potassium inwardly-rectifying channel, subfamily J, member 8 | |||||||
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Identifiers | |||||||
Symbol | KCNJ8 | ||||||
Alt. symbols | Kir6.1 | ||||||
NCBI gene | 3764 | ||||||
HGNC | 6269 | ||||||
OMIM | 600935 | ||||||
RefSeq | NM_004982 | ||||||
UniProt | Q15842 | ||||||
Other data | |||||||
Locus | Chr. 12 p12.1 | ||||||
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potassium inwardly-rectifying channel, subfamily J, member 11 | |||||||
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Identifiers | |||||||
Symbol | KCNJ11 | ||||||
Alt. symbols | Kir6.2 | ||||||
NCBI gene | 3767 | ||||||
HGNC | 6257 | ||||||
OMIM | 600937 | ||||||
RefSeq | NM_000525 | ||||||
UniProt | Q14654 | ||||||
Other data | |||||||
Locus | Chr. 11 p15.1 | ||||||
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ATP-binding cassette, sub-family C (CFTR/MRP), member 8 | |||||||
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Identifiers | |||||||
Symbol | ABCC8 | ||||||
Alt. symbols | SUR1 | ||||||
NCBI gene | 6833 | ||||||
HGNC | 59 | ||||||
OMIM | 600509 | ||||||
RefSeq | NM_000352 | ||||||
UniProt | Q09428 | ||||||
Other data | |||||||
Locus | Chr. 11 p15.1 | ||||||
|
ATP-binding cassette, sub-family C (CFTR/MRP), member 9 | |||||||
---|---|---|---|---|---|---|---|
Identifiers | |||||||
Symbol | ABCC9 | ||||||
Alt. symbols | SUR2A, SUR2B | ||||||
NCBI gene | 10060 | ||||||
HGNC | 60 | ||||||
OMIM | 601439 | ||||||
RefSeq | NM_005691 | ||||||
UniProt | O60706 | ||||||
Other data | |||||||
Locus | Chr. 12 p12.1 | ||||||
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An ATP-sensitive potassium channel (or KATP channel) is a type of potassium channel that is gated by intracellular nucleotides, ATP and ADP. ATP-sensitive potassium channels are composed of Kir6.x-type subunits and sulfonylurea receptor (SUR) subunits, along with additional components. [1] KATP channels are widely distributed in plasma membranes; [2] however some may also be found on subcellular membranes. These latter classes of KATP channels can be classified as being either sarcolemmal ("sarcKATP"), mitochondrial ("mitoKATP"), or nuclear ("nucKATP").
KATP channels were first identified in cardiac myocytes by Akinori Noma in Japan. [3] Glucose-regulated KATP channel activity was found in pancreatic beta cells by Frances Ashcroft at the University of Oxford. [4] The closure of KATP channels leads to increased insulin secretion in beta cells and reduces glucagon secretion in alpha cells. [5]
SarcKATP are composed of eight protein subunits (octamer). Four of these are members of the inward-rectifier potassium ion channel family Kir6.x (either Kir6.1 or Kir6.2), while the other four are sulfonylurea receptors (SUR1, SUR2A, and SUR2B). [6] The Kir subunits have two transmembrane spans and form the channel's pore. The SUR subunits have three additional transmembrane domains, and contain two nucleotide-binding domains on the cytoplasmic side. [7] These allow for nucleotide-mediated regulation of the potassium channel, and are critical in its roles as a sensor of metabolic status. These SUR subunits are also sensitive to sulfonylureas, MgATP (the magnesium salt of ATP), and some other pharmacological channel openers. While all sarcKATP are constructed of eight subunits in this 4:4 ratio, their precise composition varies with tissue type. [8]
MitoKATP were first identified in 1991 by single-channel recordings of the inner mitochondrial membrane. [9] The molecular structure of mitoKATP is less clearly understood than that of sarcKATP. Some reports indicate that cardiac mitoKATP consist of Kir6.1 and Kir6.2 subunits, but neither SUR1 nor SUR2. [10] [11] More recently, it was discovered that certain multiprotein complexes containing succinate dehydrogenase can provide activity similar to that of KATP channels. [12]
The presence of nucKATP was confirmed by the discovery that isolated patches of nuclear membrane possess properties, both kinetic and pharmacological, similar to plasma membrane KATP channels. [13]
Four genes have been identified as members of the KATP gene family. The sur1 and kir6.2 genes are located in chr11p15.1 while kir6.1 and sur2 genes reside in chr12p12.1. The kir6.1 and kir6.2 genes encode the pore-forming subunits of the KATP channel, with the SUR subunits being encoded by the sur1 (SUR1) gene or selective splicing of the sur2 gene (SUR2A and SUR2B). [14]
Changes in the transcription of these genes, and thus the production of KATP channels, are directly linked to changes in the metabolic environment. High glucose levels, for example, induce a significant decrease in the kir6.2 mRNA level – an effect that can be reversed by lower glucose concentration. [15] Similarly, 60 minutes of ischemia followed by 24 to 72 hours of reperfusion leads to an increase in kir6.2 transcription in left ventricle rat myocytes. [16]
A mechanism has been proposed for the cell's KATP reaction to hypoxia and ischemia. [17] Low intracellular oxygen levels decrease the rate of metabolism by slowing the TCA cycle in the mitochondria. Unable to transfer electrons efficiently, the intracellular NAD+/NADH ratio decreases, activating phosphotidylinositol-3-kinase and extracellular signal-regulated kinases. This, in turn, upregulates c-jun transcription, creating a protein which binds to the sur2 promoter.[ citation needed ]
One significant implication of the link between cellular oxidative stress and increased KATP production is that overall potassium transport function is directly proportional to the membrane concentration of these channels. In cases of diabetes, KATP channels cannot function properly, and a marked sensitivity to mild cardiac ischemia and hypoxia results from the cells' inability to adapt to adverse oxidative conditions. [18]
The degree to which particular compounds are able to regulate KATP channel opening varies with tissue type, and more specifically, with a tissue's primary metabolic substrate.
In pancreatic beta cells, ATP is the primary metabolic source, and the ATP/ADP ratio determines KATP channel activity. Under resting conditions, the weakly inwardly rectifying KATP channels in pancreatic beta cells are spontaneously active, allowing potassium ions to flow out of the cell and maintaining a negative resting membrane potential (slightly more positive than the K+ reversal potential). [19] In the presence of higher glucose metabolism, and consequently increased relative levels of ATP, the KATP channels close, causing the membrane potential of the cell to depolarize, activating voltage-gated calcium channels, and thus promoting the calcium-dependent release of insulin. [19] The change from one state to the other happens quickly and synchronously, due to C-terminus multimerization among proximate KATP channel molecules. [20]
Cardiomyocytes, on the other hand, derive the majority of their energy from long-chain fatty acids and their acyl-CoA equivalents. Cardiac ischemia, as it slows the oxidation of fatty acids, causes an accumulation of acyl-CoA and induces KATP channel opening while free fatty acids stabilize its closed conformation. This variation was demonstrated by examining transgenic mice, bred to have ATP-insensitive potassium channels. In the pancreas, these channels were always open, but remained closed in the cardiac cells. [21] [22]
Upon the onset of a cellular energy crisis, mitochondrial function tends to decline. This is due to alternating inner membrane potential, imbalanced trans-membrane ion transport, and an overproduction of free radicals, among other factors. [8] In such a situation, mitoKATP channels open and close to regulate both internal Ca2+ concentration and the degree of membrane swelling. This helps restore proper membrane potential, allowing further H+ outflow, which continues to provide the proton gradient necessary for mitochondrial ATP synthesis. Without aid from the potassium channels, the depletion of high energy phosphate would outpace the rate at which ATP could be created against an unfavorable electrochemical gradient. [23]
Nuclear and sarcolemmal KATP channels also contribute to the endurance of and recovery from metabolic stress. In order to conserve energy, sarcKATP open, reducing the duration of the action potential while nucKATP-mediated Ca2+ concentration changes within the nucleus favor the expression of protective protein genes. [8]
Cardiac ischemia, while not always immediately lethal, often leads to delayed cardiomyocyte death by necrosis, causing permanent injury to the heart muscle. One method, first described by Keith Reimer in 1986, involves subjecting the affected tissue to brief, non-lethal periods of ischemia (3–5 minutes) before the major ischemic insult. This procedure is known as ischemic preconditioning ("IPC"), and derives its effectiveness, at least in part, from KATP channel stimulation.[ citation needed ]
Both sarcKATP and mitoKATP are required for IPC to have its maximal effects. Selective mitoKATP blockade with 5-hydroxydecanoic acid ("5-HD") or MCC-134 [24] completely inhibits the cardioprotection afforded by IPC, and genetic knockout of sarcKATP genes [25] in mice has been shown to increase the basal level of injury compared to wild type mice. This baseline protection is believed to be a result of sarcKATP's ability to prevent cellular Ca2+ overloading and depression of force development during muscle contraction, thereby conserving scarce energy resources. [26]
Absence of sarcKATP, in addition to attenuating the benefits of IPC, significantly impairs the myocyte's ability to properly distribute Ca2+, decreasing sensitivity to sympathetic nerve signals, and predisposing the subject to arrhythmia and sudden death. [27] Similarly, sarcKATP regulates vascular smooth muscle tone, and deletion of the kir6.2 or sur2 genes leads to coronary artery vasospasm and death. [28]
Upon further exploration of sarcKATP's role in cardiac rhythm regulation, it was discovered that mutant forms of the channel, particularly mutations in the SUR2 subunit, were responsible for dilated cardiomyopathy, especially after ischemia/reperfusion. [29] It is still unclear as to whether opening of KATP channels has completely pro- or antiarrhythmic effects. Increased potassium conductance should stabilize membrane potential during ischemic insults, reducing the extent infarct and ectopic pacemaker activity. On the other hand, potassium channel opening accelerates repolarization of the action potential, possibly inducing arrhythmic reentry. [8]
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions across the cell membrane, controlling the flow of ions across secretory and epithelial cells, and regulating cell volume. Ion channels are present in the membranes of all cells. Ion channels are one of the two classes of ionophoric proteins, the other being ion transporters.
In biology, a connexon, also known as a connexin hemichannel, is an assembly of six proteins called connexins that form the pore for a gap junction between the cytoplasm of two adjacent cells. This channel allows for bidirectional flow of ions and signaling molecules. The connexon is the hemichannel supplied by a cell on one side of the junction; two connexons from opposing cells normally come together to form the complete intercellular gap junction channel. In some cells, the hemichannel itself is active as a conduit between the cytoplasm and the extracellular space, allowing the transference of ions and small molecules lower than 1-2 KDa. Little is known about this function of connexons besides the new evidence suggesting their key role in intracellular signaling. In still other cells connexons have been shown to occur in mitochondrial membranes and appear to play a role in heart ischaemia.
Uniporters, also known as solute carriers or facilitated transporters, are a type of membrane transport protein that passively transports solutes across a cell membrane. It uses facilitated diffusion for the movement of solutes down their concentration gradient from an area of high concentration to an area of low concentration. Unlike active transport, it does not require energy in the form of ATP to function. Uniporters are specialized to carry one specific ion or molecule and can be categorized as either channels or carriers. Facilitated diffusion may occur through three mechanisms: uniport, symport, or antiport. The difference between each mechanism depends on the direction of transport, in which uniport is the only transport not coupled to the transport of another solute.
Glibenclamide, also known as glyburide, is an antidiabetic medication used to treat type 2 diabetes. It is recommended that it be taken together with diet and exercise. It may be used with other antidiabetic medication. It is not recommended for use by itself in type 1 diabetes. It is taken by mouth.
The renal outer medullary potassium channel (ROMK) is an ATP-dependent potassium channel (Kir1.1) that transports potassium out of cells. It plays an important role in potassium recycling in the thick ascending limb (TAL) and potassium secretion in the cortical collecting duct (CCD) of the nephron. In humans, ROMK is encoded by the KCNJ1 gene. Multiple transcript variants encoding different isoforms have been found for this gene.
Inward-rectifier potassium channels (Kir, IRK) are a specific lipid-gated subset of potassium channels. To date, seven subfamilies have been identified in various mammalian cell types, plants, and bacteria. They are activated by phosphatidylinositol 4,5-bisphosphate (PIP2). The malfunction of the channels has been implicated in several diseases. IRK channels possess a pore domain, homologous to that of voltage-gated ion channels, and flanking transmembrane segments (TMSs). They may exist in the membrane as homo- or heterooligomers and each monomer possesses between 2 and 4 TMSs. In terms of function, these proteins transport potassium (K+), with a greater tendency for K+ uptake than K+ export. The process of inward-rectification was discovered by Denis Noble in cardiac muscle cells in 1960s and by Richard Adrian and Alan Hodgkin in 1970 in skeletal muscle cells.
In molecular biology, the sulfonylurea receptors (SUR) are membrane proteins which are the molecular targets of the sulfonylurea class of antidiabetic drugs whose mechanism of action is to promote insulin release from pancreatic beta cells. More specifically, SUR proteins are subunits of the inward-rectifier potassium ion channels Kir6.x. The association of four Kir6.x and four SUR subunits form an ion conducting channel commonly referred to as the KATP channel.
Kir6.2 is a major subunit of the ATP-sensitive K+ channel, a lipid-gated inward-rectifier potassium ion channel. The gene encoding the channel is called KCNJ11 and mutations in this gene are associated with congenital hyperinsulinism.
Protein kinase C epsilon type (PKCε) is an enzyme that in humans is encoded by the PRKCE gene. PKCε is an isoform of the large PKC family of protein kinases that play many roles in different tissues. In cardiac muscle cells, PKCε regulates muscle contraction through its actions at sarcomeric proteins, and PKCε modulates cardiac cell metabolism through its actions at mitochondria. PKCε is clinically significant in that it is a central player in cardioprotection against ischemic injury and in the development of cardiac hypertrophy.
ATP-binding cassette transporter sub-family C member 8 is a protein that in humans is encoded by the ABCC8 gene. ABCC8 orthologs have been identified in all mammals for which complete genome data are available.
G protein-activated inward rectifier potassium channel 2 is a protein that in humans is encoded by the KCNJ6 gene. Mutation in KCNJ6 gene has been proposed to be the cause of Keppen-Lubinsky Syndrome (KPLBS).
Potassium inwardly-rectifying channel, subfamily J, member 8, also known as KCNJ8, is a human gene encoding the Kir6.1 protein. A mutation in KCNJ8 has been associated with cardiac arrest in the early repolarization syndrome.
G protein-activated inward rectifier potassium channel 4(GIRK-4) is a protein that in humans is encoded by the KCNJ5 gene and is a type of G protein-gated ion channel.
Alpha-endosulfine is a protein that in humans is encoded by the ENSA gene.
ATP-binding cassette, sub-family C member 9 (ABCC9) also known as sulfonylurea receptor 2 (SUR2) is an ATP-binding cassette transporter that in humans is encoded by the ABCC9 gene.
HMR 1883 and its sodium salt HMR 1098, are experimental anti-arrhythmic drugs classified as sulfonylthiourea compounds. Their main purpose is to treat ventricular fibrillation caused by myocardial ischemia. They were synthesized via structural modifications to glibenclamide, an antidiabetic drug. Both HMR 1883 and glibenclamide act by inactivating the ATP-sensitive potassium channels (KATP) responsible for potassium efflux. Unlike glibenclamide, HMR 1883 has been suggested to target selectively the Kir6.2/SUR2A KATP subtype, found mostly in the membranes of cardiac cells. However, data showing that HMR 1098 inhibits the Kir6.2/SUR1 KATP subtype found in insulin-secreting pancreatic beta cells challenges this view.
Rottlerin (mallotoxin) is a polyphenol natural product isolated from the Asian tree Mallotus philippensis. Rottlerin displays a complex spectrum of pharmacology.
Diallyl trisulfide (DATS), also known as Allitridin, is an organosulfur compound with the formula S(SCH2CH=CH2)2. It is one of several compounds produced by hydrolysis of allicin, including diallyl disulfide and diallyl tetrasulfide; DATS is one of the most potent.
Colin G. Nichols FRS is the Carl Cori Endowed Professor, and Director of the Center for Investigation of Membrane Excitability Diseases at Washington University in St. Louis, Missouri.
Verrucotoxin (VTX) is a lethal venom produced by the dorsal fins of Synanceia verrucosa. This species of reef stonefish is connected to the family Synanceiidae. The venom of this species of stonefish is a tetrameric glycoprotein with cardiovascular and cytolytic effects.