Transmural concentration profiles of 125I-albumin in vivo were measured across the normal and balloon catheter-deendothelialized rabbit descending thoracic aorta as a function of time following intravenous injection. A tracer was injected 5 or 60 minutes after deendothelialization, and the animals were sacrificed after circulation times of 10, 30 or 60 minutes. The aorta was immediately excised and frozen flat between glass slides. Samples were serially sectioned parallel to the intimal surface in a refrigerated microtome, washed with trichloroacetic acid (TCA), and counted. Relative tissue concentration profiles of TCA-precipitable radioactivity from the media of control animals showed entry from both luminal and adventitial sides, as previously found with conscious normal rabbits, but spatial gradients at both luminal and medial-adventitial borders were less steep. Relative concentration levels in ballooned animals were 10- to 40-fold higher than in controls, and the profiles were flatter. Uptake rates at equivalent circulation times were greater in experiments initiated 60 minutes, as compared with 5 minutes, after deendothelialization, suggesting that progressive medial edema may have occurred following balloon injury. These results show that the intact endothelium is the dominant mass transfer resistance for 125I-albumin transport across the aortic wall. The data also suggest that the incomplete monolayer of platelets adherent to the subendothelium after balloon deendothelialization is not a substantial resistance to transport, as compared to that of the media, and that convection plays a more important role than diffusion for 125I-albumin transport across the deendothelialized aortic wall.

A method is presented for measuring the thickness of the intima-media layer of the normal rabbit descending thoracic aortic wall under both relaxed (excised) and specified simulated in vivo conditions. The in vivo conditions were simulated by maintaining the aorta in situ at its normal longitudinal extension while perfusing its lumen at the normal mean arterial pressure with a mixture of liquid silicone polymer and a catalyst, thus providing physiological radial distension. After the rubber cured, both relaxed and extended-distended tissue segments were obtained from adjacent sites on the same aorta. These tissue segments were fixed and further processed for measurement of their medial thicknesses by light microscopy. This data was used to estimate the ratio of the medial thickness of the relaxed, excised aorta to that under in vivo conditions, 1.72 +/- 0.15. This information is required for quantitative analysis of data obtained from previous studies of in vivo macromolecular transport across the rabbit thoracic aortic wall.

We have quantitated platelet adhesion to the exposed subendothelium of the balloon-deendothelialized rabbit descending thoracic aorta in vivo. Platelet adhesion was studied for elapsed times of 10 sec to 3 hr after injury. Platelet surface coverage of the thoracic aorta was measured by an intercept-counting light microscopy technique and a novel transmission electron microscopy technique. Both techniques gave similar results. Platelets adhered and spread on the exposed subendothelium forming a monolayer at all time intervals greater than 10 sec. However, platelets did not cover the entire surface. Coverage increased with time and reached a maximum of nearly 90% 3 hr after injury. Gaps between adhering platelets decreased with time and averaged approximately 1000 nm. No mural platelet thrombi were observed at any elapsed time. Comparison of our results with those reported by H. R. Baumgartner (1973, Microvasc. Res. 5, 167–179) for the rabbit abdominal aorta in vivo suggests that platelet adhesion to the exposed subendothelium may be less extensive in the thoracic aorta.