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.
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.
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.
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.
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.
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.
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.
Activation of classical Kir (K+ inwardly rectifying) channels (KCNJ2, 4, 12 and 14) results in K+ influx which contributes to the maintenance of the membrane potential (Phase 4 of the action potential). The current created by this flow of K+ is called the inward rectifying current (IK1). A channel that is inwardly-rectifying is one that passes current more easily into the cell than out of the cell. At membrane potentials negative to potassium's reversal potential, KCNJs support the flow of K+ ions into the cell, pushing the membrane potential back to the resting potential. Two factors regulate K+ permeability - cell permeability to K+ is increased at more negative membrane potentials and increasing extracellular K+ concentrations.
When the membrane potential is positive to the channel's resting potential (such as in Phase 3 of the action potential), these channels pass very little charge out of the cell. This may be due to the channel's pores being blocked by internal Mg2+ and endogenous polyamines such as spermine (Shin & Lu 2005).
Inwardly rectifying (Kir) channels contribute to potassium leak, stabilizing cells near the equilibrium reversal potential of potassium (EK). Kir channels pass small outward currents because of pore blockade by internal magnesium and polyamines; at potentials negative to EK, large inward currents are passed upon relief from blockade.
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.
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.
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.
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 (Krenz et al. 2002).
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DataNodes
channels-inwardly
rectifying (SUR2)G-protein beta-gamma and Kir3
channel complexG-protein
beta-gamma complexchannels (Kir 1.1
and Kir 4.1/5.1)Ca2+ activated
potassium channelAnnotated Interactions
channels-inwardly
rectifying (SUR2)G-protein beta-gamma and Kir3
channel complexG-protein beta-gamma and Kir3
channel complexG-protein
beta-gamma complexG-protein
beta-gamma complexchannels (Kir 1.1
and Kir 4.1/5.1)When the membrane potential is positive to the channel's resting potential (such as in Phase 3 of the action potential), these channels pass very little charge out of the cell. This may be due to the channel's pores being blocked by internal Mg2+ and endogenous polyamines such as spermine (Shin & Lu 2005).
Inwardly rectifying (Kir) channels contribute to potassium leak, stabilizing cells near the equilibrium reversal potential of potassium (EK). Kir channels pass small outward currents because of pore blockade by internal magnesium and polyamines; at potentials negative to EK, large inward currents are passed upon relief from blockade.
Ca2+ activated
potassium channel