Transport of microperoxidase across segments of cerebral arterioles under normal conditions.

Microperoxidase, injected intravenously into mice, was allowed to circulate in the blood stream for 1 min to 1 h. The amount of reaction product in the walls of cerebral arterioles was most pronounced after circulation of 40 mg peroxidase for one min, the maximum dose used. The peroxidase was transported across the endothelium in a few segments of cerebral arterioles, lying on the surface of the brain or restricted to regions of the brain parenchyma. Reaction product was observed in vesicles: 1 open to the vessel lumen, 2 freely situated in the cytoplasm of endothelial cells, and 3 open to the subendothelial basement membrane. The endothelial cells did not show leaky plasma membranes. Channels or chains or vesicles, simultaneously open to the vessel lumen and the subendothelial basement membrane, were not seen, nor were fenestrae found. Furthermore, reaction product never formed a continuous column from the first luminal to the first abluminal tight junctions between two adjacent endothelial cells. From these observations, it is suggested that vesicular transfer of intravenously injected microperoxidase may occur under normal conditions across the endothelium in segments of cerebral arterioles.