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

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).

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

“…Further studies revealed that the reduced cGMP response to NTG in aortic smooth muscle from tolerant rats was associated with a marked decrease in the activity of the cGMP generating enzyme guanylate cyclase (Axelsson & Andersson 1983); the activity of the cGMP hydrolyzing enzyme cGMP phosphodiesterase was slightly elevated. Similarly, BMA made tolerant to NTG in viho exhibited a 50 per cent reduction in cGMP stimulated guanylate cyclase activity (Axelsson & Karlsson 1984), while no change in cGMP phosphodiesterase activity could be observed in these vessels. These results, as summarized in Table 1, seem to indicate an altered cGMP metabolism as a possible explanation for NTG tolerance in experimental animals.…”

“…This denitration may occur directly in the vessel wall. The possibility that a reduced metabolism of NTG in the blood vessel wall may explain tolerance development was therefore investigated (Axelsson & Karlsson 1984) but no evidence for such an effect was obtained.…”

“…Other reducing agents, such as cysteine, glutathione and ascorbate, were without effect. Since a reduced responsiveness of guanylate cyclase to NTG may be an important factor for NTG tolerance, and with the known involvement of thiols in the activation process of this enzyme by NTG, the effect of various thiols were tested on guanylate cyclase isolated from tolerant BMA (Axelsson & Karlsson 1984). None of the tested thiols (i.e.…”

Nitrate tolerance. State of the art — Chairman's summary

“…One particular problem with nitrates is that they affect several target organs (Keith et al 1982, Sutton & Fung 1983, Trompler et al 1983, Axelsson & Karlsson 1984, Kedem et al 1985, Murad et al 1985. Table 3 shows the target organs which may be effected by nitrate therapy.…”

“…The nitrates mainly affect various vessel systems, but effects have also been found in smooth muscle of the gastrointestinal tract (Keith et al 1982), and on the ischemic heart muscle itself (Kedem et al 1985). The varying effect in different vessels may be related to the anatomical origin of endothelial or vascular smooth muscle cells (Vanhoutte & Miller 1985) or the inherent cellular ability to generate cyclic GMP (Keith et al 1982, Axelsson & Karlsson 1984. Nitrates dilate veins more than arteries and small arteries are more affected than large arteries.…”