Cell volume and rate of proliferation, but not protein expression pattern,distinguish pup/intimal smooth muscle cells from subcultured adult smooth muscle cells

Smooth muscle cells from neonatal rats and from injured blood vessels grow with a characteristic cobblestone morphology that distinguishes them from a dult smooth muscle cells. This has led to the proposition that there are tw o distinct types of smooth muscle cells with different proliferative capaci ty. Here we systematically compare the properties of subcultured adult smoo th muscle cells in culture and clonal lines of cobblestone smooth muscle ce lls from both neonatal rats and injured vessels. The cobblestone smooth mus cle cells have a significantly smaller average cell volume, estimated using two different flow cytometry measurements. However, the two types of smoot h muscle cells have indistinguishable protein expression patterns when the levels of more than 20 different proteins (including cytoskeletal proteins, matrix proteins, cytokines, cytokine receptors, adhesion molecules and enz ymes) are measured by quantitative immunofluorescence. Furthermore, in cont rast to previous observations, we demonstrate that both types of smooth mus cle cells secrete a powerful mitogenic activity. The higher cell density ac hieved by the cobblestone smooth muscle cells in culture was responsible fo r the earlier reports that this mitogenic activity was secreted only by cob blestone smooth muscle cells. We conclude that many of the differences seen between cobblestone smooth muscle cells and adult smooth muscle cells in v itro (proliferation rate, morphology, protein expression pattern, secretion of mitogenic activity) could be attributable to a stable difference in the median cell volume of the cultures.