PC zymogen increased only 5.9% with occlusion (F=6.3, P<0.02), consistent with hemoconcentration. There were no changes in radial artery PC or APC levels.
Conclusions: These findings demonstrate brain-specific protein C activation in humans during carotid occlusion and suggest a protective role for endogenous APC generation during cerebrovascular occlusion.