Potassium channels are the most widely distributed type of ion channel found in virtually all organisms. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cell types and control a wide variety of cell functions. Function Potassium channels function to conduct potassium ions down their electrochemical gradient, doing so both rapidly (up to the diffusion rate of K+ ions in bulk water) and selectively (excluding, most notably, sodium despite the sub-angstrom difference in ionic radius). Biologically, these channels act to set or reset the resting potential in many cells. In excitable cells, such as neurons, the delayed counterflow of potassium ions shapes the action potential. By contributing to the regulation of the cardiac action potential duration in cardiac muscle, malfunction of potassium channels may cause life-threatening arrhythmias. Potassium channels may also be involved in maintaining vascular tone. They also regulate cellular processes such as the secretion of hormones (e.g., insulin release from beta-cells in the pancreas) so their malfunction can lead to diseases (such as diabetes). Some toxins, such as dendrotoxin, are potent because they block potassium channels. Types There are four major classes of potassium channels: Calcium-activated potassium channel - open in response to the presence of calcium ions or other signalling molecules. Inwardly rectifying potassium channel - passes current (positive charge) more easily in the inward direction (into the cell). Tandem pore domain potassium channel - are constitutively open or possess high basal activation, such as the "resting potassium channels" or "leak channels" that set the negative membrane potential of neurons. Voltage-gated potassium channel - are voltage-gated ion channels that open or close in response to changes in the transmembrane voltage. The following table contains a comparison of the major classes of potassium channels with representative examples (for a complete list of channels within each class, see the respective class pages). For more examples of pharmacological modulators of potassium channels, see potassium channel blocker and potassium channel opener. Structure Potassium channels have a tetrameric structure in which four identical protein subunits associate to form a fourfold symmetric (C4) complex arranged around a central ion conducting pore (i.e., a homotetramer). Alternatively four related but not identical protein subunits may associate to form heterotetrameric complexes with pseudo C4 symmetry. All potassium channel subunits have a distinctive pore-loop structure that lines the top of the pore and is responsible for potassium selective permeability. There are over