Early effects of acute gamma-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 (60Co, 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 microM)-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 microM). 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 microM), 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 microM) 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.