Temporal and spatial changes in macromolecular uptake in rat thoracic aorta and relation to [3H]thymidine uptake

Leaky endothelial junctions associated with cell turnover have been suggested to be a hydrophilic pathway for the transport of macromolecules across the vascular endothelium. To demonstrate focal increases in endothelial permeability, the occurrence of localized uptake of macromolecules in the rat thoracic aorta was studied at various time periods after intravascular administration of Evans blue-albumin (EBA) complexes. With fluorescence microscopy, EBA uptake in the rat thoracic aorta was visible either as discrete spots or as larger areas in both en face and cross-sectional preparations. The average size of EBA leaky spots increased with dye circulation time, indicating that there is a continuous influx of macromolecules through the transiently leaky junctions in these foci with subsequent diffusion in the vessel wall. There was heterogeneity in EBA spot size distribution, suggesting that endothelial cells undergoing turnover in different phases of the cell cycle might exhibit different extents of junctional leakage to macromolecules. The technique of [3H]thymidine labeling autoradiography was applied to en face preparations of the rat thoracic aorta for identifying replicating endothelial cells. The correlation of EBA leakage with [3H]thymidine-labeled endothelial cells was determined. Only 26% of endothelial cells with nuclear incorporation of [3H]thymidine were shown to be associated with EBA leaky foci. This lack of correlation suggests that alterations in endothelial junctional permeability accompanying cell turnover might occur only in some limited time periods of the cell cycle, e.g., the mitotic (M) phase, rather than the whole period of [3H]thymidine labeling.