The effect of the nitric oxide synthesis inhibitor L-NAME on amitriptyline-induced hypotension in rats

Hypotension induced by tricyclic antidepressants is multifactorial. Previous animal experiments suggest a contribution from nitric oxide production. Our study aimed to evaluate the role of nitric oxide in amitriptyline-induced hypotension using N-nitro-L-arginine methyl ester, a nitric oxide synthesis inhibitor, and 3-morpholino sydnonimine, a nitric oxide donor, in anesthetized rats.

Amitriptyline intoxication was induced by the continuous infusion of amitriptyline 0.625 mg/kg/min throughout the experiment in anesthetized rats. Fifteen and 25 minutes after amitriptyline infusion began, two bolus doses of 10 mg/kg of N-nitro-L-arginine methyl ester (n = 8) or an equivalent volume of 5% dextrose solution (n = 8) was administered to each rat (Protocol 1). To investigate whether the effect of N-nitro-L-arginine methyl ester on blood pressure is counteracted by 3-morpholino sydnonimine, after the same protocol of amitriptyline infusion and 5 minutes after an N-nitro-L-arginine methyl ester bolus, a bolus of 3000 nmol/kg of 3-morpholino sydnonimine was administered (n = 8) to each rat (Protocol 2). To investigate the effect of N-nitro-L-arginine methyl ester on 3-morpholino sydnonimine induced hypotension, a group of rats received a continuous infusion of 0.54 mg/kg/h of 3-morpholino sydnonimine until 50% reduction was observed in mean arterial blood pressure followed by a bolus dose of 10 mg/kg of N-nitro-L-arginine methyl ester (n = 6) or 5% dextrose solution (n = 6) (Protocol 3). Outcome measures included mean arterial blood pressure, heart rate, and QRS duration in electrocardiogram. Student's t test and survival analysis were used for selected comparisons.

For all parameters, the treatment groups were similar at baseline and at postamitriptyline periods before therapy was rendered. Amitriptyline infusion significantly reduced mean arterial blood pressure by 50.8 +/- 2.2% and prolonged QRS by 23.9 +/- 7.2% after 15 minutes. In Protocol 1, N-nitro-L-arginine methyl ester significantly increased mean arterial blood pressure compared to dextrose-treated control animals within 30 minutes (77.9 +/- 8.5% vs. 49.7 +/- 5.0% mmHg, p < 0.01, 95% CI 57.1-98.7%). QRS duration progressively increased during the amitriptyline infusion; however, there was no significant difference in QRS width between N-nitro-L-arginine methyl ester and control groups at any time point. N-nitro-L-arginine methyl ester increased survival time compared to controls (33.4 +/- 4.1 vs. 19.9 +/- 2.7 minutes, p < 0.01, 95% CI 25.4-41.3) but did not affect mortality. In Protocol 2 of continuous infusion of amitriptyline, 3-morpholino sydnonimine counteracted the N-nitro-L-arginine methyl ester-induced increase in mean arterial blood pressure. In both protocols, heart rate decreased significantly during amitriptyline infusion but there was no difference between treatment and control groups. In Protocol 3, N-nitro-L-arginine methyl ester bolus reversed 3-morpholino sydnonimine-induced hypotension compared to dextrose bolus. (83.8 +/- 5.7% vs. 54.6 +/- 4.8%, p < 0.01, 95% CI 69.2-98.4).

N-nitro-L-arginine methyl ester is found to be effective in temporarily improving hypotension and prolonging survival time but does not affect overall mortality. Because this effect was antagonized by 3-morpholino sydnonimine, nitric oxide production appears to contribute to the pathophysiology of amitriptyline-induced hypotension.