Comparison of nitroprusside and nitroglycerin in inhibition of angiotensin II and other vasoconstrictor-mediated contraction in human coronary bypass conduits

Aims?To compare the effect of nitroprusside (SNP) and nitroglycerin (NTG) on angiotensin II (ANGII), endothelin-1 (ET-1), and α₁-adrenoceptor (phenylephrine, PE)-mediated contraction in internal mammary artery (IMA). Methods?Human IMA segments (n=120) taken from 37 patients were studied. Concentration-relaxation curves for SNP and NTG were established in IMA precontracted with these vasoconstrictors. Concentration-contraction curves were also constructed in IMA rings incubated with SNP and NTG (0.1 and 1?μm?) for 10?min. Results?Both SNP and NTG caused full relaxation with similar EC₅₀?s except NTG was four-fold more potent than SNP in PE-induced contraction (?7.92±0.06 vs?7.32±0.2 log?m, mean±s.e. mean, P<0.01; 95% confidence interval for the difference of the means: 0.19, 1.01 log?m?). Pretreatment with SNP (0.1 and 1?μm?) significantly depressed the contraction by ANGII from 56.6±7.7% (of 100?mm K⁺-contraction) to 18.3±8.6% and 3.9±2.1% (P=0.0001). In four rings treated with SNP, the contraction to ANGII was abolished whereas NTG did not depress ANGII-mediated contraction. Pretreatment with SNP (1?μm?), but not NTG, significantly depressed the magnitude of the PE-induced contraction from 4.7±1.2 to 1.7±0.4?g (P<0.05). Treatment with both SNP and NTG significantly increased the EC₅₀ (?5.09±0.17 log?m, P=0.0007 for SNP and ?5.40±0.06 log?M, P=0.02 for NTG). Pretreatment with SNP did not significantly change either the magnitude or the EC₅₀ of the ET-1-induced contraction. Conclusions?SNP may be advantageous compared with NTG in preventing coronary arterial graft contraction. However, once grafts have constricted to ANGII, α₁-adrenoceptor agonists, and ET-1, NTG may be only marginally advantageous.