Pinacidil is an antihypertensive agent that has been found to increase potassium conductance. This study examined the type of K channel affected by pinacidil in cardiac myocytes. Pinacidil shortened the action potential duration in papillary muscle. The effect was reversible upon addition of glyburide, a known IKATP blocker. The effect of pinacidil was temperature-dependent. Action potential duration was shortened more rapidly and to a greater extent at 37 degrees C than at 23 degrees C. Whole-cell experiments showed that I-V curves lost rectification after pinacidil treatment. As with the action potential experiments, the effect was more rapid at 37 degrees C than at 23 degrees C. Rectification was restored after exposure to glyburide. The I-V curve generated after pinacidil exposure was similar to that observed by others after treatment with metabolic inhibitors that activate IKATP. The effect of pinacidil was also ATP-dependent. Addition of 5 mM ATP to the internal solution prevented activation of IKATP. These data indicate that pinacidil activates IKATP.