Total prevention of the development of in vitro tolerance to organic nitrates. Experiments with antioxidants.

The nearly total inhibition of development of pharmacological tolerance to an organic nitrate is reported here for the first time. The development of in vitro tolerance in the rabbit aorta to isosorbide-5-mononitrate (CAS 87-33-2) was potently inhibited by five structurally unrelated antioxidants--diaminodurol, ascorbic acid, potassium sulphite, pyrogallol and quercetin. Diaminodurol, ascorbic acid and potassium sulphite decreased, but quercetin increased, the spasmolytic activity of isosorbide-5-mononitrate. Diaminodurol, potassium sulphite, quercetin and ascorbic acid potently inhibited the spasmolytic activity of nitric oxide (NO). Quercetin also inhibited the development of in vitro tolerance to glyceryl trinitrate. It is suggested that tolerance to organic nitrates is the result of biochemical damage caused by a reactive intermediate such as NO. To test this possibility directly the effect of pretreatment with NO on the spasmolytic activity of glyceryl trinitrate (CAS 55-63-0) was examined. This pretreatment produced a small but significant tolerance to glyceryl trinitrate and to SIN-1 (3-morpholinosydnone imine), which also acts through guanylate cyclase. There was no effect on the activity of the unrelated vasodilators nitrendipine and theophylline. It is concluded that the reaction between NO and soluble quanylate cyclase is a real but minor cause of tolerance to organic nitrates. Other possible mechanisms of tolerance development are discussed.