Differentiation by hydroquinone of relaxations induced by exogenous and endogenous nitrates in non‐vascular smooth muscle: role of superoxide anions

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Abstract
1 The influence of hydroquinone on relaxations induced by nitric oxide (NO), nitrovasodilator drugs, and non-adrenergic, non-cholinergic (NANC) field stimulation has been investigated in three tissues in which endogenous nitrates have been implicated in the NANC response; the mechanism of action of hydroquinone was also studied. 2 In mouse anococcygeus, hydroquinone (10–100 μm) produced a concentration-dependent inhibition of relaxations induced by 15 μm NO. Hydroquinone, 100 μm, which reduced responses to NO by 85%, had no effect on relaxations induced by NANC field stimulation (10 Hz; 20s trains), hydroxylamine (10 μm), sodium nitroprusside (1 μm) or sodium azide (20 μm). 3 In guinea-pig trachea, 100 μm hydroquinone reduced relaxations to 150 μm NO by 75%, but had no effect on those to NANC stimulation (10 Hz; 30 s trains) or sodium azide (5 μm). 4 In rat gastric fundus, 100 μm hydroquinone reduced relaxations to 1 μm NO by 85%, but had no effect on those to NANC stimulation (0.5 Hz; 15 s trains) or sodium azide (2 μm). 5 Superoxide dismutase (SOD; 50 u ml−1) had no effect on relaxations of the mouse anococcygeus in response to 15 μm NO or 10 Hz NANC stimulation. Further, the inhibition of responses to NO by hydroquinone was unaffected in the presence of SOD. 6 Hydroquinone (10–100 μm) failed to generate superoxide anions, as detected by a chemiluminescent assay. However, 100 μm hydroquinone, like SOD (50 u ml−1), produced almost complete inhibition of superoxide anion chemiluminescence induced by xanthine (500 μm): xanthine oxidase (0.07 u ml−1). 7 It is concluded that, in our system, hydroquinone inhibits NO by acting as a free radical scavenger rather than by generating superoxide anions. The ability of hydroquinone to block relaxations to NO, but not NANC stimulation, may suggest that the endogenous nitrate substance released by these NANC nerves may not be free NO, but may be an NO-containing, or NO-generating, molecule.