Comparison of the vasorelaxant effect of nitroprusside and nitroglycerin in the human radial artery in vitro

Aims In recent years the radial artery (RA) has been re-introduced for coronary artery bypass grafting (CABG). However, the potential for vasospasm remains a clinical problem when this vessel is employed and effective vasodilator agents are required to combat vasospastic events. This in vitro study was designed to compare the vasodilator effects of sodium nitroprusside (SNP) and nitroglycerin (NTG) in the human RA. Methods Human RA segments (n=70) were taken from vessels employed for grafting in patients undergoing CABG. Concentration-relaxation curves for SNP and NTG were established in RA which had been precontracted with various vasoconstrictors (potassium chloride [K+], the thromboxane A(2) mimetic agent U46619 or endothelin-l [ET-1]). Results Both SNP and NTG caused complete relaxation and EC(50)s were similar except that NTG was 6.2-fold more potent than SNP in U46619-induced contraction (-7.50+/-0.16 vs -6.71+/-0.38 log M, P=0.04). After treatment with verapamil and NTG solution during harvesting, the RA segments responded with reduced maximal relaxation to NTG (84.9+/-3.9%, compared with 98.8+/-10.8% in the control, P=0.004). The vessel became less sensitive to NTG (EC50: - 6.29+/-0.4 vs -7.50+/-0.16 log M, P=0.01). In investigations carried out with SNP, tolerance was only seen in the magnitude of the relaxation (87.4+/-4.7% vs 99.2+/-0.6% in the control, P=0.03). Conclusions Both NTG and SNP are potent vasodilators in the RA. NTG may have more potent effects in certain;situations (constriction related to thromboxane A2) However, tolerance to NTG may develop. A cross tolerance to SNP may exist but the effect is weak so that SNP may be preferable to NTG as a vasodilator in the RA postoperatively. Other vasodilators may be the drugs of choice under such circumstances.