ATP-sensitive potassium channel

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potassium inwardly-rectifying channel, subfamily J, member 8
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Chr. 12 p12.1
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potassium inwardly-rectifying channel, subfamily J, member 11
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Chr. 11 p15.1
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ATP-binding cassette, sub-family C (CFTR/MRP), member 8
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Chr. 11 p15.1
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ATP-binding cassette, sub-family C (CFTR/MRP), member 9
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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").

Discovery and structure

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 (

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]

Sensor of cell metabolism

Regulation of gene expression

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

promoter
.

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]

Metabolite regulation

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

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]

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]

Mitochondrial KATP and the regulation of aerobic metabolism

Upon the onset of a cellular energy crisis, mitochondrial function tends to decline. This is due to alternating inner

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]

Cardiovascular KATP channels and protection from ischemic injury

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.

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

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

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]

See also

References

Further reading

External links