Identification of ATP-sensitive potassium channel in frog ventricular myocytes

ATP-sensitive potassium channels were found in frog ventricular myocytes using the inside-out patch-clamp technique. The channel was selectively permeable to K+. Single-channel conductance was 32.6 pS at 3.0 mM of [K+]o and 132 mM [K+]i and 77.3 pS at 114 mM [K+]o and 132 mM [K+]i. ATP did not affect single-channel conductance. The open probability of the channel was decreased by intracellular application of ATP in both the presence and absence of 2 mM MgCl2. The coexistence of Mg2+ with ATP shifts the dose-response curve for the open probability of ATP-sensitive K+ channel against ATP rightward. The shift of the curve indicates that Mg-ATP is less effective than free ATP in inhibiting the channel. An open-time histogram was fitted by a single exponential function with a time constant of 1.63 +/- 0.17 msec (n = 5) in an ATP-free medium. Mean open time (1.57 +/- 0.10 msec; n = 5) was not altered but the inter-burst time (closed time between bursts) lengthened in 10 microM ATP.