The present study aimed to characterize the inotropic and vasodilatory properties of the K-ATP channel opener nicorandil (NIC) in isolated human cardiac tissue. For comparison, the Ca+2 channel blockers diltiazem (DIL) and nifedipine (NIF) have been studied. Concentration-dependent effects of NIC, DIL and NIF on the force of contraction (FOC) and the vascular tone have been studied on left ventricular papillary muscle strips (dilated cardiomyopathy, New York Heart Association Class IV, n = 20; nonfailing, donor hearts, n = 4), on right auricular trabeculae (nonfailing, n = 5) and on precontracted (prostaglandin F2 alpha: 0.3, 0.5 or 1 mumol/l) isolated human coronary artery rings (cardiac transplantation, n = 15). NIC, DIL and NIF concentration-dependently reduced the FOC of the papillary muscle preparations. However, the IC25 for the negative inotropic effect was significantly higher for NIC compared to DIL and NIF. The maximal negative inotropic effects of NIC, DIL and NIF (100 mumol/l) were -48.2 +/- 4.1, -92.9 +/- 0.9 and -93.4 +/- 1.4% of the basal FOC. The negative inotropic actions of NIC were similar in the human failing and the nonfailing ventricular and in the right atrial myocardium. Whereas pretreatment with methylene blue, an inhibitor of guanylyl cyclase, had no effect on the negative inotropic action of NIC, it was almost abolished by glibenclamide, a selective antagonist of the ATP-dependent K channels. NIC, DIL and NIF relaxed the coronary artery rings with 97.1 +/- 0.5, 90.7 +/- 0.9 and 96.4 +/- 0.7% of maximal relaxation (papaverine, 100 mumol/l). The rank order of vasodilatory potency was NIF > NIC > DIL. In conclusion, NIC is as effective as DIL and NIF in relaxing human coronary artery rings. However, NIC showed significantly lower negative inotropic effects when compared with the Ca+2 channel antagonists. The negative inotropic action of NIC is probably due to an interaction with the ATP-dependent K channels. In addition, activation of guanylyl cyclase does not seem to exert any negative inotropic action in the human myocardium.