Lung fluid restriction affects growth but not airway branching of embryonic rat lung

During the later stages of fetal life lung growth and development is dependent upon a variety of factors, including a normal amount of liquid within the lung's lumen. To investigate whether embryonic lung epithelium secretes fluid and whether lung liquid is essential for proper embryonic airway lung growth and branching, we incubated 12-day rat lung primordia (term=22 days) in submersion culture in serum-free medium for 48 h in room air (21% O2/5% CO2). Under these conditions, lung growth and branching proceeded but at a slower rate when compared to growth and branching in vivo. Neither addition of serum nor incubation in a fetal O2 concentration (=3% O2) changed the growth rate or the degree of branching in vitro. The luminal area of the explant increased progressively with time in culture. Inhibitors of active Cl- secretion (200 microM bumetanide and 1 mM furosemide) significantly reduced the lumen size compared with control. A similar effect was noted with lung explants of 13-15 days of gestation. Branching morphogenesis was not impaired by lung fluid reduction. Reduction of luminal liquid significantly increased DNA synthesis of 12-day embryonic lung explants, but this effect of bumetanide and furosemide on DNA synthesis was reversed when 13-15 day lung explants were used. These data suggest that embryonic lung epithelium secretes fluid and that the secretion is chloride dependent. Lung fluid is involved in controlling lung growth but not branching of the embryonic rat lung.