Morphological changes and smooth muscle cell proliferation after experimental excimer laser treatment. BACKGROUND. Little is known about the mechanism(s) in the development of restenosis after excimer laser angioplasty. Thus, the rationale of this study was to determine the time course of intimal and medial smooth muscle cell (SMC) proliferation and histomorphological changes after experimental excimer laser treatment. METHODS AND RESULTS. Laser ablation was performed in the right carotid artery of 34 New Zealand White rabbits after development of a fibromuscular plaque by repeated weak electrical stimulations. The vessels were excised 3, 7, 14, 21, 28, and 42 days after excimer laser treatment. Staining of alpha-actin was used to identify SMCs. In five rabbits (15%), a stenosis of more than 50% of luminal area was due to intimal proliferation of SMCs, and in four other rabbits, a total occlusion was due to organized thrombi. After the initial ablation of the performed plaque (13 +/- 6 intimal SMC layers) a continuous increase of intimal wall thickness was found from 7 +/- 6 SMC layers at 7 days to 28 +/- 5 intimal SMC layers at 28 days after excimer laser ablation (p less than 0.01). After 42 days, no additional increase of intimal thickening occurred. After bromodeoxyuridine labeling, the extent of cell proliferation (percent of cells undergoing DNA synthesis) in the intima and media was determined using a monoclonal antibody against bromodeoxyuridine. Immunohistological quantification of SMC proliferation in the intima revealed a significant increase of cells undergoing DNA synthesis at 3 (p less than 0.05) and 14 (p less than 0.01) days after laser treatment. Medial proliferation of SMCs was delayed and had a significant increase 7 days (p less than 0.05) after intervention. Twenty-one days after laser treatment, SMC proliferation in the intima as well as in the media was normalized. CONCLUSIONS. The proliferative response of SMCs after experimental excimer laser treatment will occur as a dynamic process with a maximum of SMCs undergoing DNA synthesis during 14 days after laser ablation, resulting in an increase of intimal thickening within 4 weeks after laser treatment. The extent of intimal hyperplasia due to SMC proliferation after excimer laser treatment is comparable with the effect of transluminal balloon angioplasty in this experimental model. 