Pulmonary capillary permeability to HRP in dogs: a physiological and morphological study.

Pulmonary capillary permeability was studied in 10 normal anesthetized dogs using horseradish peroxidase (HRP) as tracer. Physiological studies involved measurement of left ventricular, pulmonary arterial, and wedge pressures, and of right lymphatic duct (RLD) and thoracic duct lymph flows. Plasma and interstitial oncotic pressures were calculated from protein measurements. After 1 h of lymph collection, HRP was injected intravenously at three total dose levels, low (20-30 mg/kg), medium (55 mg/kg), and high (110-120 mg/kg) as a bolus followed by smaller doses over 1.5-3 h. HRP was measured colorimetrically in plasma and lymph and appeared in the first RLD lymph sample collected at all dose levels. Electron microscopic studies to examine the passage of HRP from plasma to RLD, while less sensitive in that HRP was seen only at the high dose, showed that it crossed pulmonary capillaries under normal physiological conditions. Freeze-cleaving studies suggest this may have occurred through pathways in the interendothelial space caused by discontinuities in the junctional strands of tight junctions.