Differentiated uptake and transcytosis of albumin in successive vascular segments.

The interaction of exogenous albumin with the continuous endothelium of large vessels and microvessels of various organs was investigated in situ in mouse. Bovine serum albumin either tagged with 5 nm gold particles (Alb-Au) or radioiodinated was perfused for 3 to 30 min. The following tissues were processed for electron microscopy: heart (coronaries and microvessels), aorta, vena cava, diaphragm (phrenic arteries, arterioles, capillaries, venules, phrenic veins), and brain cortex. Morphometric analysis showed that in all organs examined, except brain, endothelium of capillaries and postcapillary venules possesses specific binding sites for Alb-Au virtually restricted to plasmalemmal vesicles. The latter contain 1,000 times more particles than the equivalent volume of the perfusate. The Alb-Au binding is saturable and competed by monomeric albumin. Commonly, in these capillary endothelia, coated pits and coated vesicles did not bind Alb-Au. Starting with 3 min and especially at longer time points, tracer-labeled vesicles apparently discharged the ligand into the subendothelial space. At variance, in the endothelium of arteries, arterioles, muscular venules, and veins, usually few vesicles were labeled by rare particles in concentration comparable with that of the perfused tracer. In these endothelia, vesicle Alb-Au content did not increase with time and was not influenced by competition with monomeric albumin. Same differences in albumin uptake between successive vascular segments were found by light microscopy autoradiography with monomeric radioiodinated albumin perfused for 3 or 30 min. The results suggest that among vessels with continuous endothelium, albumin binds and is intensely transported in the capillaries and postcapillary venules of the diaphragm and heart. The rest of the examined vessels perform a nonspecific, low rate uptake, possibly in fluid phase.