Action of cromakalim on potassium membrane conductance in isolated heart myocytes of frog

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Abstract
1 The effects of cromakalim on membrane currents were studied at 20°C in frog atrial and ventricular cells in patch clamp recording using the whole cell configuration. 2 When cromakalim (1 μm) was applied in the external medium, a time-independent current was activated in a few minutes. Cromakalim induced a weak increase of inward membrane currents recorded during hyperpolarization and a large increase of outward membrane currents recorded during depolarization. 3 The current voltage relationship of the cromakalim-induced current reversed near EK and rectified in the outward direction. 4 The cromakalim-activated current was inhibited by external application of cesium (20 mm), barium (1.8 mm), tolbutamide (1 mm) and glibenclamide (1 μm). 5 The effects of cromakalim were insensitive to a cytosolic increase in adenosine 5′-triphosphate (ATP, 3–5 mm). Cromakalim had no effects when applied in the cell. 6 Our results confirm that cromakalim activates an IK(ATP)-like conductance and suggest that the effects of the drug are due to an action on the external side of the membrane and are independent of the ATP cell content.