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.