Fetal breathing movements (FBM) are necessary for normal fetal lung growth and maturation. Absence of FBM or lung constriction results in lung hypoplasia. The molecular mechanisms by which FBM stimulate lung growth are not known. We analyzed fetal rat lungs (E19) cultured ± lung distension and tracheal ligation. We also examined the effects of mechanical stretch on rat fetal lung type II cells, human pulmonary adenocarcinoma A549 cells, and a human pulmonary epithelial cell line (NCI-H441) that shows regulated expression of surfactant proteins SP-A and SP-B. Cells were grown to confluence on matrix-coated silastic membranes and mounted in a FX-3000 Strain Unit (Flexcell). Cyclic deformation simulating FBM was achieved by applying a vacuum of 22 kPa (5-15% radial deformation) at 50 cycles/min for 2-24h. Unstretched cells were used as controls. We separately evaluated the effects of fluid shear forces (without cellular radial elongation) and cell deformation. SP-A and SP-B mRNA abundance was assayed by northern blot. Parathyroid hormone-related protein (PTHrP, a stretch-dependent lung developmental regulator), and cyclin-dependent kinase (cdk) inhibitors were analyzed by RNase protection assay (RPA). Our results indicate that static distension for as brief as 4h decreased steady-state SP-A and SP-B mRNA levels in whole lung to 51% and 31%, respectively, of control levels (n=5-6; p<.01). In contrast, cyclic stretching of H441 cells for 24h increased SP-B>SP-A expression 2-4 fold over controls and PTHrP expression by 30-60%. These effects were not mediated by fluid shear but were partially inhibited by lanthanum (a stretch-activated channel blocker). Cyclic deformation also significantly enhanced 3H-choline incorporation into saturated phosphatidylcholine in the different epithelial culture models. Cyclic stretching for 48h altered cell cycle phase distribution, apparently by inducing a block in S→G2 progression, and did not activate apoptosis(assayed by DNA flow cytometry). The cell cycle effects do not appear to be due to transcriptional regulation of cdk inhibitor proteins. Our results in lung organ culture are consistent with observations made in vivo in tracheal ligation models, and suggest that continuous lung distension may interfere with airway epithelial cytodifferentiation. In contrast, dynamic mechanodeformation (distension-relaxation) may be a critical stimulus for fetal lung maturation. The molecular mechanisms by which mechanotransduction alters gene expression and cytodifferentiation are under investigation.