The distribution of labeled albumin across the rabbit thoracic aorta in vivo.

125I-albumin was injected intravenously into normal conscious rabbits. The rabbits were killed after 10 minutes to 24 hours, and the descending thoracic aorta was excises immediately, opened longitudinally, rinsed, and frozen. Samples of frozen aorta were sectioned paralled to the intimal surface and washed with trichloroacetic acid (TCA) prior to counting. TCA-soluble tissue radioactivity slowly increased with time, suggesting that 125I was cleaved gradually from the labeled albumin within the aortic wall. At up to 4 hours, transmural concentration profiles of TCA-precipitable radioactivity had steep gradients near the intimal surface, moderate gradients near the medial-advential border, and were relatively falt in the middle of the media. After 24 hours, the steep intimal gradient had disappeared. Concentrations were otherwise comparable to those at 4 hours. The rate of accumulation of TCA-precipitable radioactivity was rapid initially (measurable concentrations were found throughout the media after only 10 minutes) and decreased with time. The results are consistent with entry of 125I-albumin into the media from both the luminal and adventitial sides. Approximate calculations indicate that the albumin mass transfer resistance associated with the intimal endothelium is about 1 order of magnitude greater than that associated with the media.