Nitric oxide generation from nitroglycerin and other no-donors measured in the lung and studies on nitrate tolerance in the cardiovascular system

Abstract: Nitroglycerin (GTN) and compounds referred to as nitrovasodilators or NOdonors are frequently used in the treatment of ischemic heart disease The common mode of action for these drugs is liberation of nitric oxide (NO). Nitric oxide evokes relaxation of smooth muscle through activation of guanylate cyclase with subsequent formation of eGMP. In spite of the effectiveness of nitrovasodilators their utility for long-term management of disease is limited by development of tolerance, including neurohormonal effects opposing NO-mediated vasodilation (pseudotolerance) and diminished NO effect (true tolerance). Their relative importance for tolerance development in vivo remains unclear. An obstacle in solving the tolerance problem has been difficulties in measuring NO-release from GTN in vivo. An objective of the present study was to establish a model allowing on-line recording of NO generation and cardiovascular parameters upon administration of NO-donors in vivo. The established model has been applied to investigate how these parameters are affected during the course of nitroglycerin tolerance and cross-tolerance. Finally, mechanistic studies on NO generation from NO-donors have also been conducted in buffer perfused lungs. The findings demonstrate that NO generation from NO-donors in vivo can be recorded in expired air and correlates with changes in blood pressure and plasma nitrite elicited by the drug infusions. There is conspicuous distinction in the amounts and profiles of NO generation and blood pressure effects elicited by different NO donors. GTN and isosorbide dinitrate exhibited a peak and plateau pattern, GTN being the more potent, whereas SIN-1 steadily increased its NO- generation and decreased blood pressure with peak effect first after stop of the infusion. NO detected during GTN infusion was unaffected by nitric oxide synthase inhibition or infusions of thiol-containing compounds. Tachyphylaxis in NO generation from GTN was seen during continuous infusion (the peak plateau pattern described) or upon repeated infusions of the drug. Concurrent infusions of isosorbide dinitrate or isosorbide-5mononitrate also diminished NO generation from GTN whereas this was not affected by NO generation from SIN-1. Tolerance to long-term low dose GTN treatment also resulted in attenuated capacity of the drug to generate NO. Severe hypoxia potently increased NO formation from all types of NO-donors tested, including inorganic nitrite, as measured in expired air in vivo or in perfused lungs. Both endogenous L-arginine dependent NO formation and NO generation from nitrite exhibit biphasic temperature dependence. In conclusion, NO measured in expired air is a useful tool for exploration of nitrovasodilators. Tolerance and cross-tachyphylaxis is associated with decreased NO generation from GTN, and feedback from the generated NO is not the cause of decreased NO formation. Enhancement of NO formation from nitrite or organic nitrates during hypoxia might contribute to their beneficial effects in ischemic conditions. The temperature dependence of NO formation from nitrite suggests an enzymatic formation.