Nitrate tolerance: effect of thiol supplementation during prolonged nitroglycerin infusion in an in vivo rat model

Depletion of intracellular sulfhydryl groups has been considered a main reason for the development of nitrate tolerance during sustained nitrate therapy. Although administration of N-acetyl-cysteine, a sulfhydryl donor, potentiates the acute hypotensive effect of nitroglycerin (NTG), its role in the reversal of nitrate tolerance is controversial. In the present study, we developed a conscious in vivo rat model to study nitrate tolerance and nitrate-thiol interactions. Tolerance to NTG, as assessed by the blood pressure reduction in response to i.v. NTG bolus doses, developed after 24 h of i.v. NTG infusion. After 3 and 5 days of 0.2 mg/h NTG i.v., the dose-response relations for NTG-induced reduction in blood pressure were shifted to 25-fold higher doses (P less than .01). Infusion of N-acetylcysteine (0.245, 1.225 and 6.125 mmol/kg/h for 4 h) and, to a lesser extent, equimolar doses of reduced glutathione, but not N-acetylserine, significantly potentiated the hypotensive effect of NTG, in a dose-dependent manner (P less than .05). However, complete reversal of tolerance was not achieved. This animal model of nitrate tolerance is suitable for further investigations of nitrate-thiol interactions and shares similarities with nitrate tolerance development in humans. The results suggest that sulfhydryl supplementation may enhance the hypotensive effect of NTG in a dose-dependent manner. This effect is more likely to be achieved with N-acetylcysteine than with glutathione and may be related to differences in membrane permeability.