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The internal mammary artery (IMA) and saphenous vein (SV) are used routinely in coronary artery (CA) bypass graft surgery. The IMA may develop spasm during surgery, and the SV often develops spasm during removal from the leg. We sought to determine the relative reactivity of the canine CA, IMA, and SV to potential vasoconstrictor substances and especially to determine which vasodilator agents were effective in these different blood vessels. All vessels were arranged as ring segments suspended at optimal stretch in organ baths. Glyceryl trinitrate (GTN) caused relaxation of the three vessels but was less sensitive, less potent (as determined by EC 50 values), and had a reduced range of relaxation in the IMA. Papaverine was less sensitive in the IMA as compared with the CA and SV. Nifedipine, verapamil, and diltiazem were potent relaxing agents in all three vessels when precontracted by K+, but were less …