Determination of disaturated lecithin in rhesus monkey amniotic fluid as an index of fetal lung maturity.

Although increased concentrations of total lecithin in amniotic fluid allow prenatal assessment of fetal lung maturation, it has become clear that routine use of the L/S index may lead to a substantial number of inaccurate predictions. Since disaturated lecithin (DL) is a more specific marker of pulmonary surfactant than total lecithin, we developed a convenient method for measuring this phospholipid in amniotic fluid, and then evaluated its level in pregnant rhesus monkeys of 120 to 163 days of gestation. The method involves osmic acid destruction of unsaturated lipids, chromatographic isolation of disaturated lecithin, and quantitation by phosphorus assay. It can be performed in approximately 5 hours on 4 ml. of amniotic fluid and yield 67 +/- 3 per cent average recovery of added 14C-dipalmitolyl lecithin. The results of analyzing 36 rhesus amniotic fluid specimens showed the disaturated lecithin and the disaturated lecithin/sphingomyelin ratio (DL/S) increase sharply after 150 days of gestation, consistent with the pattern of lung maturation in this species. We conclude that disaturated lecithin can be readily quantitated in primate amniotic fluid and that its concentration, the DL/S ratio, and percentage of disaturated lecithin are potentially useful indices of fetal lung maturity for the clinical laboratory.

Interspecies variation in lung lavage and tissue saturated phosphatidylcholine.

We measured the saturated phosphatidylcholine in lung lavage fluid and in lung tissue after lavage in five vertebrate species. The amount of saturated phosphatidylcholine recovered by lung lavage and from lung tissue showed a direct log linear correlation with species alveolar surface area. The saturated phosphatidylcholine content of lung lavage fluid per square meter of alveolar surface area varied in the sequence: mouse greater than rat greater than rabbit greater than dog greater than cat, and showed a direct correlation with species respiratory rate. We compared the lavage (presumably mainly alveolar) and tissue saturated phosphatidylcholine with the theoretical minimum amount required to produce a monomolecular layer over an area equal to the computed alveolar surface area. The data suggest that there is an alveolar and a tissue reserve of saturated phosphatidylcholine. The size of the alveolar reserve varied in the sequence: mouse greater than rat greater than rabbit greater than dog greater than cat. We conclude that in each species studied there is an alveolar and tissue reserve of saturated phosphatidylcholine and that both reserves are larger in animals with rapid ventilatory rates and small alveoli than in animals with slower breathing rates and larger alveoli.

Influence of fasting on the lung.

We examined the following in fed rats and in rats fasted for 72 h: 1) the dipalmitoyl lecithin (DPL) content of lung lavage fluid and of the remaining lung tissue, 2) descending air and saline pressure-volume curves of excised lungs, and 3) the volume density of granular pneumocyte lamellar bodies. Lung tissue DPL was decreased by 27% and lavage DPL was decreased by 40% in lungs of fasted rats. The decreased lung DPL content was associated with a 20% decrease in the volume density of lamellar bodies of granular pneumocytes. In spite of the decrease in lavage DPL content, air pressure-volume curves of excised lungs were the same as curves of lungs of fed rats. Saline pressure-volume curves of excised lungs were also the same in fed and fasted rats. The amount of lavage DPL obtained from both fed (1.1 +/- 0.1 mg, n=6) and fasted (0.7 +/- 0.1 mg, n=7) rats exceeded the theoretical minimum amount of DPL (0.5 mg) required for a monomolecular film to cover the alveolar surface of the rat at functional residual capacity. If we assume that lavage DPL represents mainly DPL lining the alveolus (surface film and hypophase) the data suggest that there is an alveolar reserve of DPL above that amount needed to maintain normal alveolar stability.

Oxygen consumption by rat lung after in vivo hyperoxia.

We studied the influence of 48 and 96 hours of in vivo hyperoxia (O2 greater than 98 per cent) on O2 consumption by rat lung slices. After 48 hours of hyperoxia, lung O2 consumption expressed per mg of deoxyribonucleic acid or per left lung decreased to approximately 70 per cent of that of lungs from rats exposed to compressed air. After 96 hours of hyperoxia, there was no difference in lung O2 consumption per mg of deoxyribonucleic acid between rats exposed to O2 and those exposed to air, but lung O2 consumption per left lung was higher in rats exposed to O2 than in those exposed to compressed air. Lung ribonucleic acid content and the ratio of ribonucleic acid to deoxyribonucleic acid were significantly increased after 96 hours of hyperoxia. We concluded that the rate of lung metabolism is altered after in vivo exposure to high PO2.

Influence of fasting on lung oxygen consumption and respiratory quotient.

We measured the oxygen consumption (QO2) of lung slices from rats and rabbits and the respiratory quotient (RQ) of lung slices from fed and fasted rats. The QO2 of lung slices is lowered within 24 h after the onset of food deprivation; this decrease in QO2 lasts during at least 2 additional days of fasting and is not eliminated by addition of glucose to the reaction medium. In fed rats the RQ of lung slices after 30 min of incubation without glucose is 0.75 +/- 0.01 (mean +/- SE) and 0.96 +/- 0.02 with glucose present. Fasting for 72 h lowers the RQ of lung slices after 30 min of incubation without glucose to 0.68 +/- 0.03; addition of glucose raises the RQ of lung slices from 72-h-fasted rats to 0.76 +/- 0.02. We conclude that fasting depresses lung oxidative metabolism. In the fed rat glucose is a major substrate for oxidative processes but in the fasting rat the oxidation of glucose is impaired and lipids are an important source of lung energy.

Influence of cycloheximide on the lung.

We examined the time course of the influence of cycloheximide on descending pressure-volume curves of excised lungs and on protein and lecithin synthesis and oxygen consumption by lung slices. We also looked at the influence of cycloheximide on granular pneumocyte ultrastructure. Excised lungs from cycloheximide-treated animals are more compliant than controls. After ventilation with air, lungs from control and cycloheximide animals show increased retractive forces and a shift to the right of the deflation P-V curve. Incubation at 38 degrees C for 30 min reverses these changes in control lungs, but not in lungs from cycloheximide-treated rabbits. There is no change in liquid delfation P-V curves after cycloheximide. Cycloheximide causes an immediate decrease of 50% in incorporation of radioactive leucine into protein by lung slices. Incorporation of radioactive palmitate into lecithin and oxygen consumption are also decreased by 50% 6 h after cycloheximide. Lamellar bodies in granular pneumocytes are smaller after cycloheximide. Cycloheximide causes a significant increase in the surface density of the lamellar body envelope. Cytoplasmic area of granular pneumocytes is increased after cycloheximide.

Lung oxygen consumption and mitochondria of alveolar epithelial and endothelial cells.

We examined oxygen consumption by lung slices and measured the volume density of mitochondria of granular pneumocytes, alveolar type I cells, and alveolar capillary endothelial cells in several species. We found that lung oxygen consumption (mu-1 02 times h-1 times mg DNA-1) varies inversely with the log of animal body weight and with the species alveolar diameter and directly with the species respiratory rate. The volume density of granular pneumocyte mitochondria show a direct linear correlation with the lung's oxygen consumption and the species respiratory rate, and an inverse linear correlation with the species alveolar diameter. The volume density of mitochondria in type I alveolar epithelial cells and capillary endothelial cells, considered together, did not differ in the two species studied (mouse and rat). We conclude that there are interspecies differences in oxygen consumption by lung cells and that granular pneumocytes contribute to these differences. We suggest that, at least part of these differences, are related to interspecies differences in surfactant secretory activity.

Intraspecies differences in lung metabolism and granular pneumocyte mitochondria.

The authors used waltzing and nonwaltzing mice to examine granular pneumocyte mitochondria and lung oxygen consumption and protein synthesis. They found that the oxygen consumption of lung slices from waltzing mice is higher than that of lung slices from nonwaltzing mice. The volume density of granular pneumocyte mitochondria is higher in waltzing than nonwaltzing mice as is their number per 100 mum-3 of cytoplasm. The mean, length, width, volume and surface density of individual mitochondria are the same in both groups. The incorporation of [14-C]leucine by lung slices into protein in a surface active lung fraction is greater in lung slices from waltzing than nonwaltzing mice. This difference occurs in the face of similar levels of free leucine in both groups. The authors conclude that there are intraspecies differences in lung oxygen consumption and protein synthesis and in the volume density and number of granular pneumocyte mitochondria.