Uptake of low molecular weight materials in an in situ preparation of the rabbit thoracic aorta.

1. An in situ preparation of the rabbit thoracic aorta is described in which the integrity of the intimal endothelium is shown to be preserved. An extensive system of vasa vasorum and lymphatic vessels distributed throughout the adventitia, but not penetrating the media, are demonstrated using resin casts and lead chromate precipitation techniques and are shown to be patent in this preparation. 2. The role of the vasa vasorum in the medial uptake of low molecular weight materials is investigated as a function of luminal hydrostatic pressure and perfusion time. Comparison of uptake in vessels with and without patent vasa vasorum indicates that at physiological pressures medial uptake of 125I ions is dominated by entry from the adventitial surface. Analysis of the transmural distribution of 125I demonstrates that iodide is not retarded at the endothelial surface but a barrier exists within the intima which is tentatively identified as the internal elastic lamina. Qualitative studies show that Patent Blue V whose coloured anion has a molecular weight of 500 is able, at longer perfusion times, to penetrate this barrier. There is no circumferential variation in intimal uptake, but in contrast, adventitial uptake is rather patchy and generally more intense on the dorsal surfaces of the aorta. 3. Iodide transport across the intimal surface is argued to be predominantly diffusive in short duration experiments with an effective diffusion coefficient of (1.6 +/- 0.9) X 10(-6) cm2 min-1.