In this study, vasodilator effect of iloprost on KCl-induced contraction in human internal mammary artery (IMA) was studied and compared with other vasodilators papaverin and diltiazem. Ring segments of IMA were studied in organ baths for measurement of isometric tension. After the tissues has reached their baseline tension, precontraction was induced by 100 mm KCl and cumulative concentration-relaxation was measured by the application of iloprost (10(-9)-10(-6) m), papaverine (10(-5)-10(-4) m), diltiazem (10(-9)-10(-4) m) or ethanol; a solvent for iloprost; alone. The maximal relaxation of IMA segments to iloprost was 13.5 +/- 2.2%. Iloprost caused significantly limited relaxation when compared with papaverin (106.0 +/- 2.9%) and diltiazem (93.6 +/- 2.5%) (P < 0.001). Papaverin produced the greatest maximal relaxation to KCl-induced contraction of IMA. The potency of iloprost (-log EC(50) = 6.59 +/- 0.19) was significantly higher than those of papaverine (-log EC(50) = 4.21 +/- 0.11) and diltiazem (-log EC(50) = 5.63 +/- 0.06) (P < 0.001). In addition, -log EC(50) of diltiazem was significantly greater than papaverin (P < 0.001). Iloprost appears to be more potent than those of papaverine and diltiazem but it was inefficient in maximal inhibition on KCl-induced contraction. Iloprost may have little benefit in KCl-related vasoconstriction on human IMA segments.