Regulation of lung surfactant secretion by intracellular pH

We investigated secretion of lung surfactant phosphatidylcholine (PC) using isolated perfused rat lung preparation after labeling the lung lipids in vitro with [methyl-3H]choline. The perfusion medium was Krebs-Ringer bicarbonate buffer (pH 7.4) containing 10 mM glucose and 3% fatty acid-poor bovine serum albumin. After ventilation of lungs with air containing 5% CO2 (control) for 1 h, 0.91% +/- 0.04 (mean +/- SE, n = 6) of total lung lipid radioactivity (greater than 95% in PC) was recovered in the cell-free lavage fluid. The secretion of PC was increased with terbutaline (50 microM), 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP, 100 microM), phorbol L2-myristate 13-acetate (30 ng/ml), and ATP (1 mM), in each case by approximately 150%. Secretion of PC was also increased by 160% if the lungs were ventilated with air containing 0% CO2. The low CO2-mediated PC secretion was time and concentration dependent. The dose-response curve for 0-10% CO2 was S-shaped. The low CO2-induced increase in PC secretion could be largely reversed with diffusible weak acids (25 mM, acetate or butyrate) in the perfusion medium. An increase (70%) in secretion was also induced with 10 mM NH4Cl, suggesting a role for intracellular alkalosis. These observations suggest that intracellular alkalosis stimulates lung surfactant secretion. Alkalosis-stimulated secretion of PC was additive with that with terbutaline (5 X 10(-7) to 5 X 10(-4) M) or 10(-4) M 8-BrcAMP, suggesting that alkalosis effect was not mediated through the beta-adrenergic pathway of surfactant secretion.(ABSTRACT TRUNCATED AT 250 WORDS)