Inhibition of rat smooth muscle proliferation by radiation after arterial injury: Temporal characteristics in vivo and in vitro

Although several studies have suggested that inhibition of arterial narrowi ng by radiation after angioplasty is dependent on both time and dose, littl e is known regarding the temporal aspects of this effect and the mechanisms by which radiation affects the response of smooth muscle cells to injury. To determine the time course of inhibition of intimal hyperplasia by radiat ion, 135 rats were given single-fraction external gamma irradiation (1-10 G y) to one carotid artery at intervals from 5 days before to 5 days after bi lateral carotid artery balloon catheter injury, and intimal cross-sectional area was determined from histological sections at 20 days after injury. Th ere was a prominent time- and dose-dependent inhibition of intimal hyperpla sia by radiation when it was administered before or after balloon injury, w ith the greatest effect noted within 24 h before or after injury. To invest igate the effect of radiation on smooth muscle cell growth (by cell countin g) and proliferation, cell cycle kinetics (by BrdU incorporation), and cell killing (by clonogenic assay), smooth muscle cell cultures derived from ra t aortic explants were seeded in equine plasma to induce quiescence, and ra diation (2.5-10 Gy) was administered at various intervals before or after s ynchronous growth stimulation by 10% whole blood serum. A similar time and dose dependence was noted in growth kinetics, BrdU incorporation and cell k illing for smooth muscle cells irradiated in vitro; in each case, the effec t was most prominent for radiation administered in temporal proximity to st imulation with whole blood serum. By Western blot analysis, cultured smooth muscle cells showed a rapid time-dependent increase in Cdkn1a (formerly kn own as p21) protein expression, followed by a delayed increase in Tp53 (for merly known as p53) expression after irradiation, Activation of intracellul ar caspases, manifest by proteolytic poly(ADP-ribose) polymerase (PARP) cle avage, was not detected in smooth muscle cell cultures after irradiation. T hese observations suggest that radiation limits intimal hyperplasia in vivo by a transient, reversible process. Although apparent cytotoxic injury occ urs in vitro, apoptosis of smooth muscle cells is not apparent. Both inhibi tion of proliferation of smooth muscle cells and cell cycle delay may contr ibute to inhibition of intimal hyperplasia in vivo by radiation. (C) 2000 b r Radiation Research Society.