Differential expression of SM1 and SM2 myosin isoforms in cultured vascular smooth muscle

Ribonuclease protection assays were used to measure expression of smooth muscle (SM) specific myosin heavy chain (MHC) isoforms SM1 (204 kDa) and SM2 (200 kDa) and also nonmuscle MHC-A in cultured smooth muscle cells isolated from rat aorta. In cells grown in 10% serum for 3-5 days until subconfluent, SM1 MHC mRNA decreased by 30% and SM2 MHC mRNA decreased by 80%. In cells grown in confluency for 7-11 days, SM1 MHC mRNA decreased by 45% and SM2 MHC mRNA decreased by 80%. Similar reductions were found in passaged cells. Serum withdrawal for 1-2 days from confluent cultures had little or no effect on SM1 or SM2 MHC mRNA levels. In contrast, nonmuscle MHC-A mRNA increased 10-fold in subconfluent cultures but increased only threefold higher than controls in quiescent cells. Myosin protein analysis using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting indicated that SM1 MHC protein was detectable at a reduced level in confluent cultured cells, whereas SM2 MHC protein was absent in confluent cells. The decrease in SM2 was much greater than SM1, indicating differential regulation. An apparently new isoform of SM1 MHC migrating with a mobility similar to SM2 type MHC was detected by immunoblot analysis in cultured cells.