Balloon catheter injury to rabbit carotid artery. I. Changes in smooth muscle phenotype.

Stereology was used to investigate the changes in ultrastructure of smooth muscle cells during the formation of an experimental intimal thickening induced by injury with an inflated balloon catheter. The volume density of myofilaments in the cell cytoplasm was measured in smooth muscle cell-lined areas (which are freely permeable to Evans blue dye and, hence, stain blue) and in re-endothelialized areas (which remain white after injection of Evans blue) of the rabbit carotid artery. Two weeks after injury, the volume densities of myofilaments in the intimal smooth muscle cells in both white and blue areas were significantly less than that for control medial smooth muscle (67.9% +/- 3.6%; mean +/- SE), being 38.8% +/- 1.0% and 35.9 +/- 3.3%, respectively. By 6 weeks after injury, the volume density had increased significantly in both white (55.1% +/- 3.4%) and blue areas (53.5% +/- 3.0%), and these values did not change significantly by 18 weeks. The volume density of myofilaments in the luminal (lining) smooth muscle cells in the blue areas was significantly less than that of control medial cells and remained low (26.7% +/- 2.1%) up to 18 weeks after injury. The initial balloon-induced injury caused considerable damage to the smooth muscle cells in the media, and the remaining medial cells underwent similar changes in ultrastructure to the cells in the neointima. At 2 weeks, the cells had a low volume density of myofilaments (44.9% +/- 2.4%), which returned to a level not significantly different from the control artery by 6 weeks after injury. There were no differences in the estimates of the volume density of myofilaments between the inner and outer media of the injured arteries. These findings suggest that, after injury produced by a balloon catheter, the smooth muscle cells in both the media and the resultant intimal thickening undergo a reversible change in ultrastructure.