Tolerance towards nitroglycerin, induced in vivo, is correlated to a reduced cGMP response and an alteration in cGMP turnover

Abstract

Rats were made tolerant towards nitroglycerin (GTN) by subcutaneous injections of 50 mg/kg GTN, 3 times daily for 3 consecutive days. The effects of a test dose of GTN on tension and on the cGMP response were studied in aortic preparations. The activities of the enzymes regulating cGMP turnover were also investigated. In noradrenaline (2.5 μM)-contracted tissue from GTN-treated animals, the relaxant response towards the test dose of GTN (44 μM) was reduced by about 75% as compared to the ethanol-treated control rats. The cGMP-elevating action of GTN was reduced by 55%, while no significant effect on the cAMP level could be detected. The cyclic GMP-phosphodiesterase (G-PDE) activity was increased from 770 pmol/min × mg prot (ethanol-pretreated rats) to 1340 pmol/min × mg prot (GTN-pretreated animals). The guanylate cyclase activity, stimulated with 1 mM nitroprusside, was determined with both MnGTP and MgGTP as substrate and found to be reduced by about 75% in aortic homogenates from the GTN-pretreated animals. A slight depression in cGMP-dependent protein kinase activity was detected in aortas from GTN-treated animals. However, this depression seemed to be due to an increased breakdown of the activator (cGMP) since the inclusion of 2.5 mM theophylline in the assay solution abolished the difference. These results strongly support the suggestion that cGMP acts as a mediator of GTN-induced vascular smooth muscle relaxation. The tolerance towards the pharmacological action of GTN after repeated exposure could well be explained by the reduced formation and increased rate of breakdown of cGMP as demonstrated in this study.