Leukocytes are required for increased lung microvascular permeability after microembolization in sheep.

We studied the effects of uneven pulmonary artery obstruction by microemboli on steady state transvascular fluid and protein exchange in normal and leukopenic sheep. We measured pulmonary artery and left atrial pressures, cardiac output, lung lymph flow, and lymph plasma protein concentrations. Sheep were made profoundly leukopenic by administration of intra-arterial mechlorethamine hydrochloride (0.4 mg/kg, two doses) and colchicine (0.1-0.2 mg/kg, anesthetized sheep only). In anesthetized sheep, we injected glass beads 200 micrometers in diameter via the right atrium to raise pulmonary vascular resistance to 2-3 times baseline values. With normal levels of circulating leukocytes, sheep developed an increased protein-rich lymph flow from the lung characteristic of increased permeability edema. Leukopenic sheep had a significantly attenuated response after embolization for equivalent degrees of vascular obstruction. In unanesthetized sheep, we continuously infused air bubbles 1 mm in diameter via the right atrium to raise pulmonary vascular resistance to about 2 times baseline values. Each sheep served as its own control. With normal circulating leukocyte levels, there was an increase in protein-rich lymph flow from the lung during embolization. When the air infusion ended, the sheep recovered to the baseline condition in 24 hours. We induced emboli with the same amount of air when the sheep were profoundly leukopenic; lymph and protein flow from the lung were significantly less for equivalent degrees of obstruction. We conclude that circulating leukocytes are essential for the microvascular injury that results in increased permeability in the lungs of sheep after microembolization.