cAMP increases synthesis of surfactant-associated protein A by perfused rat lung

Synthesis and secretion of surfactant-associated protein were studied in isolated rat lungs perfused with [3H]phenylalanine or [35S]methionine in synthetic medium. Surfactant was isolated by lung lavage and density-gradient centrifugation followed by dialysis to remove unincorporated amino acid and extraction with ethanol-ether to yield a delipidated protein fraction. Incorporation of [3H]phenylalanine into the delipidated surfactant protein fraction showed a lag phase of approximately 3 h followed by progressive increase over the next 3 h at a rate of 1.6 nmol.mg protein-1.h-1. With 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP, 0.1 mM) added to the perfusate, the incorporation rate between 3 and 6 h was increased by 75%. 3H specific activity in a delipidated lamellar body-rich fraction isolated from lung homogenates was unchanged by 8-BrcAMP at 3 h but was increased by 45% at 6 h. The major peak of radioactivity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of surfactant and lamellar bodies corresponded to proteins of 27-36 kDa that were identified as surfactant protein A (SP-A) by immunoblot. In the presence of 8-BrcAMP during 6 h of perfusion, specific activity of 35S-labeled SP-A in immunoprecipitated protein was increased by 93% and the SP-A mRNA content of lung was increased 145%. These results show that isolated perfused lungs synthesize and secrete surfactant-associated proteins and that the presence of a permeable cAMP analogue in the lung perfusate leads to increased secretion followed by induction of synthesis for SP-A.