The endothelial vesicular system in cardiac capillaries subjected to hypoxia

The precise role of the numerous plasmalemmal vesicles in capillary endothelia is unclear. They undoubtedly play a part in macromolecular transport across the endothelium but, as serial section studies have shown the vast majority of vesicles to be attached to either the luminal or abluminal membrane, it is possible that transport is not their primary role. The suggestion that vesicles form a membrane reserve was explored in capillaries of isolated perfused rat hearts subjected to hypoxia, which has been reported to cause endothelial cell swelling. Luminal, "cytoplasmic," and abluminal vesicles were counted and their diameters measured in well-oxygenated and hypoxic hearts and a number of other endothelial cell parameters were measured. Total capillary and luminal cross-sectional area and luminal and abluminal membrane length decreased in hypoxia. There was no change in endothelial cell cross-sectional area despite an appearance of swelling. The numbers of abluminal and "cytoplasmic" vesicles did not change as a result of hypoxia. Luminal vesicles were, however, reduced in number but increased in size and density. The results are consistent both with the theory that the vesicles are a membrane reserve and that they may be present in order to expose membrane enzymes and receptors on the cell surface or internalise them in response to a variety of external stimuli.