Gillett, A. M., M. J. Wallace, M. T. Gillespie, and S. B. Hooper. Increased expansion of the lung stimulates calmodulin 2 expression in fetal sheep. Am J Physiol Lung Cell Mol Physiol 282: L440–L447, 2002. First published October 19, 2001; 10.1152/ajplung.00202.2001.—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 RTPCR, 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.