Early effects of acute γ-radiation on vascular arterial tone

1 To determine the acute effects of irradiation on the functionality of vessel, rat aortic rings were mounted in an organ bath for isometric tension measurements and irradiated (Co-60, 1 Gy min(-1), 15 min). 2 Irradiation, which is without effect on non-contracted or endothelium-denuded vessels, led to an immediate and reversible increase in vascular tone on (-)-phenylephrine (1 mu M)-precontracted aortic rings. The tension reached a plateau about 5 min after the beginning of irradiation. 3 The maximal radiation-induced contraction occurred on aortic rings relaxed by acetylcholine (ACh) (1 mu M). In this condition, the addition of catalase (1000 u ml(-1)), which reduces hydrogen peroxide, and DMSO (0.1% v/v), which scavenges hydroxyl radical, had no influence on tension level while superoxide dismutase (SOD) (100 u ml(-1)), a superoxide anion scavenger, reduced the observed contraction. A similar result was obtained in the presence of indomethacin (10 mu M), a cyclo-oxygenase blocker. 4 Pretreatment of rings with the nitric oxide synthase inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME) (10-100 mu M) inhibited the radiation-induced contraction. 5 This effect was dose rate-dependent and even occurred for a very low dose rate (0.06 Gy min(-1)). 6 The present results indicate that gamma-radiation induces an instantaneous vascular tone increase that is endothelium and dose rate-dependent. This effect is (i) maximal when nitric oxide (NO) is produced, (ii) greatly reduced by SOD and (iii) inhibited by L-NAME, suggesting a major involvement of complexes between NO and superoxide anion.