Effects of a nitrate-free interval on tolerance, vasoconstrictor sensitivity and vascular superoxide production

OBJECTIVES In the present study, we tested whether a nitrate-free interval is able to prevent increases in vascular superoxide (O2•−) and the development of hypersensitivity to vasoconstrictors and whether this may result in restoration of vascular nitroglycerin (NTG) sensitivity. BACKGROUND Intermittent NTG-patch treatment (12 h patch on/patch off) has been shown to increase ischemic periods in patients with stable coronary arteries, suggesting a rebound-like situation during the patch-off period. Recently, we demonstrated that long-term treatment with NTG induces tolerance, which was in part related to increases in vascular O2•− and increased vasoconstrictor sensitivity. METHODS New Zealand white rabbits received a continuous application of NTG patches (0.4 mg/h) or an intermittent application of NTG patches (12 h patch on, 12 h patch off) for three days. Isometric tension studies were performed with aortic rings, and vascular O2•− was estimated using lucigenin-derived chemiluminescence (5 μmol/liter). Expression of the copper/zinc (Cu/Zn) superoxide dismutase (SOD) was assessed by Western blotting, and SOD activity was measured by autooxidation of 6-hydroxydopamine. RESULTS Continuous treatment with NTG caused tolerance to NTG, cross-tolerance to the endothelium-dependent vasodilator acetylcholine, increased vascular O2•−, reduced Cu/Zn SOD expression and increased sensitivity to vasoconstrictors such as phenylephrine, serotonin and angiotensin II. On/off treatment with NTG improved tolerance, corrected endothelial dysfunction and decreased vascular O2•−. In addition the reduction in SOD expression was less pronounced, whereas increases in the sensitivity to vasoconstrictors such as phenylephrine and serotonin remained nearly unchanged. CONCLUSIONS Enhanced vasoconstrictor sensitivity may explain, at least in part, the rebound phenomena observed in patients during a 12-h NTG patch-off period.