Analysis of α-smooth-muscle actin mRNA expression in rat aortic smooth-muscle cells using a specific cDNA probe

Abstract

Abstract We constructed two cDNA probes, the first of which hybridizes with all rat actin mRNAs while the second is specific for α-smooth muscle (SM) actin mRNA. Northern hybridization using these probes showed that, in normal rat aortic media, the proportion of α-SM actin mRNA expression increases during development, reaching about 90% of the total actin mRNA level in adult animals. As compared to the situation in normal aortic media, the proportion of α-SM actin mRNA was found to decrease significantly in intimal thickening 15 days after endothelial injury, i.e. when SM cells (SMCs) are actively replicating. At 60 days after injury, the SMCs were observed to have stopped dividing and to have recovered a normal content of α-SM actin mRNA. The content of α-SM actin mRNA was also selectively decreased (as compared to controls) in the hypotensive abdominal aortic media located below an aortic ligature, while it was not modified in the thoracic hypertensive segment above the same ligature. Primary cultures of rat aortic SMCs synthesize and contain low amounts of α-SM actin, but their α-SM actin mRNA content is similar to that of SMCs in vivo. As compared to primary cultures, the proportion of α-SM actin mRNA was found to be significantly decreased in SMCs at the fifth passage, at which stage it became comparable to the level of synthesized α-SM actin. Thus, the synthesis and expression of α-SM actin in SMCs appear to be regulated predominantly at the level of gene transcription in certain situations (e.g. aortic ligature in vivo and culture at the fifth passage), and predominantly at a post-transcriptional level in other situations (e.g. primary culture).