BACKGROUND: To evaluate the role of free fatty acids on the ischemic myocardium, influences of various free fatty acids upon transmembrane action potential and ATP-sensitive K+(KATP) channel activity were examined in the ventricular myocardium and single cardiac myocytes. METHODS: KATP channel activities were measured in the enzymatically (collagenase) isolated single rat ventricular cardiac myocytes by the method of the excised inside-out and the cell-attached patch clamp, and transmembrane action potentials were recorded using the conventional 3M-KCl microelectode techniques in the rat ventricular myocardium. RESULTS: Free fatty acids [FFAs; arachidonic acid (AA), linoleic acid (LA) and lysophosphatidylcholine (LPC)] reduced the KATP channel activity in a dose-dependent manner in the inside-out patch, and 50%-inhibition concentrations (IC50) were 88 +/- 11.2, 49 +/- 12.5, and 188 +/- 17.4 M respectively. Both frequency of channel opening and the mean open-burst duration were markedly decreased, but the amplitude of single channel currents were not changed by the FFAs. AA (50 micrometer) and LPC (50 micrometer) did not affect the dinitrophenol (DNP, 50 micrometer)-induced KATP channel activity, whereas LA (50 micrometer) had a tendency to reduce the activity. The channel inhibition effects by 10 micrometer AA in the inside-out patch were significantly augmented by diclofenac (10 micrometer), but was not changed by nordihydroguaiaretic acid. FFAs never stimulated KATP channel activity, even in the inside-out patch where KATP channel activity reduced in the presence of internal ATP (100 micrometer). Time for 90% repolarization (APD90) significantly increased during superfusion of the FFAs, to 22 (50 micrometer AA), 24 (50 micrometer LA), and 18 (50 micrometer LPC) % from those of the contol at the time of 10 min superfusion, but the other action potential characteristics were not changed by the FFAs. AA (10 micrometer) attenuated cromakalim (10 micrometer)-induced APD90 shortening effects. CONCLUSION: It was inferred that FFAs inhibit the KATP channel activity directly by themselves and/or indirectly by their metabolites in the rat ventricular cardiomyocytes, and therefore, duration of action potential lengthens to be a burden over the ischemic myocardium accounting for the injury of myocardium at the late stage of ischemia.