RAT AORTIC SMOOTH-MUSCLE CELLS ISOLATED FROM DIFFERENT LAYERS AND AT DIFFERENT TIMES AFTER ENDOTHELIAL DENUDATION SHOW DISTINCT BIOLOGICAL FEATURES IN-VITRO

Endothelial denudation by balloon injury of the rat aorta induces the development of a neointima as a consequence of the migration and proliferation of smooth muscle cells (SMCs). Initially, intimal SMCs show a dedifferentiated phenotype, which reverts to a normal differentiated phenotype after endothelial cells have resurfaced the vessel lumen. We investigated in vitro the proliferative and phenotypic features of SMCs from different layers of rat aorta isolated 15 and 60 days after endothelial denudation. Freshly isolated intimal cells 15 days after balloon injury (IT-15) appeared rounded and showed a decreased content of alpha-smooth muscle actin, smooth muscle myosin, and desmin compared with intimal cells isolated 60 days after balloon injury (IT-60). No morphological and cytoskeletal differences were observed among freshly isolated IT-60 cells and other medial populations, which included medial SMCs that underlie the intimal thickening. In culture, IT-15 cells showed increased proliferative activity both in monolayers and in free-floating collagen lattices. Decreased expression of alpha-smooth muscle actin and smooth muscle myosin was documented in IT-15 cells compared with IT-60 cells and other medial SMC populations in monolayer. Moreover, IT-15 cells suspended in collagen lattices were poor at contracting these collagen lattices compared with IT-60 and control SMCs. IT-60 cells were equivalent to control SMCs at lattice contraction except for a temporary delay at day 1. Cells from the media underlying the intimal thickening isolated 15 and 60 days after balloon injury proliferated less, had an increased content of alpha-smooth muscle actin, and had a greater percentage of alpha-smooth muscle actin mRNA per total actin mRNA compared with IT-60 and control SMCs. Our model appears suitable to investigate the adaptation of differently derived SMC populations to various stimuli and factors involved in SMC phenotypic modulation.