Stability of pulmonary alveoli at end expiration requires a very low air-water surface tension (e.g., less than 10 mN.m-1). Another important requirement is that the surface film maintain this low surface tension for a sufficiently long time at fixed lung volume. We measured monolayer collapse rates at 37 degrees C of lung surface-active material (SAM) and certain lipids found in this material and compared them with alveolar monolayer collapse rates calculated from published lung compliance changes. We found collapse rates for purified SAM or a mixture of dipalmitoyl lecithin (DPPC):monoenoic lecithin (PC):cholesterol (CHOL) (3.03:1.65:1 molar ratios) to be much greater than collapse rates of alveolar films estimated from indirect measurements. Monolayers of pure DPPC or DPPC with 10 mol% monoenoic PC and/or CHOL had collapse rates equal to or less than those estimated from lungs. We conclude that the alveolar monolayer is enriched in DPPC to the extent of 90 mol% or greater. Enrichment may exclude more mobile components from the monolayer during expiration when surface tension reaches verry low values.

Because increased ventilation has been associated with an acceleration of lung surfactant turnover, we investigated the effect of fluid and air inflations on the release of surfactant into the air spaces. We found that excised rat lungs, initially lavaged three times at 23 degrees C, release approximately 40-90 micrograms of phospholipid/g wet lung wt into the air spaces in response to a further infusion of fluid into the airway equal to total lung capacity. A single air inflation to the same volume, followed by degassing and lavage, contributes approximately 230 micrograms to the yield of phospholipid. We estimated basal release of phospholipid as 112 micrograms wet lung wt-1 . h-1, which is far less than the 2,050 micrograms -1 . h-1 retrieved during a series of air and fluid inflations. The above findings are consistent with the hypothesis that air inflation to total lung capacity is a major physiological stimulus to release of lung surfactant into the alveolar space. The lung lavage process itself also causes the release of surfactant.

Knowledge of the dynamics of collagen turnover requires information regarding rates of synthesis of this group of connective-tissue proteins. The relationship of various amino acid pools to the tRNA precursor pool used for protein synthesis is known to vary between different cell types and tissues, even for essential amino acids. We studied extracellular, intracellular and tRNA-proline pools in cultured human lung IMR-90 fibroblasts to determine the relationship between them as candidate proline precursor pools for total protein and collagen synthesis. Time-course experiments showed that the three proline pools attained distinctly different steady-state specific radioactivities (extracellular greater than intracellular greater than tRNA) at the extracellular proline concentration of 0.2 mM. The kinetics of radioisotope incorporation into cell protein and collagenase-digestible protein indicated that the intracellular free proline pool could not be used reliably as a precursor for calculating synthetic rates. However, tRNA-proline behaved isotopically as if it were the precursor and provided synthesis rates 2-3-fold higher than those calculated by using either free proline pool. The incorporation of labelled lysine and leucine was constant over a wide range of extracellular proline concentrations. Fractional rates of protein synthesis based on tRNA-amino acid were the same with [3H]phenylalanine as with [3H]proline. The specific radioactivity of cell-associated hydroxyproline reached a steady-state value 8-10h after radioisotope administration which matched the mean tRNA-proline specific radioactivity, suggesting that tRNA-proline is not isotopically compartmentalized. A model of cellular proline-pool relationship is presented and discussed.

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