In cardiac autorhythmic cells it is the influx of calcium that is responsible for opening potassium channels. The efflux of potassium causes repolarization. The channels close as calcium is pumped out of the cells.
This channel is shown in its open state as a pair of ovals separated by a block arrow passing between them. This arrow represents the diffusion of potassium to the outside of the cell. The closed state is shown as a touching pair of ovals. There is a thick arrow labeled -Ca++ pointing from the open to the closed state. This indicates that potassium channels close when calcium ions are removed from the cell.
The potassium gradient favors the efflux of potassium (block arrow) because the ATP-driven sodium-potassium pump continually moves potassium into the cell. This is represented as a circle with a block arrow.
As with all the elements depicted in this map there is a population of these channels. The number of channels in each state depends on several factors:
The gated potassium channels are calcium dependent. This means that they assume the open state when intracellular calcium ions (Ca++) bind to them. This allows potassium efflux (block arrow) producing section C of the action potential graph--repolarization. The number of channels in this state depends on the concentration of free calcium ions (double-headed dashed arrow) in the cytoplasm.
The activity of the membrane-bound ATP-dependent calcium pump (circle with Ca block arrow)lowers the concentration of free calcium ions. Calcium ions bound to open potassium channels are released to replace those that have been pumped out. As free calcium ions are removed more potassium channels change to their closed stated. This event traps potassium ions--and their positive charges-- in the cell thus contributing to the prepotential shown in section A of the graph.
Last updated:7/16/2005