Effect of hypoxia on endothelial morphology and interendothelial junctions in the isolated perfused rat heart.

Isolated perfused rat hearts were used to assess the effect of 30 or 60 mins of hypoxia on the ultrastructure of the capillary endothelium and particularly on the interendothelial junctions. Perfusions were carried out both in the presence and absence of albumin. Albumin had no effect on ultrastructure or membrane spacing in the interendothelial clefts, neither in oxygenated controls nor in hypoxic hearts. After 30 and 60 mins of hypoxia some capillaries showed endothelial swelling while after 60 mins the endothelium of others was attenuated. The wide regions of the intercellular cleft were not affected by hypoxia but the "narrow zone" gap between membranes became significantly smaller. We conclude that factors within the clefts other than the "tightness" of the narrow zones are responsible for changes in permeability in hypoxia and with albumin.