Search

En face autoradiography of the endothelium was used to quantify the distribution, area, and permeability of sites with enhanced permeability to 125I-low-density lipoprotein (125I-LDL) around the intercostal and celiac arteries and at the iliac bifurcation of normal rabbit aortas. The density of such sites was highest in the upper thoracic aorta and around the celiac and superior mesenteric branches and was lowest in the lower abdominal aorta. Permeable sites occurred more frequently distal to the intercostal branch orifices and both lateral and distal to the orifice at the celiac branch. At the intercostal branch orifices, these sites were larger, with a lower permeability and higher frequency than those away from the branch. At the celiac flow divider, sites of elevated autoradiographic grain density were more permeable and larger than at other locations in the abdominal aorta. Mean regional permeabilities were obtained by weighted area averages of low- and high-permeability sites. Mean regional permeabilities around the intercostal branches were 1.5 times higher than values away from the intercostal branches. Within 0.25 and 1 mm away from the celiac flow divider, mean regional permeability was 3.1 and 1.3 times higher, respectively, than those away from the flow divider. Few sites of elevated permeability were found distal to the aortoiliac bifurcation, and the permeabilities at the medial and lateral walls of the iliac arteries were not different. Mitotic cells were associated with 13 +/- 8% of all sites with elevated permeability to 125I-LDL. The frequency of mitotic endothelial cells was not increased at branch sites, suggesting that mechanisms other than cell replication were responsible for increased LDL permeability in the rabbit. These results suggest that the permeability and frequency of occurrence of sites with elevated permeability around the celiac and intercostal branches may influence the distribution and severity of early lesions in rabbits fed a hypercholesterolemic diet.

A procedure of en face quantitative autoradiography of the endothelium (Hautchen preparations) was developed to examine regional variations in 125I-low density lipoprotein (125I-LDL) permeability in the arterial wall in vivo. Endothelial preparations from fixed arterial tissue and calibration standards consisting of known concentrations of 125I-albumin were dipped in nuclear emulsion, exposed for 1-3 months, developed, and stained with hematoxylin. Digital image analysis was used to analyze dark-field images of autoradiographs. Background grain densities on cold endothelial preparations were 30-100% higher than on glass, but the variability in grain densities on the two different surfaces was similar. Regression slopes of grain density versus concentration for calibration standards were the same for sections placed on cold tissue or glass. For 1-5-microns-thick calibration standards of the same concentration, the grain density was proportional to the total amount of radioactivity per unit area. The results indicated that errors arising from nonuniformities in preparation thickness were minimal, and permeabilities and intimal concentrations could be determined. Rabbits were killed 10 minutes after injection of 125I-LDL, and endothelial preparations were made. For regions of uniformly low grain density in the rabbit aorta, the 125I-LDL permeability was 1.9 +/- 0.8 x 10(-8) cm/sec, and the effective diffusion coefficient was 5.4 +/- 3.1 x 10(-10) cm2/sec. Errors in the estimated permeability arising from nonuniformities in tissue thickness were the same as the reported experimental variability. Analysis of elevated regions of permeability suggested that 125I-LDL was binding to the extracellular matrix. Approximately 25% of the sites of elevated grain density were associated with mitotic endothelial cells, and such regions had higher permeabilities than sites associated with nonmitotic cells. Around intercostal arteries, sites of highest permeability were distal and lateral to the vessels and occurred where lesions first develop in hypercholesterolemic rabbits.