Volume and protein concentration of epithelial lining liquid in perfused in situ postnatal sheep lungs

Previous methods of estimating the volume of epithelial lining liquid (ELL) in the air-filled lung may be said to have suffered from some theoretical and practical deficiencies. To estimate the ELL volume (VELL) more accurately, a known amount of artificial lung liquid (LL) containing the impermeant tracer 125I-albumin was instilled intraluminally in 56 in situ perfused postnatal sheep lungs (aged 36 h to 12 wk), and the rate of change of LL volume was measured by changes in the tracer concentration. Linear regression of LL volume against time allowed calculation of the VELL at the time of instillation; it was found to be 0.37 +/- 0.15 ml/kg body wt, which is equivalent to a film of 0.15 +/- 0.06 micron mean depth. The median ELL protein concentration was 36.8 mg/ml ELL, i.e., 0.60 times the plasma concentration, which agrees with previously published estimates and which is similar to the interstitial protein concentration. We conclude that the VELL is very small and that there is unlikely to be a protein osmotic gradient across the pulmonary epithelium.