We characterized the stimulatory effects of both glucocorticoids and thyroid hormones on the surfactant system in human fetal lung. Synthesis of phosphatidylcholine (PC) and morphology were examined in explant cultures (15-24 weeks gestation) maintained 1-7 days in serum-free Waymouth's medium in a 95%-air-5% CO2 atmosphere. Control explants (no hormones) had the same rate of choline incorporation into PC between 1 and 7 days, but a significant increase in tissue PC content [82 +/- 21%, (+/- SEM), day 6 vs. 1], consistent with slow turnover of PC. [3H]Choline incorporation was stimulated 36%, 137%, and 192% by T3 (2 nM), dexamethasone (Dex; 10 nM), and T3 plus Dex, respectively, after 6 days of exposure (optimal response) compared to 19%, 38%, and 84% after 2 days of exposure. Thus, a supra-additive response occurred in the presence of both hormones and was greater at a shorter exposure time. Dex increased the percent saturation of newly synthesized PC (28.9 +/- 0.9% vs. 17.8 +/- 0.8% for control), but T3 did not, whereas both hormones increased tissue PC content (74.4 +/- 7.3% and 18.7 +/- 7.8% increase vs. control, respectively). Pulse-chase experiments with [3H]choline suggest that remodeling of unsaturated PC to saturated PC occurred during culture and was stimulated by Dex. Incorporation of [3H]acetate and [3H]glycerol into PC was stimulated by Dex (830% and 77%, respectively), but not by T3; the distribution of incorporated radioactivity among phospholipids was changed by Dex (increased counts per min into PC and phosphatidylglycerol with acetate and glycerol, respectively), but not by T3. Half-maximal stimulation of choline incorporation occurred at concentrations of Dex (2.1 nM) and T3 (0.03 nM) that are similar to the Kd values for receptor binding (5 and 0.05 nM, respectively). The relative potencies of thyroid hormones were T3 greater than T4 greater than rT3, and for corticosteroids, Dex much greater than corticosterone greater than 11-dehydrocorticosterone = cortisol greater than cortisone. Stimulation by either T3 or cortisol was reversed within 24-48 h of hormone removal. Initial treatment of explants with Dex enhanced the subsequent response to T3, but not vice versa. Culture for 4-5 days in the absence of hormones produced some morphological maturation of the epithelial cells, whereas treatment with T3 plus Dex markedly increased the number and size of lamellar bodies in epithelial cells, caused extensive proliferation of apical microvilli, and reduced glycogen deposits. Our findings are consistent with receptor-mediated stimulation of surfactant synthesis in human lung by both glucocorticoids and thyroid hormones.(ABSTRACT TRUNCATED AT 400 WORDS)