Kinetics and block of the ATP-sensitive potassium channel of frog skeletal muscle.

The patch clamp technique was used to record the activity of single ATP-sensitive potassium channels of frog skeletal muscle sarcolemma and examine the kinetics and ionic block of the channel. The channel has at least two open states and four closed states. Because of the multiple closed states channel openings occur in bursts. Bursts can be classified into two types by duration and short and long bursts are grouped together. There are also two components in the histogram of the number of openings in a burst. ATP and voltage modify burst kinetics. Depolarising the membrane reduces the mean number of openings in a burst. ATP promotes short bursts and reduces the mean duration of both long and short bursts. The kinetics of the channel are also modified by permeant ions - as external potassium ions are removed the openings at a given potential become longer. A voltage- and concentration-dependent block of unitary currents by external barium and caesium ions and by internal sodium and magnesium ions is described. Block by sodium ions is too rapid to resolve, that by barium is slow enough for individual blocking events to be seen, and that by caesium is of intermediate rate, reducing single channel amplitude and increasing open channel current noise. The results were interpreted as a blocking ion entering the channel in response to an electric field and obstructing potassium ion flow. Block by external caesium ions is highly voltage-dependent and provides evidence that the channel can contain more than one ion at a time. Internal sodium and magensium ions block the channel at physiological concentrations, and sodium ions can permeate the channel when the driving force is sufficiently great