Thiol compounds and organic nitrates

Organic nitrates are widely used in the treatment of ischemic heart disease. The magnitude and duration of their circulatory and ischemic effects are, however, rapidly reduced during continuous treatment. The specific mechanisms underlying this tolerance development are not clear. According to the most widely accepted theory, tolerance is due to an intracellular depletion of thiol compounds (GSH and/or cysteine) involved in the conversion of nitrates to vasoactive intermediates. This presentation deals with aspects of in vivo thiol/nitrate interactions in different experimental and clinical conditions. The major results and conclusions are: The acute hypotensive effect of NTG is decreased by lowering of intracellular GSH levels. This finding emphasizes that normal intracellular thiol levels are required for optimal conversion of nitrates. Thus, intracellular GSH plays a critical role in the metabolism of NTG. Despite development of tolerance to the hypotensive effect of NTG, arterial and venous thiol levels are similar in nitrate tolerant and non-tolerant animals, suggesting that depletion of vascular thiol compounds may not be the cause of nitrate tolerance in vivo. The effect of exogenous thiol administration on intravascular thiol levels are different in nitrate tolerant and non-tolerant conscious rats. Exogenous thiol compounds (e.g. NAC) augments the hypotensive effect of NTG by a tolerance nonspecific mechanism. This effect is most likely mediated by an extracellular and/or membrane-related nitrate/thiol interaction and formation of NO. N-acetylcysteine inhibits angiotensin converting enzyme and counteracts nitrate-induced stimulation of the renin angiotensin system in vivo. Therefore, in addition to an effect on nitrate metabolism, thiol compounds may modify tolerance development by attenuating nitrate-induced counter-regulatory mechanisms. In the clinical setting, co-administration of NAC and ISDN delays and partially prevents tolerance to the antianginal and antiischemic effects normally seen in patients with stable angina pectoris during treatment with ISDN. N-acetylcysteine treatment in humans, potentiates and preserves nitrate induced venodilation and augments the effect of nitrates on small resistance vessels without affecting the response to nitrates in larger sized arteries. Thus, administration of NAC may change the normal vasodilator profile of nitrates. In conclusion, changes in cellular thiol levels may modify the hemodynamic effect of organic nitrates and the cellular handling of thiols and/or thiol related enzymes is altered after development of nitrate tolerance. In addition, a tolerance unrelated thiol/nitrate interaction, potentiating the effect of nitrates, may occur after administration of exogenous thiol compounds. In the clinical setting administration of thiols results in a characteristic change in the vasodilator profile of nitrates and an attenuation of the nitrate-induced stimulation of the renin-angiotensin system. The combination of these effects probably contributes to the improvement in antianginal and antiischemic parameters which may be seen during continuous and prolonged treatment with nitrates and thiol compounds.