Rabbit arterial endothelium and subendothelium. A change in interfacial free energy that may promote initial platelet...

Endothelial damage and exposure of the subendothelium is accompanied by platelet adhesion and aggregation. Thermodynamically, adhesion will occur if this process decreases the total free energy of the system. In this investigation, we have developed a contact angle technique to measure relative changes in the surface free energy of the arterial wall, before and after the endothelium has been removed. Sections of descending thoracic aorta from seven rabbits were tested with an equilibrium two-phase system of 4% polyethylene glycol (mol wt: 20,000)/4% dextran (mol wt: 2,000,000) in Hepes-buffered physiological saline. The tissue was split longitudinally, immersed in the polyethylene glycol phase, and test droplets of the denser dextran phase were placed on its surface. Advancing contact angles of the drop were measured from photomicrographs taken with polarizing optics. After testing, the endothelium was removed with a saline jet, which we had calibrated previously with scanning electron microscopy, and the tissue was retested. We measured angles of 86.0 +/- 1.1 degree (SEM), n = 64 for the intact endothelium and 20.0 +/- 0.8 degree (SEM), n = 61 on the subendothelial surface. Since the physical behavior of blood is probably similar to our polyethylene glycol bathing medium the subendothelium may also represent a surface of high interfacial free energy in vivo. Assuming that the surface energy of platelets is low, platelet adhesion to the subendothelium could reduce the total free energy of the system, by covering this high interfacial free energy surface.