CHANGES WITH AGE IN THE INFLUENCE OF ENDOGENOUS NITRIC-OXIDE ON TRANSPORT-PROPERTIES OF THE RABBIT AORTIC-WALL NEAR BRANCHES

Uptake of circulating albumin by the aortic wall is greater downstream than upstream of branches in immature rabbits, but the opposite patte rn occurs in mature animals. We investigated the role of NO in determi ning these variations. Descending thoracic aortas of rabbits were cann ulated using techniques that avoid depressurization, overstretching, a nd excessive fluid dynamic stresses at the endothelial surface. They w ere perfused in situ at a constant pressure and flow rate with oxygena ted, protein-containing physiological buffer, with or without N omega- monomethyl-L-arginine, an inhibitor of NO synthesis. Aortas were fixed 7 to 8 minutes after the addition of rhodamine-labeled albumin to thi s perfusate, and uptake of the tracer near intercostal ostia was measu red by digital imaging fluorescence microscopy of sections through the wall. Despite the absence of pulsatile flow, blood cells, and many pl asma components, patterns of transport in control experiments were the same as those occurring in vivo; uptake was greatest downstream of os tia in immature vessels and upstream in mature ones, although mean upt ake was higher than previously reported. In the presence of the inhibi tor, mean uptake in immature arteries was elevated threefold and the m aximum tracer concentration occurred deeper in the wall, but there was no change in the fractional difference between regions. Conversely, t he reverse of the control pattern of transport was observed ir mature arteries exposed to the inhibitor, but there was no change in mean upt ake. The reversal was almost entirely prevented by adding excess L-arg inine to the perfusate and was largely stereospecific. Endogenous NO t hus appears to determine the mature pattern of transport near branches and helps to maintain the barrier function of the immature wall.