Asymmetric dimethylarginine (ADMA) inhibits endothelium-dependent relaxation induced by superoxide dismutase or the PDE5 inhibitor, T0156, but not that induced by calcium ionophore A23187 in rat aorta

Mohammed Al-Zobaidy, William Martin. Glasgow university/FBLS, Glasgow/G12 8QQ, United Kingdom.

Asymmetric dimethylarginine (NG, NG-dimethyl-L-arginine; ADMA) is an endogenously produced inhibitor of nitric oxide synthase (NOS; Leiper & Vallance, 2006). We have previously shown in rat aorta that ADMA inhibits the basal activity of NO that suppresses vasoconstrictor tone, but paradoxically does not inhibit acetylcholine-induced activity of NO (Al-Zobaidy et al., 2010). The aim of the present study was to determine if the endothelium-dependent relaxation to calcium ionophore A23187, which stimulates NO production by a receptor-independent mechanism, was also insensitive to blockade by ADMA. In addition, we wished to determine the effects of ADMA on endothelium-dependent relaxation induced by superoxide dismutase (SOD) or the PDE5 inhibitor, T0156, which occur through the potentiation of basal NO activity by scavenging destructive superoxide anions and by suppressing cGMP hydrolysis, respectively (Mian & Martin, 1995; Mochida et al., 2004)

Aortic rings from female rats were mounted under 10 mN resting tension in tissue baths containing Krebs solution at 37°C. Submaximal tension was induced using phenylephrine (PE, 30-60 nM) before obtaining relaxation to A23187 (1 nM-1 µM), SOD (0.1-300 U/ml) or T0156 (1-300 nM). Relaxation induced by A23187, SOD or T0156 was also determined following treatment with ADMA (100 μM, 60 min). Since ADMA enhances vasoconstrictor tone through blockade of basal NO activity, lower concentrations of PE were employed in these experiments to ensure the tone was comparable to that of control tissues. When SOD was used as the relaxant agent, tissues were treated with 3600 U/ml catalase for 30 min to prevent the accumulation of hydrogen peroxide. Relaxations are expressed as % of PE-induced tone (mean ± SEM of n ≥ 6 observations); with statistical differences determined by ANOVA followed by the Bonferroni test.

Calcium ionophore A23187 produced concentration-dependent relaxation (Emax 96.6 ± 1.0%) that was unaffected by ADMA (Emax 91.0 ± 1.9%). SOD-induced relaxation was, however, significantly blocked by ADMA (Emax 59.6 ± 6.2% and 15.9 ± 4.2% in the absence and presence of ADMA, respectively, P<0.001). Moreover, relaxation induced by T0156 was also significantly blocked by ADMA (Emax 89.9 ± 3.2% and 39.7 ± 4.8% in the absence and presence of ADMA, respectively, P< 0.001).

In conclusion, the ability of ADMA to block relaxation induced by A23187, but not by SOD or T0156, supports our previous conclusion that this agent blocks basal but not agonist-stimulated activity of NO in rat aorta (Al-Zobaidy et al., 2010).

Al-Zobaidy, M.J., et al., 2010. Br. J. Pharmacol., 160: 1476-1483.

Leiper, J.M. & Vallance, P. 2006. Eur. J. Clin. Pharmacol., 62: 33-38.

Mian, K.B. & Martin, W. 1995. Br. J. Pharmacol., 115: 993-1000.

Mochida, H., et al., 2004. Eur. J. Clin. Pharmacol., 485: 283-288.