Passage of extravascular tracers into canine jugular veins and carotid arteries

Biologically active substances arising in the interstitial space (cell-fiber matrix) of a medium-sized blood vessel or in surrounding tissue might reach the lumen by diffusion. Substances so delivered to the vessel wall-blood interface would be situated to effectively influence the endothelium and to initiate deposition of blood elements on the luminal surface of vessels, thereby contributing to thrombosis and atherosclerosis. This study showed that 125I, 125I-albumin and 125I-fibrinogen passed across the walls of segments of canine jugular veins and carotid arteries that were maintained under 20 cm H2O presure while being suspended in solutions containig the radioactive substances. The ratio of 125I to 125I-protein (albumin, fibrinogen) was predictably greatly increased by diffusion across the vessel wall. Frozen sections cut parallel with the luminal surface of flattened segments of vessels showed a gradient of radioactivity from adventitial to luminal surface of the vessels. Part of the fibrogen (but no albumin) that had reached the inside of the vessel had been broken down into fragments. These observations show that ions and proteins originating in perivascular fluid transverse the wall of medium-sized veins and arteries, presumably by diffusion across complex water-filled channels. Thus biologically active substances arising in perivascular tissue or in the vessel wall itself can be expected to reach the luminal surface of the vessel wall where they can influence endothelium and/or initiate deposition of blood elements on the vessel wall. Furthermore, the action of proteolytic enzymes on protein molecules such as zymogens might give rise to active substances not previously present.