Evidence for the rapid onset of apoptosis in medial smooth muscle cells after balloon injury

Background: Vascular myocyte apoptotic cell death has been reported in human atherectomy and endarterectomy specimens and for neointimal smooth muscle cells (SMCs) in balloon-injured rat carotid arteries between 7 and 30 days after injury. However, the immediate effect of balloon injury on medical SMC viability has not been examined. Methods and Results: Rat carotid arteries were harvested at the time of balloon injury (T=0) and at 0.5, 1.2, and 4 hours after injury. Uninjured vessels or vessels harvested at the time of injury (T=0) did not display evidence of apoptosis. However, as early as 30 minutes after injury, 70% of medial SMCs appeared apoptotic by TdT- mediated dUTP nick end labeling (TUNEL) analysis and by the appearance of condensed chromatin. High frequencies of TUNNEL-positive cells were also observed at 1 and 2 hours after injury but not at 4 hours. Transmission electron microscopy revealed many cells with morphological characteristics of apoptosis in the injured sections. A marked decrease in bcl-X expression was detected in the most luminal layers of the media. To corroborate these findings in a second animal model, rabbit external iliac arteries were analyzed after balloon angioplasty. Apoptotic cell death was evident in rabbit arteries at 30 minutes and at 4 hours after injury. Conclusions: As early as 30 minutes after balloon injury, myocytes appear to undergo apoptosis cell death at a high frequency as shown by TUNEL staining, chromatin condensation, and the appearance of morphological features in electron micrographs. The induction of apoptosis coincides with a marked downregulation of bcl-X expression.