Kinetics of ATP-sensitive K+ channel revealed with oil-gate concentration jump method

Kinetics for gating the ATP-sensitive K+ channel was studied by exposing the inside-out patch to instantaneous changes in the intracellular concentration of ATP ( [ATP]i) using the oil-gate concentration jump technique in guinea pig ventricular cells. The closing time course of the channel after increasing [ATP]i was exponential with a time constant (tau), which decreased with increasing [ATP]i. The linear 1/tau - [ATP]i relation revealed two different binding (closing) rate constants (mu) of 51.7 and 5.6 mM-1.s-1 and predicted a common unbinding (opening) rate constant (lambda) of 3.2 s-1. A variable latent period was observed before channel opening when [ATP]i was decreased. The mechanism of latency is not clear. Once the channel started to open at the change lowering [ATP]i, the opening time course was exponential. Measurements of the exponential tau obtained at 0 mM [ATP]i were divided into two groups with corresponding lambda of 2.8 and 20.1 s-1, respectively. The former agrees with the predicted value of 3.2 s-1, but in the latter case, tau for opening increased as [ATP]i was increased. This increase in tau was attributed to a decrease of lambda, which approached an asymptotic value of 3.2 s-1. We conclude that binding and unbinding of one molecule of ATP determine the gating of ATP-sensitive K+ channel. Different pairs of mu and lambda result in four types of gating patterns and practically two states of sensitivities to ATP.