Although the inferior epigastric artery has been used as an alternative arterial graft for coronary artery bypass grafting, little is known about the contractile and relaxation characteristics of this artery. This study was designed to compare the pharmacologic reactivity of the two arterial conduits--the inferior epigastric artery and the internal mammary artery. Forty-one inferior epigastric artery ring segments from eight patients undergoing coronary grafting and 62 internal mammary artery ring segments were set up in organ baths under physiologic pressure. The contractility was determined from the contraction induced by the depolarizing agent potassium and receptor-mediated vasoconstrictor agents, norepinephrine, U46619, and endothelin-1. Endothelium-dependent relaxation was induced by the calcium ionophore A23187, a non-receptor agonist for endothelium-derived relaxing factor, and acetylcholine, a receptor agonist for endothelium-derived relaxing factor. Glyceryl trinitrate was used to study endothelium-independent relaxation. The maximal response (either contraction or relaxation) and the effective concentration causing 50% of the maximal response for these two arteries were compared. There was no difference (p > 0.05) either in the maximal contraction force (5.30 +/- 0.87 versus 4.76 +/- 0.89 gm for potassium, 5.13 +/- 0.67 versus 4.47 +/- 1.15 gm for norepinephrine, 8.04 +/- 1.23 versus 6.23 +/- 0.99 gm for U46619, and 4.88 +/- 0.69 versus 5.57 +/- 0.93 for endothelin-1 (n = 6 to 10 for each vasoconstrictor) or in the maximal relaxation induced by glyceryl trinitrate (86.46% versus 92.98%, n = 6) or by acetylcholine (20.72% versus 45.51%, n = 5) between the inferior epigastric artery and internal mammary artery. The effective concentration causing half maximal response to all vasoconstrictors and vasodilators was similar between the two arteries (p > 0.05). However, A23187 induced significantly less relaxation in the inferior epigastric artery (38.42 +/- 15.49%, n = 6) than in the internal mammary artery (71.89 +/- 7.17%, n = 9, p < 0.05). We conclude that contractility, endothelium-independent relaxation, and receptor-mediated endothelium-dependent relaxation are similar in the inferior epigastric artery and the internal mammary artery. However, the endothelium of this arterial graft has less ability to respond to the non-receptor-mediated endothelium-derived relaxing factor stimulant. The influence of this difference on the prevalence of atherosclerosis and long-term patency rate in the inferior epigastric artery remains to be studied.