Activation of the ATP-sensitive K+ channel by decavanadate in guinea-pig ventricular myocytes

To evaluate the effects of decavanadate on the ATP-sensitive K+ (KATP) channel, we applied the inside-out membrane patch-clamp technique to ventricular myocytes isolated from guinea-pig hearts. Decavanadate increased the probability of the KATP channel being open in a dose-dependent manner over the range of 0.1 to 5 mM in the presence of 0.3 mM ATP. Half-maximal activation occurred at 540 microM decavanadate and a Hill coefficient of 1.3 was obtained when the Hill equation was used to fit the dose-dependent activation for the channel by decavanadate. The half-maximum inhibition for the channel by ATP (K1/2) in the presence of 2 mM Mg2+ was 19 and 74 microM in its absence. In the presence of decavanadate, both curves shifted toward the higher concentration of ATP without a change in steepness of the slope (Hill coefficient = 2). The effect of decavanadate could be expressed by a model in which its binding prevents ATP binding from closing the channel. The estimated dissociation constant of decavanadate was 1.5 microM in the presence and 22.8 microM in the absence of Mg2+. Decavanadate reactivated the rundown channel in the absence of Mg2+ and ATP. Neither the single channel slope conductance nor the mean open and closed lifetime within the bursts of channel openings were affected by decavanadate. We conclude that internal Mg2+ is not required for the modulation produced by decavanadate, but this ion influences the channel and changes the dissociation constant of both ATP and decavanadate to the channel.