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Tolerance to nitrates and simultaneous upregulation of platelet activity prevented by enhancing antioxidant state
1Institut für Angewandte Physiologie, Universität Freiburg, Germany.
Abstract:
We analysed the induction of tolerance to nitrates both in the vasculature (in vivo) and platelets (ex vivo). Simultaneously, we tested mechanisms underlying the induction of tolerance and interventions to prevent or overcome this phenomenon. For this purpose nitroglycerin (GTN 1.5 micrograms/kg per min i.v.), alone or in combination with ascorbate (55 micrograms/kg per min i.v.) as antioxidant, was infused continuously for a period of 5 days into chronically instrumented dogs. Along with haemodynamic parameters, ex vivo platelet function was continuously monitored. Following the start of GTN infusions there was a maximal coronary dilator response (245 +/- 15 microm) and, as an index of venodilation, a fall of left ventricular end-diastolic pressure (by 2.3 +/- 0.4 mmHg). Both responses declined progressively and disappeared during the infusion period. However, in combination with ascorbate as antioxidant the dilator responses were maintained fully throughout the infusion period. With GTN alone there was a progressive, unexpected upregulation of platelet activity demonstrated by enhanced thrombin-stimulated intracellular Ca2+ levels and increases in the microviscosity of platelet membranes (indicating enhanced receptor expression) associated with a progressive impairment in basal, unstimulated cGMP levels. These changes could also be prevented completely by i.v. co-administration of ascorbate. From these results it is concluded that vascular tolerance is closely reflected by simultaneous changes in platelet function and further, that both can be prevented completely by appropriate antioxidants such as ascorbate.
Insights
Nitroglycerin tolerance develops rapidly in blood vessels and platelets. Antioxidants like ascorbate prevent this tolerance by maintaining nitrate effectiveness and preventing increased platelet activity.
Area of Science:
- Pharmacology
- Cardiovascular Physiology
- Biochemistry
Background:
- Nitrate tolerance, a decrease in drug effectiveness, is a common clinical issue.
- Understanding the mechanisms of nitrate tolerance is crucial for optimizing cardiovascular therapy.
Purpose of the Study:
- To investigate the development of tolerance to nitroglycerin (GTN) in both vascular and platelet functions.
- To explore the underlying mechanisms of GTN tolerance and the potential of antioxidants to prevent it.
Main Methods:
- Continuous intravenous infusion of GTN (1.5 µg/kg/min) alone or with ascorbate (55 µg/kg/min) for 5 days in chronically instrumented dogs.
- Monitoring of hemodynamic parameters (coronary vasodilation, left ventricular end-diastolic pressure) and ex vivo platelet function (thrombin-stimulated intracellular Ca2+ levels, membrane microviscosity, cGMP levels).
Main Results:
- GTN infusion led to progressive loss of vascular dilator responses and impaired platelet function (decreased cGMP, increased activity).
- Co-administration of ascorbate completely prevented the development of tolerance, maintaining vascular responses and normal platelet function.
- Ascorbate effectively counteracted the enhanced platelet activity and preserved basal cGMP levels during GTN infusion.
Conclusions:
- Vascular tolerance to nitrates is closely mirrored by simultaneous alterations in platelet function.
- Antioxidants, such as ascorbate, can effectively prevent the induction of nitrate tolerance in both vascular and platelet systems.