Anatomic pathway of pulmonary fluid leakage in endotoxemia induced in rats. The red blood cell packing method and its application.

The anatomic site of pulmonary fluid leakage in endotoxemia in rats was investigated using the red blood cell packing method and low-dose horseradish peroxidase as a tracer. To differentiate between arterioles and venules in a given section by light and electron microscopy, human red blood cells fixed with 4% paraformaldehyde were administered to the rat pulmonary arterial trunk at a pressure of 40 cm water. Fixed red blood cells were packed in the lumina of arteries, arterioles, and a few capillaries surrounding arterioles, while veins, venules, and almost all capillaries were void of red blood cells in the lumina. Fifteen minutes after the intravenous administration of 3 and 30 mg/kg of Escherichia coli endotoxin, extravascular leakage of horseradish peroxidase from venules (nonmuscular veins) was evident. Two hours after the intravenous injection of the same doses of endotoxin, some arterioles (nonmuscular arteries) and venules (non-muscular veins) showed extravascular leakage of horseradish peroxidase, while few capillaries showed this leakage. These results suggest that pulmonary fluid leakage occurs predominantly through venules in the early phase of endotoxemia (at 15 minutes), while the arterioles contribute to fluid leakage in addition to venules in the late phase of endotoxemia (at 2 hours).