Obstruction of the fetal trachea causes the lungs to expand with accumulated liquid. Although this is a potent stimulus for lung growth, the mechanisms involved are unknown. Our aim was to identify genes that are differentially expressed as a result of increased fetal lung expansion. Using differential display RT-PCR, we isolated a cDNA fragment partially encoding calmodulin 2 (CALM2) and identified the remainder of the coding region by 5′-rapid amplification of cDNA ends. Differential expression of CALM2 was confirmed by Northern blot analysis; CALM2 mRNA levels were increased to 161 ± 5% of control at 2 days of increased lung expansion, induced by tracheal obstruction (TO), and had returned to control levels at days 4 and 10. Using in situ hybridization analysis, we found that the proportion of CALM2-labeled cells increased from 10.3 ± 1.0% to 21.4 ± 6.8% by 2 days of TO. This increase in CALM2 expression was reflected by a tendency for calmodulin protein levels to increase from 122.7 ± 17.3 to 156.5 ± 17.7 at 2 days of TO. Thus increases in fetal lung expansion result in time-dependent changes in CALM2 mRNA levels, which closely parallels the changes in lung DNA synthesis rates. As calmodulin is essential for cell proliferation, increased CALM2 mRNA levels may reflect an important role for calmodulin in expansion-induced fetal lung growth.

Increased fetal lung expansion, induced by tracheal obstruction (TO), is a potent stimulus for fetal lung growth, but rapidly reduces surfactant protein (SP) mRNA levels. Our aim was to determine the time course for the re‐expression of the surfactant proteins in fetal lung tissue following the release of a TO and to relate these to the changes in lung liquid volume. Fetal sheep were exposed to either: (1) no treatment (controls); (2) 4 days of TO; (3) 4 days of TO, followed by release of the obstruction for 24 h; (4) 4 days of TO followed by release of the obstruction for 3 days. Four days of TO increased lung liquid volumes from 26.8 ± 1.9 to 72.0 ± 5.6 ml kg−1 and reduced SP‐A, SP‐B and SP‐C mRNA levels to 38.5 ± 10.7, 56.8 ± 10.3 and 18.3 ± 5.3% of control values, respectively. One day after TO release, lung liquid volumes were reduced to 17.4 ± 5.3 ml kg−1 (control 128 days, 31.0 ± 3.8 ml kg−1) and SP‐A and SP‐B mRNA levels were not different from control levels. In contrast, SP‐C mRNA levels only increased to 45.4 ± 17.3% of control. Three days after TO release, lung liquid volumes increased to 48.0 ± 8.5 ml kg−1 and SP‐A and SP‐B mRNA levels were reduced to 48.8 ± 10.2% and 71.5 ± 19.8% of control, respectively; SP‐C mRNA levels remained at 35.3 ± 12.3% of control. Following the release of a TO, SP‐A, SP‐B and SP‐C mRNA levels were closely and inversely related to the volume of lung liquid. Based on these relationships, the lung liquid volumes that equate to 100% expression were considerably less than control lung volumes (< 10 vs. 30‐40 ml kg−1) in fetuses of this age. Thus, the changes in fetal lung SP‐A, SP‐B and SP‐C mRNA levels following the release of a TO are variable, differ between the proteins and are closely related to the changes in lung liquid volumes. We conclude that the re‐expression of surfactant proteins following TO is variable and that the change in lung liquid volume is potentially a good indicator for surfactant protein re‐expression.