[14C]urea and [14C]sucrose as permeability indicators in histamine pulmonary edema

To see whether, by avoiding red blood cell transport effects, [14C]-sucrose would be a more sensitive lung vascular permeability indicator than [14C]urea, we compared the effects of 4 microgram/kg-min intravenous histamine phosphate infusions on lung vascular permeability-surface area products (PS) for each indicator in the same unanesthetized sheep. Histamine caused hematocrit, PaCO2 and pHa to increase and pulmonary arterial pressure and Pa02 to decrease. [14C]Urea PS (integral extraction calculation) increased from a base line value of 14.7 +/- 1.5 SE ml/s to 17.7 +/- 1.3 (P less than 0.05) with histamine, but [14C]sucrose PS did not change (base line, 6.5 +/- 2.9; histamine, 5.8 +/- 1.2). We conclude that [14C]urea is probably a more useful permeability indicator in the lung circulation than [14C]sucrose, because urea is more sensitive to moderate increases in permeability.