ATP-sensitive potassium channel

An ATP-sensitive potassium channel (or K_ATP channel) is a type of potassium channel that is gated by intracellular nucleotides, ATP and ADP. ATP-sensitive potassium channels are composed of K_ir6.x-type subunits and sulfonylurea receptor (SUR) subunits, along with additional components. K_ATP channels are widely distributed in plasma membranes; however some may also be found on subcellular membranes. These latter classes of K_ATP channels can be classified as being either sarcolemmal ("sarcK_ATP"), mitochondrial ("mitoK_ATP"), or nuclear ("nucK_ATP").

Discovery and structure

K_ATP channels were first identified in cardiac myocytes by Akinori Noma in Japan. Glucose-regulated K_ATP channel activity was found in pancreatic beta cells by Frances Ashcroft at the University of Oxford. The closure of K_ATP channels leads to increased insulin secretion in beta cells and reduces glucagon secretion in alpha cells.

SarcK_ATP are composed of eight protein subunits (octamer). Four of these are members of the inward-rectifier potassium ion channel family K_ir6.x (either K_ir6.1 or K_ir6.2), while the other four are sulfonylurea receptors (SUR1, SUR2A, and SUR2B). The K_ir 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. 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 sarcK_ATP are constructed of eight subunits in this 4:4 ratio, their precise composition varies with tissue type.

MitoK_ATP were first identified in 1991 by single-channel recordings of the inner mitochondrial membrane. The molecular structure of mitoK_ATP is less clearly understood than that of sarcK_ATP. Some reports indicate that cardiac mitoK_ATP consist of K_ir6.1 and K_ir6.2 subunits, but neither SUR1 nor SUR2. More recently, it was discovered that certain multiprotein complexes containing succinate dehydrogenase can provide activity similar to that of K_ATP channels.

The presence of nucK_ATP was confirmed by the discovery that isolated patches of nuclear membrane possess properties, both kinetic and pharmacological, similar to plasma membrane K_ATP channels.

Sensor of cell metabolism

Regulation of gene expression

Four genes have been identified as members of the K_ATP 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 K_ATP channel, with the SUR subunits being encoded by the sur1 (SUR1) gene or selective splicing of the sur2 gene (SUR2A and SUR2B).

Changes in the transcription of these genes, and thus the production of K_ATP 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. 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.

A mechanism has been proposed for the cell's K_ATP reaction to hypoxia and ischemia. 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.

One significant implication of the link between cellular oxidative stress and increased K_ATP production is that overall potassium transport function is directly proportional to the membrane concentration of these channels. In cases of diabetes, K_ATP 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.

Metabolite regulation

The degree to which particular compounds are able to regulate K_ATP 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 K_ATP channel activity. Under resting conditions, the weakly inwardly rectifying K_ATP 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). In the presence of higher glucose metabolism, and consequently increased relative levels of ATP, the K_ATP 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. The change from one state to the other happens quickly and synchronously, due to C-terminus multimerization among proximate K_ATP channel molecules.

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 K_ATP 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.

Mitochondrial K_ATP 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 membrane potential, imbalanced trans-membrane ion transport, and an overproduction of free radicals, among other factors. In such a situation, mitoK_ATP 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.

Nuclear and sarcolemmal K_ATP channels also contribute to the endurance of and recovery from metabolic stress. In order to conserve energy, sarcK_ATP open, reducing the duration of the action potential while nucK_ATP-mediated Ca²⁺ concentration changes within the nucleus favor the expression of protective protein genes.

Cardiovascular K_ATP 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 K_ATP channel stimulation.

Both sarcK_ATP and mitoK_ATP are required for IPC to have its maximal effects. Selective mitoK_ATP blockade with 5-hydroxydecanoic acid ("5-HD") or MCC-134 completely inhibits the cardioprotection afforded by IPC, and genetic knockout of sarcK_ATP genes 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 sarcK_ATP's ability to prevent cellular Ca²⁺ overloading and depression of force development during muscle contraction, thereby conserving scarce energy resources.

Absence of sarcK_ATP, 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. Similarly, sarcK_ATP regulates vascular smooth muscle tone, and deletion of the kir6.2 or sur2 genes leads to coronary artery vasospasm and death.

Upon further exploration of sarcK_ATP'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. It is still unclear as to whether opening of K_ATP 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.

Stimulation of hair growth

ATP-sensitive potassium channel openers including minoxidil (via its active metabolite minoxidil sulfate), diazoxide, and pinacidil are associated with hypertrichosis in humans. Other ATP-sensitive potassium channel openers, like cromakalin and P-1075 (an analogue of pinacidil), stimulate hair growth in balding stump-tailed macaques, although another ATP-sensitive potassium channel opener, RP-49356, was not efficacious. The ATP-sensitive potassium channel openers naminidil (BMS-234303) and P-1075 were under formal development for treatment of hair loss and reached phase 2 clinical trials for this indication but were never marketed. Minoxidil also has other actions, and it is not fully clear whether opening of ATP-sensitive potassium channels is responsible for the hair growth-stimulatory effects of minoxidil and other ATP-sensitive potassium channel openers. In any case, K_ATP channel activation via minoxidil and other agents has been found to directly stimulate follicular growth in cultured hair follicles ex vivo. This can be reversed by the K_ATP inhibitor tolbutamide. In addition, Cantú syndrome, which involves gain-of-function mutations in K_ATP channel subunits (specifically SUR2 and K_IR6.1), is associated with hypertrichosis.

See also

• Cardiac action potential § Channels

References

Further reading

External links