Nitroglycerin Tolerance in Vitro: Effect on cGMP Turnover in Vascular Smooth Muscle

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Abstract
Bovine mesenteric arteries (BMA) were made tolerant to nitroglycerin (GTN) by incubation with high concentrations of GTN at elevated pH. This treatment has previously been shown to reduce the relaxant and cGMP‐elevating action of a challenging dose of GTN. The stimulatory action of nitroprusside (NP) or GTN/cysteine on guanylate cyclase (GC) was reduced by 50–60% in GTN‐tolerant vessels as compared to control vessels. The stimulatory action of GTN and NP on GC has been suggested to occur through formation of S‐nitrosothiols, probably with a previous denitration step required for GTN. However, tolerance induction to GTN was not found to change the rate of nitrite formation from GTN, and exogenous addition of thiols in the GC assay, in order to increase S‐nitrosothiol formation, did not restore the GC activity in tolerant vessels back to control level. This is suggested to indicate a direct effect of GTN tolerance on GC. Since the cGMP‐phosphodiesterase activity was not affected in GTN‐tolerant vessels, the reduced GC activity may be of a crucial importance for the reduced cGMP response in GTN‐tolerant BMA as found earlier (Axelsson et al. 1982).