Potassium channels are tetrameric ion channels that are widely distributed and are found in all cell types. Potassium channels control resting membrane potential in neurons, contribute to regulation of action potentials in cardiac muscle and help release of insulin form pancreatic beta cells. Broadly K+ channels are classified into voltage gated K+ channels, Hyperpolarization activated cyclic nucleotide gated K+ channels (HCN), Tandem pore domain K+ channels, Ca2+ activated K+ channels and inwardly rectifying K+ channels.
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BK channels (also called Maxi-K or slo1) are potassium ion channels. They are activated by changes in membrane electrical potential and increases in intracellular [Ca2+]. Opening of BK channels results in cell membrane hyperpolarization. BK channels are tetramers of dimer subunits formed by the association of a pore-forming alpha subunit, always derived from the same gene KCNMA1, and a modulatory beta subunit, dervied from one of 4 human genes KCNMB11-4. Intracellular calcium regulates the physical association between the alpha and beta subunits.
GABA B receptor G-protein beta-gamma and Kir3 channel complex
In muscle cells such as cardiac, skeletal, vascular and nonvascular smooth muscle, ATP sensitive K+ channels assemble as octamers of four Kir 6.x subunits and four low-affinity sulfonyl urea receptor 2 subunits (SUR2). The human gene encoding SUR2 gives rise to two splice variants, SUR2A and SUR2B. These channels are blocked by excess intracellular levels of ATP. When the ATP is low, ATP dissociates and the channel opens to allow K+ efflux ().
Binding of G beta gamma activates the GIRK/Kir3 channels that allow the efflux of K+ out of the cell resulting in a hyperpolarized membrane potential. This negative membrane potential prevents the activation of voltage dependent Ca2+ channels.
Increase in intracellular concentration of Ca2+ ions and membrane depolarization cooperatively activates BKca, which exhibit large unitary conductance. Ca2+ activated potassium channels. Activation leads to K+ efflux which changes the membrane potential, which leads to inactivation voltage activated Ca2+ channels. BKca are involved in regulation of smooth muscle tone, microbial killing in leukocytes and modulation of neurotransmitter release.
Activation of BKca channel with increase in intracellular concentration of Ca2+ leads to efflux of K+ into the extracellular space, which contributes to hyperpolarization of the membrane potential.
THIK subfamily has 2 members, THIK1 and THIK 2. THIK 1 forms functional homodimers whereas THIK2 function has not been demonstrated. THIK1 channels are inhibited by halothane. THICK1 channels form K+ leak channels and are not regulated by acidity or alkalanity changes.
Intermediate conductance K+ channels are restricted to non neuronal tissues like epithelia, blood cells and are activated by intracellular Ca2+ ion concentration.
Activation of Ca2+ activated K+ channels with intermediate conductance leads to K+ efflux in to the extracellular space.
Homotetramers of Kir 1.1 function as inwardly rectifying potassium transport channels. Ki 1.1 are found on the apical side of the cells in the ascending limp of loop of henle and upon activation transport K+ into the extracellular space. Heterotetramers of Kir 4.1 and Ki 5.1 are found on the basolateral side of cells in the distal convoluted tube. Activation of kir 4.1 and 5.1 heterotetramers leads to efflux of K+ into the extracellular space.
Small conductance Ca2+ activated potassium channels (SKca) are solely activated by intracellular Ca2+ concentration. SKca channels form functional tetramers. SKca channels control the contractility of uterus, maintian vascular tone, modulate hormone secretion, control cell volume in red blood cells and activation of microglia and lymphocytes.
Actiavtion of SKca channels is triggered by increase in the intracellular Ca2+ ion concentration. Activation of Skca channels leads to relatively small K+ ion effluxes.
TREK channels are activated by mechanical stretch, pH temperature and arachidonic acid which leads to efflux of K+ into the extracellular space resulting in membrane hyperpolarization.
Activation of voltage gated potassium channel is triggered by membrane potential changes that is sensed by the channel assembly. Activation of voltage-gated potassium channel leads to selective outward current of K+ ions.
In neuroendocrine cells such as pancreatic alpha-, beta-, and delta-cells and in the brain, ATP sensitive K+ channels assemble as octamers of four Kir 6.1, 6.2 subunits and four high-affinity sulfonyl urea receptor 1 subunits (SUR1). These channels are blocked by excess intracellular levels of ATP. When the ATP is low, ATP dissociates and the channel opens to allow K+ efflux.
Broadly K+ channels are classified into voltage gated K+ channels, Hyperpolarization activated cyclic nucleotide gated K+ channels (HCN), Tandem pore domain K+ channels, Ca2+ activated K+ channels and inwardly rectifying K+ channels.
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=1296071
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