New approaches to overcome tolerance to nitrates.

In isolated cells (vascular smooth muscle, endothelium, platelets), perfused hearts, in vivo experiments, conscious instrumented animals, and in human subjects the induction of tachyphylaxis and tolerance to various exogenous NO-donors was analyzed. Various ways to circumvent tolerance were successfully tested. Different nitrovasodilators were associated with different rates and magnitudes of generation of tolerance and reactive oxygen radicals (ROS) in all models tested, beginning with PETN (pentaerithrityltetranitrate) (lowest rate) and concluding with GTN (highest rate). This pattern was found in all models tested (isolated cells, perfused organs, and in vivo experiments). The observed changes in ROS production in isolated cells were identical to changes in ROS production in vascular smooth muscle, endothelial cells, and platelets. Thus, blood cells such as washed platelets could be used as marker cells to identify induction of tolerance and rise in platelet activity, closely reflecting changes in the rate of tolerance generation to nitrates associated with enhanced oxidant stress (ROS generation). Generation of tachyphylaxis could be suppressed or even avoided by supplementation of appropriate antioxidants (SOD, vitamin C, DMSO, beta-blockers with antioxidant capacity, modulators of prostanoid metabolism such as ASS) in all models tested, including human subjects. Even fully developed tolerance (during non-intermittent GTN-administration) could be reversed by starting an appropriate antioxidant supplementation. This indicates that other potential factors involved in the generation of nitrovasodilator-associated tolerance (reducing the intended vasodilation and the concovactent decreases in blood pressure, namely augmented sympathetic and RAS-activity, changes in the activity of soluble guanylyl cyclase, protein kinase C, phosphodiesterase, etc.) are of minor importance. Thus, the treatment of tolerance under clinical conditions should closely target changes in redox potential and antioxidant capacity.