['The alpha adrenoceptor antagonist phentolamine prevents ischaemia related arrhythmias in rat, guinea pig, and cat heart. This effect has been related to the attenuation of ischaemia induced shortening of the action potential and has been ascribed to its alpha adrenoceptor antagonist properties. The aim of this study was to examine the effect of phentolamine on the ATP sensitive potassium channel (KATP), because this channel seems to be involved in action potential shortening during ischaemia.', 'Single channel experiments were performed on inside-out and outside-out patches of isolated rabbit ventricular cells at room temperature. Cells were isolated with conventional isolation techniques. Pipette and bath solution contained (in mmol.litre-1): K-gluconate 140, KCl 10, and HEPES-KOH 10 (pH 7.4).', 'Excision of the patch always resulted in KATP channel activity [single channel conductance 60(SD 2.8) pS n = 4], which could be completely blocked by 5 mM ATP. In 22 of 26 patches the addition of 5 microM phentolamine to the intracellular side of the membrane reduced KATP channel activity. In 17 of these patches the effect was reversible. In four patches no effect was observed. Open probability decreased by 94% (n = 12). Addition of 50 microM phentolamine resulted in the disappearance of channel activity in six of eight patches which was reversible in four patches. In outside-out patches 5 microM phentolamine was only effective in 50% of the patches, reducing open probability by 98 to 100%.'] Does phentolamine block ATP sensitive potassium channels in cardiac ventricular cells?