TRANSCYTOSIS OF ALPHA(1)-ACIDIC GLYCOPROTEIN IN THE CONTINUOUS MICROVASCULAR ENDOTHELIUM

By using perfusions and bolus administration, coupled with postembeddi ng immunocytochemical procedures, we have identified the structures in volved in the transport of derivatized orosomucoid (alpha(1)-acidic gl ycoprotein) across the continuous microvascular endothelium of the mur ine myocardium. Our findings indicate that: (i) monomeric orosomucoid binds to the luminal surface of the endothelium; (ii) it is restricted to caveolae during its transport across the endothelium; (iii) it is detected in the perivascular spaces at early time points (by 1 min) an d in larger quantities at later time points (>5 min) from the beginnin g of its perfusion or its intravascular administration; (iv) no orosom ucoid molecules are found in the intercellular junctions or at the abl uminal exits of interendothelial spaces; and (v) the vesicular transpo rt of orosomucoid is strongly inhibited by N-ethylmaleimide (>80%). Be cause, by size and shape, the orosomucoid qualifies as a preferential probe for the postulated small pore system, our results are discussed in relation to the pore theory of capillary permeability.