Tolerance to organic nitroesters in experimental animals. An assessment of in vitro and in vivo investigations

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Abstract
This paper will present a short survey of the literature dealing with nitrate tolerance in experimental animals. Tolerance towards organic nitroesters has been experimentally established in animal models both in vivo and in vitro. However the dose of drug used to provoke tolerance has invariably been very high, which makes it difficult to apply experiences from animal experiments to the clinical setting. Several different hypothesis has been put forward in order to explain the mechanism of tolerance. Today, most evidence seem to point towards an altered metabolism of cGMP as a possible cause of tolerance. Cyclic GMP is the second messenger suggested to mediate the vasodilatory effect of organic nitroesters. The proposed role of critical tissue sulfhydryl groups in mediating the smooth muscle relaxant effect of nitroglycerin (NTG) have made experiments concerning reversal and prevention of tolerance to center on the use of various sulfhydryl reactive compounds. The results of these experiments are as yet ambiguous. Several studies show the existence of cross tolerance between NTG and various other vasodilatory organic nitroesters. In contrast no cross tolerance between NTG and chemically unrelated vasodilator drugs has been observed. The endogenous vasodilators acetylcholine and atrial natriuretic factor are known to induce cGMP increases. In view of this, their effect on NTG-tolerant bovine mesenteric arteries was investigated. However no cross-tolerance appears to exist under in vitro conditions.