Abstract 1813 Poster Session IV, Tuesday, 5/4 (poster 318)

Fetal lung growth is determined mainly by mechanical factors, especially distension of the lung. In fetal sheep, increased lung distension accelerates lung growth but decreases surfactant phospholipids, surfactant protein (SP) A and the number of type II cells. The following study was designed to examine the effects of changes in distension of the lung in fetal rats on mRNA for SP-A, -B, and -C.

To increase lung distension, fetuses were decapitated (DCP) at 17d by rapidly tightening a purse string suture around the neck below the mandible. This causes tracheal scarring and obstruction; the lung becomes distended with fetal lung liquid. To control for removal of the brain, we partially decapitated fetuses (P-DCP) by placing the suture above the mandible and through the mouth. In these, the site of decapitation is above the larynx so that the trachea is neither scarred nor obstructed. To decrease lung distension, we induced congenital diaphragmatic hernia (CDH) by administering nitrofen (100 mg intra-gastrically) to pregnant rats at 9.5d gestation. This causes CDH in 40 to 60% of fetuses. Controls (N-C) were nitrofen exposed fetuses without CDH. Fetuses were delivered at 22d and weighed; the lungs were removed, weighed and snap frozen in liquid N2. RNA was extracted by the RNAzol method and mRNA for SP-A, B and C were measured by RNase Protection Assay. To correct for differences in loading, we normalized results to 18S mRNA.

Increased lung distension (DCP) reduced mRNA content for SP-A to 66% (P<0.02), SP-B to 84% and SP-C to 57% (P<0.05) of P-DCP. In contrast, decreased lung distension (CDH) increased mRNA content for SP-A to 161%, SP-B to 148% (P<0.03 for both) and SP-C to 211% (P=0.07) of N-C. Intra-assay coefficient of variation (CV) was 9.6% and inter-assay CV was 7.5%.

We conclude that increased lung distension reduces mRNA content for SP-A, -B and -C and decreased lung distension has the opposite effect. These results indicate that mechanical factors not only influence growth of the fetal lung but can have marked effects on surfactant protein mRNA.