EFFECT OF AGE ON THE PATTERN OF SHORT-TERM ALBUMIN UPTAKE BY THE RABBIT AORTIC-WALL NEAR INTERCOSTAL BRANCH OSTIA

Lipid deposition occurs more frequently downstream than upstream of br anches in immature human aorta but the opposite pattern is seen in mat ure vessels. These distributions may reflect variation in the uptake o f plasma macromolecules by the aortic wall. We have recently shown tha t the quasi-steady state uptake of albumin is greater downstream than upstream of branches in immature rabbit aortas and that the opposite p attern occurs in mature animals. Additionally, there is a sharp drop i n the mean uptake shortly after weaning. In the present study, the mec hanisms underlying these phenomena were investigated by examining the short-term uptake of albumin and its distribution across the wall. Alb umin was labeled with a fluorescent dye and introduced into the circul ation of conscious New Zealand White rabbits. Thoracic aortas were fix ed in situ 10 minutes later and were sectioned through the center of i ntercostal ostia. Fluorescence from sections was measured by using dig ital imaging fluorescence microscopy and was converted to tracer conce ntrations after appropriate autofluorescence levels had been subtracte d. In animals aged 45 days, more tracer was detected in the wall downs tream than upstream of branches; the difference between regions was >1 00% of the mean value. This percentage halved and the mean uptake decr eased almost threefold by 75 days. In mature animals, the mean value r emained at the 75-day level but the converse distribution was seen; 22 % more trader was detected upstream than downstream. These trends were insensitive to the depth of the intimal-medial layer examined. Tn eac h region, the maximum tracer concentration occurred close to the lumin al surface but not always within the first 2.9-mu m-thick layer of the wall. Maxima were similar in magnitude to those observed at quasi-ste ady state, but the fall with increasing distance into the wall was muc h sharper. In many cases concentrations remained constant over most of the media, and rises toward the adventitial boundary were rarely seen . Uptake after 10 minutes predominantly reflects the rate at which tra cer enters the wall. The concentration profiles were consistent with m ost of the tracer having entered from the luminal surface and with the involvement of convective transport. The trends observed with age clo sely paralleled those occurring at quasi-steady state. Consequently, t he latter are also likely to be determined by changes in the resistanc e of the wall to macromolecule influx.