Metabolism of exogenously administered natural surfactant in the newborn lamb.

[3H]-Palmitate labeled natural lamb surfactant and free [14C]-choline were mixed with the lung fluid of 11 term lambs at cesarean section, before the first breath. After receiving the isotope, the lambs were delivered, allowed to breathe spontaneously, and were subsequently sacrificed from 5 min to 96 h of age. Alveolar washes, lung homogenates, microsomal and lamellar body fractions of lungs, and pulmonary alveolar macrophages were examined for the presence of labeled phosphatidylcholine. Analysis of the labeled natural surfactant kinetic data revealed an apparent t 1/2 of phosphatidylcholine in the whole lung of 6.0 days. This half-life can be interpreted only as a rough estimate. Appearance of considerable [3H] labeled phosphatidylcholine in the lung homogenates demonstrated uptake of phosphatidylcholine from alveoli into lung tissue. The surfactant-associated label in homogenates was localized preferentially to lamellar body fractions. Some of the administered [14C]-choline appeared in phosphatidylcholine. Almost all of this labeled phosphatidylcholine was associated with the homogenate. Extremely small % of administered [3H] and [14C] were found in pulmonary alveolar macrophages.