IN UTERO LUNG LECITHIN METABOLISM IN TERM FETAL RABBITS

Abstract

Fetal rabbits at 30 days gestation received lecithin precursors ¹⁴C-palmitic acid, ³H-choline and ³²p simultaneously by injection of pregnant does. Fetuses were delivered by cesarean section from 11 min to 18 hrs after isotope injection. Part of each litter was sacrificed at birth, and the remainder allowed to air breathe for 40 minutes. Alveolar wash surfactant was recovered by lavage from the newborn lungs. The lungs were homogenized, an aliquot was reserved for parenchymal lecithin analysis, and microsomal and lamellar body (LB) fractions were isolated from the remainder by density gradient centrifugation. Lecithin labeled with palmitic acid and choline appeared very rapidly in microsomal and parenchymal fractions while ³²P labeled lecithin appeared in the same fractions after approximately 1/2 hour. The appearance of labeled lecithin in the LB or alveolar wash fractions was delayed 3 hours, a delay identical to that found in other studies in newborn rabbits. The specific activity (SA) of LB lecithin exceeded the SA of alveolar wash lecithin at each time point, and maximal SA in these fractions was not achieved by 18 hours. The SA of lecithin in the lung fractions from the new-borns allowed to air breathe approximated the SA of the lecithin from the non-breathing animals. This data for in utero term rabbits and previous studies with newborn rabbits indicate that newly formed lecithin slowly appears in the alveolar space - after a 3 hour delay - kinetics that are uniquely characteristic of lung phospholipid metabolism in the perinatal animal.

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