Effect of intestinal bypass on the expression of actin mRNA in ileal smooth muscle

In this study, messenger RNAs (mRNAs) for actin isoforms were assessed in longitudinal smooth muscle from the ileum of unoperated rats and from rats that had undergone bypass of the middle 70% of the small intestine. The plasmid clone pGEM 10C, which contains a DNA insert complementary to the 3' untranslated region and the region of mRNA that codes for the synthesis of alpha-smooth muscle actin protein, was used to synthesize two riboprobes. One probe, complementary to the coding region of the insert, hybridizes to most, if not all, actin isoform mRNAs. The second probe, complementary to the 3' untranslated region of the insert, hybridizes only to alpha-smooth muscle actin mRNA. RNA was isolated from animals 4 to 5 days after operation, size fractionated by denaturing gel electrophoresis, transferred to nylon membranes, and exposed to the two 32P-labeled riboprobes. Both probes hybridized to RNA of about 1.3 kilobases long. Longitudinal muscle from both groups of animals contained alpha-smooth muscle actin mRNA as well as mRNA for other actin isoforms. Dot blots of varying amounts of RNA were hybridized to the riboprobes to determine the proportions of actin mRNAs. The content and concentration of mRNAs for all actins, and of mRNA for alpha-smooth muscle actin, were significantly greater in muscle from the functioning ileum of bypassed animals 4-5 days after the operation. Thus the operation induces a rapid, specific activation of these contractile protein genes.