Mutation spectra of ABCC8 gene in Spanish patients with Hyperinsulinism of Infancy (HI)

Hyperinsulinism of Infancy (HI) is a clinical disorder characterized by deregulation of insulin secretion that leads to profound hypoglycemia. Mutations in genes encoding the ATP-regulated potassium channels of the pancreatic beta-cell, namely ABCC8 (SUR1) and KCNJ11 (Kir6.2), are the major genetic known cause of the disease. To elucidate the genetic etiology of HI in the uncharacterized Spanish population, we conducted extensive sequencing analysis of the ABCC8 (83.5Kb) and KCNJ11 (1.7Kb) genes in 34 Spanish HI patients. Mutations in ABCC8 were detected for both alleles in 13 patients, while ten patients carried only one mutation in one of the ABCC8 alleles. We have detected 22 novel and seven previously described mutations in ABCC8, approximately 60% of them lead to a premature termination signal, which would result in truncated SUR1 proteins. No mutations were found in the KCNJ11 gene. In addition, we report for the first time a 3914bp macrodeletion associated with the HI disorder. The potential pathogenicity of several additional variants is discussed. The spatial pattern of three pathological mutations suggests possible geographical founder effects. This work reveals for first time the involvement of KATP channels in the pathogenesis of an important proportion (approximately 68%) of Spanish HI patients. The spectrum of mutations in Spanish HI patients provides an important tool for diagnosis and prognosis of HI patients in the Spanish population, as well as for genetic counseling of HI families.     

Genetics of congenital hyperinsulinism

Congenital hyperinsulinism (CHI) is a clinically and genetically heterogeneous entity and causes severe hypoglycemia in neonates and infants. The clinical heterogeneity is manifested by severity ranging from extremely severe, life-threatening disease to very mild clinical symptoms, which may even be difficult to identify. Furthermore, clinical responsiveness to medical and surgical management is extremely variable. Recent discoveries have begun to clarify the molecular etiology of this disease in about 50% of cases. Mutations in five different genes have been identified in patients with this clinical syndrome. Most cases are caused by mutations in the genes ABCC8 and KCNJ11 coding for either of the two subunits of the beta-cell KATP channel (SUR1 and Kir6.2). Recessive mutations of the beta-cell K(ATP) channel genes cause diffuse HI, whereas loss of heterozygosity together with inheritance of a paternal mutation causes focal adenomatous HI. In other cases, CHI is caused by mutations in genes coding for the beta-cell enzymes glucokinase (GK), glutamate dehydrogenase (GDH), and SCHAD. However, for as many as 50% of the cases, no genetic etiology has yet been determined. The study of the genetics of this disease has provided important new information regarding beta-cell physiology.     

Unbalanced expression of 11p15 imprinted genes in focal forms of congenital hyperinsulinism: association with a reduction to homozygosity of a mutation in ABCC8 or KCNJ11

Congenital hyperinsulinism (CHI), previously named persistent hyperinsulinemic hypoglycemia of infancy, is characterized by profound hypoglycemia because of excessive insulin secretion. CHI presents as two different morphological forms: a diffuse form with functional abnormality of islets throughout the pancreas and a focal form with focal islet cell adenomatous hyperplasia, which can be cured by partial pancreatectomy. Recently, we have shown that focal adenomatous hyperplasia involves the specific loss of the maternal 11p15 region and a constitutional mutation of a paternally inherited allele of the gene encoding the regulating subunit of the K(+)(ATP) channel, the sulfonylurea receptor (ABCC8 or SUR1). In the present study on a large series of 31 patients, describing both morphological features and molecular data, we report that 61% of cases (19 out of 31) carried a paternally inherited mutation not only in the ABCC8 gene as previously described but also in the second gene encoding the K(+)(ATP) channel, the inward rectifying potassium channel (KCNJ11 or KIR6.2), in 15 cases and 4 cases, respectively. Moreover our results are consistent with the presence of a duplicated paternal 11p15 allele probably because of mitotic recombination or reduplication of the paternal chromosome after somatic loss of the maternal chromosome. In agreement with the loss of the maternal chromosome, the level of expression of a maternally expressed tumor suppressor gene, H19, was greatly reduced compared to the level of expression of the paternally expressed growth promoter gene, IGF2. The expression of IGF2 was on average only moderately increased. Thus, focal forms of CHI can be considered to be a recessive somatic disease, associating an imbalance in the expression of imprinted genes in the 11p15.5 region to a somatic reduction to homozygosity of an ABCC8- or KCNJ11-recessive mutation. The former is responsible for the abnormal growth rate, as in embryonic tumors, whereas the latter leads to unregulated secretion of insulin.     

A heterozygous activating mutation in the sulphonylurea receptor SUR1 (ABCC8) causes neonatal diabetes

Neonatal diabetes is a genetically heterogeneous disorder with nine different genetic aetiologies reported to date. Heterozygous activating mutations in the KCNJ11 gene encoding Kir6.2, the pore-forming subunit of the ATP-sensitive potassium (K(ATP)) channel, are the most common cause of permanent neonatal diabetes. The sulphonylurea receptor (SUR) SUR1 serves as the regulatory subunit of the K(ATP) channel in pancreatic beta cells. We therefore hypothesized that activating mutations in the ABCC8 gene, which encodes SUR1, might cause neonatal diabetes. We identified a novel heterozygous mutation, F132L, in the ABCC8 gene of a patient with severe developmental delay, epilepsy and neonatal diabetes (DEND syndrome). This mutation had arisen de novo and was not present in 150 control chromosomes. Residue F132 shows evolutionary conservation across species and is located in the first set of transmembrane helices (TMD0) of SUR1, which is proposed to interact with Kir6.2. Functional studies of recombinant K(ATP) channels demonstrated that F132L markedly reduces the sensitivity of the K(ATP) channel to inhibition by MgATP and this increases the whole-cell K(ATP) current. The functional consequence of this ABCC8 mutation mirrors that of KCNJ11 mutations causing neonatal diabetes and provides new insights into the interaction of Kir6.2 and SUR1. As SUR1 is expressed in neurones as well as in beta cells, this mutation can account for both neonatal diabetes and the neurological phenotype. Our results demonstrate that SUR1 mutations constitute a new genetic aetiology for neonatal diabetes and that they act by reducing the K(ATP) channel's ATP sensitivity.     

Update of mutations in the genes encoding the pancreatic beta-cell K(ATP) channel subunits Kir6.2 (KCNJ11) and sulfonylurea receptor 1 (ABCC8) in diabetes mellitus and hyperinsulinism

The beta-cell ATP-sensitive potassium (K(ATP)) channel is a key component of stimulus-secretion coupling in the pancreatic beta-cell. The channel couples metabolism to membrane electrical events bringing about insulin secretion. Given the critical role of this channel in glucose homeostasis it is therefore not surprising that mutations in the genes encoding for the two essential subunits of the channel can result in both hypo- and hyperglycemia. The channel consists of four subunits of the inwardly rectifying potassium channel Kir6.2 and four subunits of the sulfonylurea receptor 1 (SUR1). It has been known for some time that loss of function mutations in KCNJ11, which encodes for Kir6.2, and ABCC8, which encodes for SUR1, can cause oversecretion of insulin and result in hyperinsulinism of infancy, while activating mutations in KCNJ11 and ABCC8 have recently been described that result in the opposite phenotype of diabetes. This review focuses on reported mutations in both genes, the spectrum of phenotypes, and the implications for treatment on diagnosing patients with mutations in these genes.     

The spectrum of ABCC8 mutations in Norwegian patients with congenital hyperinsulinism of infancy

Potassium channels in the plasma membrane of the pancreatic beta cells are critical in maintaining glucose homeostasis by responding to ATP and coupling metabolic changes to insulin secretion. These channels consist of subunits denoted the sulfonylurea receptor SUR1 and the inwardly rectifying ion channel KIR6.2, which are encoded by the genes ABCC8 and KCNJ11 , respectively. Activating mutations in the subunit genes can result in monogenic diabetes, whereas inactivating mutations are the most common cause of congenital hyperinsulinism of infancy (CHI). Twenty-six Norwegian probands with CHI were analyzed for alterations in ABCC8 and KCNJ11 . Fifteen probands (58%) had mutations in the ABCC8 gene. Nine patients were homozygous or compound heterozygous for the mutations, indicating diffuse pancreatic disease. In five patients, heterozygous and paternally inherited mutations were found, suggesting focal disease. One patient had a de novo mutation likely to cause a milder, dominant form of CHI. Altogether, 16 different ABCC8 mutations (including the novel alterations W231R, C267X, IVS6-3C>G, I462V, Q917X and T1531A) were identified. The mutations IVS10+1G>T, R1493W and V21D occurred in five, three and two families, respectively. KCNJ11 mutations were not found in any patients. Based on our mutation screening, we estimate the minimum birth prevalence of ABCC8 -CHI in Norway to 1:70,000 during the past decade. Our results considerably extend the knowledge of the molecular genetics behind CHI in Scandinavia.     

Cantú Syndrome Resulting from Activating Mutation in the KCNJ8 Gene

ATP‐sensitive potassium (K_ATP) channels, composed of inward‐rectifying potassium channel subunits (Kir6.1 and Kir6.2, encoded by KCNJ8 and KCNJ11, respectively) and regulatory sulfonylurea receptor (SUR1 and SUR2, encoded by ABCC8 and ABCC9, respectively), couple metabolism to excitability in multiple tissues. Mutations in ABCC9 cause Cantú syndrome (CS), a distinct multiorgan disease, potentially via enhanced K_ATP channel activity. We screened KCNJ8 in an ABCC9 mutation‐negative patient who also exhibited clinical hallmarks of CS (hypertrichosis, macrosomia, macrocephaly, coarse facial appearance, cardiomegaly, and skeletal abnormalities). We identified a de novo missense mutation encoding Kir6.1[p.Cys176Ser] in the patient. Kir6.1[p.Cys176Ser] channels exhibited markedly higher activity than wild‐type channels, as a result of reduced ATP sensitivity, whether coexpressed with SUR1 or SUR2A subunits. Our results identify a novel causal gene in CS, but also demonstrate that the cardinal features of the disease result from gain of K_ATP channel function, not from a Kir6‐independent SUR2 function.     

Mutations in the genes encoding the pancreatic beta-cell KATP channel subunits Kir6.2 (KCNJ11) and SUR1 (ABCC8) in diabetes mellitus and hyperinsulinism

The beta-cell ATP-sensitive potassium channel is a key component of stimulus-secretion coupling in the pancreatic beta-cell. The channel couples metabolism to membrane electrical events, bringing about insulin secretion. Given the critical role of this channel in glucose homeostasis, it is not surprising that mutations in the genes encoding for the two essential subunits of the channel can result in both hypo- and hyperglycemia. The channel consists of four subunits of the inwardly rectifying potassium channel Kir6.2 and four subunits of the sulfonylurea receptor 1. It has been known for some time that loss of function mutations in KCNJ11, which encodes for Kir6.2, and ABCC8, which encodes for SUR1, can cause oversecretion of insulin and result in hyperinsulinemia (HI) of infancy; however, heterozygous activating mutations in KCNJ11 that result in the opposite phenotype of diabetes have recently been described. This review focuses on reported mutations in both genes, the spectrum of phenotypes, and the implications for treatment when patients are diagnosed with mutations in these genes.     

Novel ABCC8 (SUR1) Gene Mutations in Asian Indian Children with Congenital Hyperinsulinemic Hypoglycemia

Congenital hyperinsulinemic hypoglycemia (HI) is a heterogeneous genetic disorder of insulin secretion characterized by persistent hypoglycemia, most commonly associated with inactivating mutations of the β‐cell ATP‐sensitive K⁺ channel (K^ATP channel) genes ABCC8 (encoding SUR1) and KCNJ11(encoding Kir6.2). This study aimed to screen the mutations in the genes associated with congenital HI in Asian Indian children. Recessive mutations of these genes cause hyperinsulinism that is unresponsive to treatment with channel agonists like diazoxide. Dominant K^ATP mutations have been associated with diazoxide‐responsive disease. The KCNJ11, ABCC8, GCK, HNF4A, and GLUD1 genes were analyzed by sequence analysis in 22 children with congenital HI. We found 10 novel mutations (c.1delA, c.61delG, c.267delT, c.619–629delCCCGAGGACCT, Gln444*, Leu724Pro, Ala847Thr, Trp898*, IVS30–2A>C, and Leu1454Arg) and two known mutations (Gly111Arg and Arg598*) in the ABCC8 gene. This study describes novel and known ABCC8 gene mutations in children with congenital HI. This is the first large genetic screening study on HI in India and our results will help clinicians in providing optimal treatment for patients with hyperinsulinemia and in assisting affected families with genetic counseling.     

Clinical and molecular characterization of neonatal diabetes and monogenic syndromic diabetes in Asian Indian children

Mutations in the pancreatic ATP sensitive K⁺ channel proteins [sulfonyluea receptor 1 (SUR1) and inward rectifier K⁺ channel Kir6.2 (Kir6.2), encoded by ATP‐binding cassette transporter subfamily C member 8 (ABCC8) and potassium channel J11 (KCNJ11), respectively], are the most common cause of neonatal diabetes. We describe the clinical presentation and molecular characterization of Asian Indian children with neonatal diabetes mellitus and monogenic syndromes of diabetes. We sequenced KCNJ11, ABCC8 and insulin (INS) genes in 33 unrelated Indian probands with onset of diabetes below one year of age. A total of 12 mutations were identified which included ABCC8 mutations in seven, KCNJ11 mutations in three and INS mutations in two children. The Asp212Tyr mutation in ABCC8 was novel. We also detected two novel mutations (Val67Met and Leu19Arg) in children with syndromic forms of diabetes like Berardinelli Seip syndrome [1‐acyl‐sn‐glycerol‐3‐phosphate acyltransferase beta (AGPAT2)] and Fanconi Bickel syndrome [solute carrier family 2A2 (SLC2A2)]. Children carrying the KCNJ11 (Cys42Arg, Arg201Cys) and ABCC8 (Val86Ala, Asp212Tyr) mutations have been successfully switched over from insulin therapy to oral sulfonylurea. Our study is the first large genetic screening study of neonatal diabetes in India.     

Mutation of the pancreatic islet inward rectifier Kir6.2 also leads to familial persistent hyperinsulinemic hypoglycemia of infancy

Closure of ATP-sensitive potassium channels in pancreatic islet beta- cells initiates a cascade of events that leads to insulin secretion. beta-Cell ATP-sensitive potassium currents can be reconstituted by coexpression of the inward rectifier Kir6.2 and the sulfonylurea receptor (SUR), a member of the ATP-binding cassette superfamily. Mutations in SUR have been identified in individuals affected with familial persistent hyper-insulinemic hypoglycemia of infancy (PHHI), an autosomal recessive disorder of glucose metabolism which is linked to chromosome 11p15.1 and characterized by unregulated secretion of insulin and profound hypoglycemia. Because the Kir6.2 locus is within 5 kilobases (kb) of the SUR gene on chromosome 11p15.1 and it is a necessary member of the beta-cell KATP channel, we considered Kir6.2 as a candidate gene for PHHL we identified a homozygous point mutation in Kir6.2 in the genomic DNA of a child, severely affected with PHHI, from a consanguineous family. This mutation is predicted to disrupt the conserved alpha-helical second transmembrane (M2) domain of the inward rectifier by substitution of a proline for a leucine residue (L147P). Mutation of Kir6.2, like SUR, appears to lead to the PHHI phenotype suggesting that Kir6.2 is necessary, although not sufficient, for normal regulation of insulin release.      

Dominantly acting ABCC8 mutations in patients with medically unresponsive hyperinsulinaemic hypoglycaemia

Flanagan SE, Kapoor RR, Banerjee I, Hall C, Smith VV, Hussain K, Ellard S. Dominantly acting ABCC8 mutations in patients with medically unresponsive hyperinsulinaemic hypoglycaemia. Recessive inactivating mutations in the ABCC8 and KCNJ11 genes encoding the adenosine triphosphate‐sensitive potassium (K_ATP) channel subunit sulphonylurea receptor 1 (SUR1) and inwardly rectifying potassium channel subunit (Kir6.2) are the most common cause of hyperinsulinaemic hypoglycaemia (HH). Most of these patients do not respond to treatment with the K_ATP channel agonist diazoxide. Dominant inactivating ABCC8 and KCNJ11 mutations are less frequent, but are usually associated with a milder form of hypoglycaemia that is responsive to diazoxide therapy. We studied five patients from four families with HH who were unresponsive to diazoxide and required a near total pancreatectomy. Mutations in KCNJ11 and ABCC8 were sought by sequencing and dosage analysis. Three novel heterozygous ABCC8 mis‐sense mutations (G1485E, D1506E and M1514K) were identified in four probands. All the mutations affect residues located within the Nucleotide Binding Domain 2 of the SUR1 subunit. Testing of family members showed that the mutations had arisen de novo with dominant inheritance in one pedigree. This study extends the clinical phenotype associated with dominant K_ATP channel mutations to include severe congenital HH requiring near total pancreatectomy in addition to a milder form of diazoxide responsive hypoglycaemia. The identification of dominant vs recessive mutations does not predict clinical course but it is important for estimating the risk of HH in future siblings and offspring.     

ATP敏感性钾通道突变与新生儿糖尿病iDEND综合征

ATP敏感性钾通道(ATP-sensitive K+chan-nels,KATP)由SUR1和Kir6.2亚基组成,是葡萄糖刺激胰岛β细胞分泌胰岛素的关键部位。新生儿糖尿病iDEND综合征(intermediate developmental delay,epilepsy,and neonatal diabetes syndrome)是由KATP通     

Congenital hyperinsulinism: Molecular basis of a heterogeneous disease

Congenital hyperinsulinism (CHI) is a disease phenotype characterized by increased, usually irregular, insulin secretion leading to hypoglycemia, coma, and severe brain damage, left untreated. Hyperinsulinism may be caused by a range of biochemical disturbances and molecular defects. In pancreatic β cells, insulin secretion is stimulated by closure of the ATP‐dependent potassium channel (K_ATP channel). K_ATP channel is a complex composed of at least two subunits: the sulfonylurea receptor SUR1 and Kir6.2, an inward rectifier K⁺ channel member. Mutations in both subunits have been identified in patients with the autosomal recessive form of hyperinsulinism, including 28 different mutations in the SUR1 gene and two mutations in the Kir6.2 gene. These mutations co‐segregated with disease phenotype, also known as persistent hyperinsulinemic hypoglycemia of infancy (PHHI), and with attenuated K_ATP channel function. Inadequately high insulin secretion in one family with an autosomal dominant mode of inheritance is caused by a mutation in the glucokinase gene, resulting in increased affinity of the enzyme for glucose. Five different mutations have been identified in the glutamate dehydrogenase gene, resulting in overactivity of this enzyme and causing a syndrome of hyperinsulinism and hyperammonemia. In 13 cases, hyperinsulinism was caused by one or more focal pancreatic lesions with specific loss of maternal alleles of the imprinted chromosome region 11p15. In five patients, this loss of heterozygosity unmasked a paternally inherited recessive SUR1 mutation. The new molecular approaches in PHHI give further insight into the mechanism of pancreatic β cell insulin secretion. The heterogeneous group of patients with CHI may now be classified according to their basic defects in the four different genes, with potential implications for a more specific treatment. Hum Mutat 13:351–361, 1999. © 1999 Wiley‐Liss, Inc.     

Sulfonylurea therapy in two Korean patients with insulin-treated neonatal diabetes due to heterozygous mutations of the KCNJ11 gene encoding Kir6.2

Permanent neonatal diabetes (PND) is a rare form of diabetes characterized by insulin-requiring hyperglycemia diagnosed within the first three months of life. In most cases, the causes are not known. Recently, mutations in the KCNJ11 gene encoding the Kir6.2 subunit of the ATP-sensitive K+ channel have been described in patients with PND. We report the first two Korean cases with PND due to a lysineto- arginine substitution at position 170 (K179R) and a valine-to-methionine substitution at position 59 (V59M) mutations of KCNJ11 encoding Kir6.2, respectively. After several years of insulin therapy, these patients were managed by oral glibenclamide therapy at a daily dose of 0.8-0.9 mg/kg. Their basal c-peptide levels increased after one week of glibenclamide therapy, and one month later, the insulin and c-peptide levels were in the normal ranges without any episodes of hyper- or hypoglycemia. These cases demonstrate that oral sulfonylurea may be the treatment of choice in PND patients with KCNJ11 mutations even at a young age.     

Functional analysis of six Kir6.2 (KCNJ11) mutations causing neonatal diabetes

ATP-sensitive potassium (K_ATP) channels, composed of pore-forming Kir6.2 and regulatory sulphonylurea receptor (SUR) subunits, play an essential role in insulin secretion from pancreatic beta cells. Binding of ATP to Kir6.2 inhibits, whereas interaction of Mg-nucleotides with SUR, activates the channel. Heterozygous activating mutations in Kir6.2 (KCNJ11) are a common cause of neonatal diabetes (ND). We assessed the functional effects of six novel Kir6.2 mutations associated with ND: H46Y, N48D, E227K, E229K, E292G, and V252A. K_ATP channels were expressed in Xenopus oocytes and the heterozygous state was simulated by coexpression of wild-type and mutant Kir6.2 with SUR1 (the beta cell type of SUR). All mutations reduced the sensitivity of the K_ATP channel to inhibition by MgATP, and enhanced whole-cell K_ATP currents. Two mutations (E227K, E229K) also enhanced the intrinsic open probability of the channel, thereby indirectly reducing the channel ATP sensitivity. The other four mutations lie close to the predicted ATP-binding site and thus may affect ATP binding. In pancreatic beta cells, an increase in the K_ATP current is expected to reduce insulin secretion and thereby cause diabetes. None of the mutations substantially affected the sensitivity of the channel to inhibition by the sulphonylurea tolbutamide, suggesting patients carrying these mutations may respond to these drugs.     

Functional effects of KCNJ11 mutations causing neonatal diabetes: enhanced activation by MgATP

Recent studies have shown that heterozygous mutations in KCNJ11, which encodes Kir6.2, the pore-forming subunit of the ATP-sensitive potassium (K(ATP)) channel, cause permanent neonatal diabetes either alone (R201C, R201H) or in association with developmental delay, muscle weakness and epilepsy (V59G,V59M). Functional analysis in the absence of Mg2+, to isolate the inhibitory effects of ATP on Kir6.2, showed that both types of mutation reduce channel inhibition by ATP. However, in pancreatic beta-cells, K(ATP) channel activity is governed by the balance between ATP inhibition via Kir6.2 and Mg-nucleotide stimulation mediated by an auxiliary subunit, the sulphonylurea receptor SUR1. We therefore studied the MgATP sensitivity of KCNJ11 mutant K(ATP) channels expressed in Xenopus oocytes. In contrast to wild-type channels, Mg2+ dramatically reduced the ATP sensitivity of heterozygous R201C, R201H, V59M and V59G channels. This effect was predominantly mediated via the nucleotide-binding domains of SUR1 and resulted from an enhanced stimulatory action of MgATP. Our results therefore demonstrate that KCNJ11 mutations increase the current magnitude of heterozygous K(ATP) channels in two ways: by increasing MgATP activation and by decreasing ATP inhibition. They further show that the fraction of unblocked K(ATP) current at physiological MgATP concentrations correlates with the severity of the clinical phenotype.     

The properties of the Kir6.1–6.2 tandem channel co-expressed with SUR2A

Functional ATP-sensitive K (KATP) channels have an octameric subunit structure with four pore-forming subunits (Kir6.x) and four sulfonylurea receptors (SURx). In the present study, the properties of the heteromeric KATP channel whose pore subunits are composed of Kir6.1 and Kir6.2 were examined using a heterologous expression system. In COS7 cells co-transfected with Kir6.1, Kir6.2 and SUR2A at a ratio of 1:1:2, KATP channels showed various unitary conductances between those of Kir6.1/SUR2A (33.6+/-4.2 pS) and Kir6.2/ SUR2A (67.1+/-1.6 pS). Kir6.1-6.2 tandem protein, constructed by fusing the C-terminus of Kir6.1 to the N-terminus of Kir6.2 with a ten glutamine linker sequence, also formed a channel with an intermediate conductance (58.9+/-1.5 pS). Kir6.2 and Kir6.1-6.2 showed similar sensitivity to ATP4-: half-maximal inhibition (IC50) was obtained at 14.1+/-12.8 microM and 17.6+/-9.6 microM, respectively. In the presence of Mg2+, Kir6. 1-6.2 was significantly less sensitive than Kir6.2 to MgATP (IC50=95.5+/-49.6 microM versus 18.9+/-5.0 microM). These results suggest that Kir6.1 and Kir6.2 are endowed with the potential to form a heteromeric KATP channel, which has a low sensitivity to MgATP.     

ATP-sensitive potassium channels: structures, functions, and pathophysiology

ATP-sensitive potassium channels (KATP channels) play important roles in various tissues by coupling cell metabolic status to electrical activity. Recently, molecular biological and electrophysiological techniques have revealed the molecular basis of the KATP channels to be a complex of the Kir6.0 subunit, a member of the inwardly rectifying K+ channel subfamily Kir6.0, and the sulfonylurea receptor (SUR) subunit, a member of ATP-binding cassette (ABC) superfamily; the functional diversity of the various KATP channels is being determined by a combination of the Kir6.0 subunit (Kir6.1 or Kir6.2) and the SUR subunit (SUR1 or SUR2) comprising it. Recent studies of the KATP channels have suggested mechanisms of KATP channel regulation and pathophysiology and also a new model in which ABC proteins regulate the functional expression of ion channels.     

Characterisation of sulphonylurea and ATP-regulated K+ channels in rat pancreatic A-cells

We have monitored whole-cell and single channel ATP-sensitive K+ (KATP) currents in isolated rat glucagon-secreting pancreatic A-cells. Tolbutamide produced a concentration-dependent decrease in the whole-cell KATP conductance (Ki = 6 microM) and initiated action potential firing. The K+ channel opener diazoxide, but not cromakalim or pinacidil, inhibited electrical activity and increased the whole-cell K+ conductance fourfold. ATP applied to the intracellular face of the membrane inhibited KATP channel activity with a Ki of 17 microM, an effect that could be counteracted by Mg-ADP and Mg-GDP. GTP and UTP did not affect KATP channel activity. Phosphatidylinositol 4,5-bisphosphate activated KATP channels inhibited by ATP after a delay of 90 s. In situ hybridisation demonstrated the expression of the mRNA encoding KATP channel subunits Kir6.2 and SUR1 but not Kir6.1 and SUR2. We conclude that rat pancreatic A-cells express KATP channels with the nucleotide-, sulphonylurea- and K+ channel-opener sensitivities expected for a channel formed by Kir6.2 and SUR1 subunits.