ENDOCARDIAL ENDOTHELIUM IN THE RAT - JUNCTIONAL ORGANIZATION AND PERMEABILITY

Selective permeability of endocardial endothelium has been suggested a s a mechanism underlying the modulation of the performance of subjacen t myocardium. In this study, we characterized the organization and per meability of junctional complexes in ventricular endocardial endotheli um in rat heart. The length of intercellular clefts viewed en face per unit endothelial cell surface area was lower, and intercellular cleft s were deeper in endocardial endothelium than in myocardial vascular e ndothelium, whereas tight junctions had a similar structure in both en dothelia. On this basis, endocardial endothelium might be less permeab le than capillary endothelium. However, confocal scanning laser micros copy showed that intravenously injected dextran 10000 coupled to Lucif er Yellow penetrated first the endocardial endothelium and later the m yocardial capillary endothelium. Penetration of dextran 10000 in myoca rdium occurred earlier through subepicardial capillary endothelium tha n through subendocardial capillary endothelium. Penetration of tracer might thus be influenced by hydrostatic pressure. Dextran of MW 40000 did not diffuse through either endocardial endothelium or capillary en dothelium. The ultrastructure df endocardial endothelium may constitut e an adaptation to limit diffusion driven by high hydrostatic pressure in the heart. Differences in paracellular diffusion of dextran 10000, between endocardial endothelium and myocardial vessels, may result fr om differing permeability properties of the endocardium and underlying myocardium.