Adenosine diphosphate as an intracellular regulator of insulin secretion

Octameric stoichiometry of the KATP channel complex

ATP-sensitive potassium (KATP) channels link cellular metabolism to electrical activity in nerve, muscle, and endocrine tissues. They are formed as a functional complex of two unrelated subunits-a member of the Kir inward rectifier potassium channel family, and a sulfonylurea receptor (SUR), a member of the ATP-binding cassette transporter family, which includes cystic fibrosis transmembrane conductance regulators and multidrug resistance protein, regulators of chloride channel activity. This recent discovery has brought together proteins from two very distinct superfamilies in a novel functional complex. The pancreatic KATP channel is probably formed specifically of Kir6.2 and SUR1 isoforms. The relationship between SUR1 and Kir6.2 must be determined to understand how SUR1 and Kir6.2 interact to form this unique channel. We have used mutant Kir6.2 subunits and dimeric (SUR1-Kir6.2) constructs to examine the functional stoichiometry of the KATP channel. The data indicate that the KATP channel pore is lined by four Kir6.2 subunits, and that each Kir6.2 subunit requires one SUR1 subunit to generate a functional channel in an octameric or tetradimeric structure.

Regulation of KATP channel activity by diazoxide and MgADP. Distinct functions of the two nucleotide binding folds of the sulfonylurea receptor

KATP channels were reconstituted in COSm6 cells by coexpression of the sulfonylurea receptor SUR1 and the inward rectifier potassium channel Kir6.2. The role of the two nucleotide binding folds of SUR1 in regulation of KATP channel activity by nucleotides and diazoxide was investigated. Mutations in the linker region and the Walker B motif (Walker, J.E., M.J. Saraste, M.J. Runswick, and N.J. Gay. 1982. EMBO [Eur. Mol. Biol. Organ.] J. 1:945-951) of the second nucleotide binding fold, including G1479D, G1479R, G1485D, G1485R, Q1486H, and D1506A, all abolished stimulation by MgADP and diazoxide, with the exception of G1479R, which showed a small stimulatory response to diazoxide. Analogous mutations in the first nucleotide binding fold, including G827D, G827R, and Q834H, were still stimulated by diazoxide and MgADP, but with altered kinetics compared with the wild-type channel. None of the mutations altered the sensitivity of the channel to inhibition by ATP4-. We propose a model in which SUR1 sensitizes the KATP channel to ATP inhibition, and nucleotide hydrolysis at the nucleotide binding folds blocks this effect. MgADP and diazoxide are proposed to stabilize this desensitized state of the channel, and mutations at the nucleotide binding folds alter the response of channels to MgADP and diazoxide by altering nucleotide hydrolysis rates or the coupling of hydrolysis to channel activation.

The interaction of nucleotides with the tolbutamide block of cloned ATP-sensitive K+ channel currents expressed in Xenopus oocytes: a reinterpretation

1. We have examined the mechanism by which nucleotides modulate the tolbutamide block of the beta-cell ATP-sensitive K+ channel (KATP channel), using wild-type and mutant KATP channels heterologously expressed in Xenopus oocytes. This channel is composed of sulphonylurea receptor (SUR1) and pore-forming (Kir6.2) subunits. 2. The dose-response relation for tolbutamide block of wild-type KATP currents in the absence of nucleotide showed both a high-affinity (Ki = 2.0 microM) and a low-affinity (Ki = 1.8 mM) site. 3. The dose-response relation for tolbutamide block of Kir6.2 delta C36 (a truncated form of Kir6.2 which is expressed independently of SUR1) was best fitted with a single, low-affinity site (Ki = 1.7 mM). This indicates that the high-affinity site resides on SUR1, whereas the low-affinity site is located on Kir6.2. 4. ADP (100 microM) had a dual effect on wild-type KATP currents: the nucleotide enhanced the current in the presence of Mg2+, but was inhibitory in the absence of Mg2+. Kir6.2 delta C36 currents were blocked by 100 microM ADP in the presence of Mg2+. 5. For wild-type KATP currents, the blocking effect of 0.5 mM tolbutamide appeared greater in the presence of 100 microM MgADP (84 +/- 2%) than in its absence (59 +/- 4%). When SUR1 was mutated to abolish MgADP activation of KATP currents (K719A or K1384M), there was no difference in the extent of tolbutamide inhibition in the presence or absence of MgADP. 6. The Ki for tolbutamide interaction with either the high- or low-affinity site was unaffected by 100 microM MgADP, for both wild-type and K719A-K1384M currents. 7. MgGDP (100 microM) enhanced wild-type KATP currents and was without effect on K719A-K1384M currents. It did not affect the Ki for tolbutamide block at either the high- or low-affinity site. 8. Our results indicate that interaction of tolbutamide with the high-affinity site (on SUR1) abolishes the stimulatory action of MgADP. This unmasks the inhibitory effect of ADP and leads to an apparent increase in channel inhibition. Under physiological conditions, abolition of MgADP activation is likely to constitute the principal mechanism by which tolbutamide inhibits the KATP channel.

Therapy for persistent hyperinsulinemic hypoglycemia of infancy. Understanding the responsiveness of beta cells to diazoxide and somatostatin

The neonatal disorder persistent hyperinsulinemic hypoglycemia of infancy (PHHI) arises as the result of mutations in the subunits that form the ATP-sensitive potassium (KATP) channel in pancreatic beta cells, leading to insulin hypersecretion. Diazoxide (a specific KATP channel agonist in normal beta cells) and somatostatin (octreotide) are the mainstay of medical treatment for the condition. To investigate the mechanism of action of these agents in PHHI beta cells that lack KATP currents, we applied patch clamp techniques to insulin-secreting cells isolated from seven patients with PHHI. Five patients showed favorable responses to medical therapy, and two were refractory. Our data reveal, in drug-responsive patients, that a novel ion channel is modulated by diazoxide and somatostatin, leading to termination of the spontaneous electrical events that underlie insulin hypersecretion. The drug-resistant patients, both of whom carried a mutation in one of the genes that encode KATP channel subunits, also lacked this novel K+ channel. There were no effects of diazoxide and somatostatin on beta cell function in vitro. These findings elucidate for the first time the mechanisms of action of diazoxide and somatostatin in infants with PHHI in whom KATP channels are absent, and provide a rationale for development of new therapeutic opportunities by K+ channel manipulation in PHHI treatment.

MgADP antagonism to Mg2+-independent ATP binding of the sulfonylurea receptor SUR1

Pancreatic beta-cell ATP-sensitive potassium (KATP) channels play an important role in the regulation of glucose-induced insulin secretion. The beta-cell KATP channel comprises two subunits, the sulfonylurea receptor SUR1, a member of the ATP-binding cassette (ABC) superfamily, and Kir6.2, a member of the inward rectifier K+ channel family. The activity of the KATP channel is under complex regulation by the intracellular ATP and ADP. To understand the roles of the two nucleotide-binding folds (NBFs) of SUR1 in the regulation of KATP channel activity, we introduced point mutations into the core consensus sequence of the Walker A or B motif of each NBF of SUR1 and characterized ATP binding and ADP or MgADP antagonism to it. SUR1 was efficiently photolabeled with 8-azido-[alpha-32P]ATP and 8-azido-[gamma-32P]ATP in the presence or absence of Mg2+ or vanadate. NBF1 mutations impaired ATP binding, but NBF2 mutations did not. MgADP strongly antagonized ATP binding, and the NBF2 mutation reduced MgADP antagonism. These results show that SUR1, unlike other ABC proteins, strongly binds ATP at NBF1 even in the absence of Mg2+ and that MgADP, through binding at NBF2, antagonizes the Mg2+-independent high affinity ATP binding at NBF1.

Somatic deletion of the imprinted 11p15 region in sporadic persistent hyperinsulinemic hypoglycemia of infancy is specific of focal adenomatous hyperplasia and endorses partial pancreatectomy

Sporadic persistent hyperinsulinemic hypoglycemia of infancy (PHHI) or nesidioblastosis is a heterogeneous disorder characterized by profound hypoglycemia due to inappropriate hypersecretion of insulin. An important diagnostic goal is to distinguish patients with a focal hyperplasia of islet cells of the pancreas (FoPHHI) from those with a diffuse abnormality of islets (DiPHHI) because management strategies differ significantly. 16 infants with sporadic PHHI resistant to diazoxide and who underwent pancreatectomy were investigated. Selective pancreatic venous sampling coupled with peroperative surgical examination and analysis of extemporaneous frozen sections allowed us to identify 10 cases with FoPHHI and 6 cases with DiPHHI. We show here that in cases of FoPHHI, but not those of DiPHHI, there was specific loss of maternal alleles of the imprinted chromosome region 11p15 in cells of the hyperplastic area of the pancreas but not in normal pancreatic cells. This somatic event is consistent with a proliferative monoclonal lesion. It involves disruption of the balance between monoallelic expression of several maternally and paternally expressed genes. Thus, we provide the first molecular explanation of the heterogeneity of sporadic forms of PHHI such that it is possible to perform only partial pancreatectomy, limited to the focal somatic lesion, so as to avoid iatrogenic diabetes in patients with focal adenomatous hyperplasia.

Spontaneous cell fusion in macrophage cultures expressing high levels of the P2Z/P2X7 receptor

Mouse and human macrophages express a plasma membrane receptor for extracellular ATP named P2Z/P2X7. This molecule, recently cloned, is endowed with the intriguing property of forming an aqueous pore that allows transmembrane fluxes of hydrophylic molecules of molecular weight below 900. The physiological function of this receptor is unknown. In a previous study we reported experiments suggesting that the P2Z/P2X7 receptor is involved in the formation of macrophage-derived multinucleated giant cells (MGCs; Falzoni, S., M. Munerati, D. Ferrari, S. Spisani, S. Moretti, and F. Di Virgilio. 1995. J. Clin. Invest. 95:1207- 1216). We have selected several clones of mouse J774 macrophages that are characterized by either high or low expression of the P2Z/P2X7 receptor and named these clones P2Zhyper or P2Zhypo, respectively. P2Zhyper, but not P2Zhypo, cells grown to confluence in culture spontaneously fuse to form MGCs. As previously shown for human macrophages, fusion is inhibited by the P2Z/P2X7 blocker oxidized ATP. MGCs die shortly after fusion through a dramatic process of cytoplasmic sepimentation followed by fragmentation. These observations support our previous hypothesis that the P2Z/P2X7 receptor is involved in macrophage fusion.

Activation and inhibition of K-ATP currents by guanine nucleotides is mediated by different channel subunits

The ATP-sensitive potassium channel (K-ATP channel) plays a key role in insulin secretion from pancreatic beta-cells. It is closed by glucose metabolism, which stimulates secretion, and opened by the drug diazoxide, which inhibits insulin release. Metabolic regulation is mediated by changes in ATP and MgADP concentration, which inhibit and potentiate channel activity, respectively. The beta-cell K-ATP channel consists of a pore-forming subunit, Kir6.2, and a regulatory subunit, SUR1. The site at which ATP mediates channel inhibition lies on Kir6.2, while the potentiatory action of MgADP involves the nucleotide-binding domains of SUR1. K-ATP channels are also activated by MgGTP and MgGDP. Furthermore, both nucleotides support the stimulatory actions of diazoxide. It is not known, however, whether guanine nucleotides mediate their effects by direct interaction with one or more of the K-ATP channel subunits or indirectly via a GTP-binding protein. We used a truncated form of Kir6.2, which expresses independently of SUR1, to show that GTP blocks K-ATP currents by interaction with Kir6.2 and that the potentiatory effects of GTP are endowed by SUR1. We also showed that mutation of the lysine residue in the Walker A motif of either the first (K719A) or second (K1384M) nucleotide-binding domain of SUR1 abolished both the potentiatory effects of GTP and GDP on K-ATP currents and their ability to support stimulation by diazoxide. This argues that the stimulatory effects of guanine nucleotides require the presence of both Walker A lysines.

Control of rectification and gating of cloned KATP channels by the Kir6.2 subunit

KATP channels are a functional complex of sulphonylurea receptor (SUR1, SUR2) and inward rectifier K+ (Kir6.1, Kir6.2) channel subunits. We have studied the role of the putative pore forming subunit (Kir6.2) in regulation of rectification and gating of KATP channels generated by transfection of SUR1 and Kir6.2 cDNAs in COSm6 cells. In the absence of internal polyvalent cations, the current-voltage relationship is sigmoidal. Mg2+ or spermine4+ (spm) each induces a mild inward rectification. Mutation of the asparagine at position 160 in Kir6.2 to aspartate (N160D) or glutamate (N160E) increases the degree of rectification induced by Mg2+ or spermine4+, whereas wild-type rectification is still observed after mutation to other neutral residues (alanine-N160A, glutamine-N160Q). These results are consistent with this residue lining the pore of the channel and contributing to the binding of these cations, as demonstrated for the equivalent site in homomeric ROMK1 (Kir1.1) channels. Since Kir6.2 contains no consensus ATP binding site, whereas SUR1 does, inhibition by ATP has been assumed to depend on interactions with SUR1. However, we found that the [ATP] causing half-maximal inhibition of current (Ki) was affected by mutation of N160. Channels formed from N160D or N160Q mutant subunits had lower apparent sensitivity to ATP (Ki,N160D = 46.1 microM; Ki,N160Q = 62.9 microM) than wild-type, N160E, or N160A channels (Ki = 10.4, 17.7, 6.4 microM, respectively). This might suggest that ATP binding to the channel complex was altered, although examination of channel open probabilities indicates instead that the residue at position 160 alters the ATP-independent open probability, i.e., it controls the free energy of the open state, thereby affecting the "coupling" of ATP binding to channel inhibition. The results can be interpreted in terms of a kinetic scheme whereby the residue at Kir6.2 position 160 controls the rate constants governing transitions to and from the open state, without directly affecting ATP binding or unbinding transitions.

Evidence that GCN1 and GCN20, translational regulators of GCN4, function on elongating ribosomes in activation of eIF2alpha kinase GCN2

In the yeast Saccharomyces cerevisiae, phosphorylation of translation initiation factor eIF2 by protein kinase GCN2 leads to increased translation of the transcriptional activator GCN4 in amino acid-starved cells. The GCN1 and GCN20 proteins are components of a protein complex required for the stimulation of GCN2 kinase activity under starvation conditions. GCN20 is a member of the ATP-binding cassette (ABC) family, most of the members of which function as membrane-bound transporters, raising the possibility that the GCN1/GCN20 complex regulates GCN2 indirectly as an amino acid transporter. At odds with this idea, indirect immunofluorescence revealed cytoplasmic localization of GCN1 and no obvious association with plasma or vacuolar membranes. In addition, a fraction of GCN1 and GCN20 cosedimented with polysomes and 80S ribosomes, and the ribosome association of GCN20 was largely dependent on GCN1. The C-terminal 84% of GCN20 containing the ABCs was found to be dispensable for complex formation with GCN1 and for the stimulation of GCN2 kinase function. Because ABCs provide the energy-coupling mechanism for ABC transporters, these results also contradict the idea that GCN20 regulates GCN2 as an amino acid transporter. The N-terminal 15 to 25% of GCN20, which is critically required for its regulatory function, was found to interact with an internal segment of GCN1 similar in sequence to translation elongation factor 3 (EF3). Based on these findings, we propose that GCN1 performs an EF3-related function in facilitating the activation of GCN2 by uncharged tRNA on translating ribosomes. The physical interaction between GCN20 and the EF3-like domain in GCN1 could allow for modulation of GCN1 activity, and the ABC domains in GCN20 may be involved in this regulatory function. A human homolog of GCN1 has been identified, and the portion of this protein most highly conserved with yeast GCN1 has sequence similarity to EF3. Thus, similar mechanisms for the detection of uncharged tRNA on translating ribosomes may operate in yeast and human cells.

The ABCs of ATP-sensitive potassium channels: more pieces of the puzzle

ATP-sensitive potassium channels, KATP channels are critical for the normal regulation of insulin secretion. The cloning of cDNAs encoding the subunits of these channels shows that they are a novel combination of an ATP-binding protein and a small inward rectifier. Loss of pancreatic beta-cell KATP channels has been shown to cause familial hyperinsulinism.

Intracellular diadenosine polyphosphates: a novel family of inhibitory ligands of the ATP-sensitive K+ channel

Intracellular diadenosine polyphosphates (Ap(n)A) are signal molecules that alert the cell under stress conditions. Herein, we review evidence that has recently identified a novel target for Ap(n)A, namely the ATP-sensitive K+ (K(ATP)) channel. These channels are abundant in pancreatic beta-cells and cardiac myocytes where they are essential in coupling the cellular metabolic state with membrane excitability. The potency and efficacy of Ap(n)A to inhibit K(ATP) channel activity were first described in cardiac K(ATP) channels, and appear similar to those of intracellular ATP, the primary ligand of K(ATP) channels. Also, the inhibitory ligand action of Ap(n)A is dependent upon the operative condition of K(ATP) channels and the presence of nucleotide diphosphates. In addition to a direct antagonism of channel opening, an indirect effect of Ap(n)A on K(ATP) channel activity has been associated with inhibition of adenylate kinase, a catalytic system believed essential for the regulation of channel opening. At present, the precise role for Ap(n)A-induced K(ATP) channel inhibition remains to be established. Yet, it is known that, under glucose challenge of pancreatic beta-cells, intracellular concentrations of Ap(n)A do increase to micromolar levels necessary to block K(ATP) channels, leading to insulin secretion. Thus, the Ap(n)A-mediated K(ATP) channel gating represents a previously unrecognized pathway of channel regulation.

Evidence for a unique long chain acyl-CoA ester binding site on the ATP-regulated potassium channel in mouse pancreatic beta cells

The mechanism by which long chain acyl-CoA (LC-CoA) esters affect the ATP-regulated potassium channel (KATP channel) was studied in inside-out patches isolated from mouse pancreatic beta cells. Addition of LC-CoA esters dramatically increased KATP channel activity. The stimulatory effect of the esters could be explained by the induction of a prolonged open state of the channel and did not involve alterations in single channel unitary conductance. Under control conditions, absence of adenine nucleotides, the distribution of KATP channel open time could be described by a single exponential, with a time constant of about 25 ms. Exposing the same patch to LC-CoA esters resulted in the appearance of an additional component with a time constant of >>150 ms, indicating a conformational change of the channel protein. LC-CoA esters were also able to potently activate channel activity at different ratios of ATP/ADP. Simultaneous additions of MgADP and LC-CoA esters resulted in a supra-additive effect on channel mean open time, characterized by openings of very long duration. Following modification of the KATP channel by a short exposure of the patch to the protease trypsin, the stimulatory effect of ADP on channel activity was lost while activation by LC-CoA esters still persisted. This indicates that LC-CoA esters and MgADP do not bind to the same site. We conclude that LC-CoA esters may play an important role in the physiological regulation of the KATP channel in the pancreatic beta cell by binding to a unique site and thereby inducing repolarization of the beta cell-membrane potential.

KATP-channel on the somata of spiny neurones in rat caudate nucleus: regulation by drugs and nucleotides

1. The aim of the present study was to characterize the pharmacological properties of the adenosine 5'-triphosphate(ATP)-sensitive K+ channel (KATP-channel) on the somata of spiny neurones in rat caudate nucleus and to compare them with those of beta-cells. For that purpose we tested the effects of several KATP-channel-inhibiting and -activating drugs on the opening activity of the KATP-channel in caudate nucleus by use of the patch-clamp technique. In addition, the modulation of drug responses by cytosolic nucleotides was examined. 2. When KATP-channels in caudate nucleus were activated in cell-attached patches by inhibition of mitochondrial energy production, meglitinide (a benzoic acid derivative), Hoe36320 (a sulphonylurea of low lipophilicity) and glipizide reduced KATP-channel activity half-maximally at 0.4 microM, 0.4 microM and about 0.5 nM, respectively. 3. In inside-out patches (presence of 0.7 mM free Mg2+ at the cytoplasmic membrane side), tolbutamide (0.1 mM) caused only partial inhibition of KATP-channels in the absence of cytosolic nucleotides but complete inhibition in the simultaneous presence of the channel-activating nucleotide guanosine 5'-diphosphate (GDP; 1 mM) and the channel-inhibiting nucleotide adenylyl-imidodiphosphate (AMP-PNP; 0.2 mM). 4. Diazoxide (0.3 mM) strongly increased channel activity in the presence of ATP (0.1 mM) or GDP (0.03 mM), but was ineffective in the presence of AMP-PNP (0.1 mM). In the absence of cytosolic nucleotides diazoxide even decreased channel activity. 5. In the presence of 0.1 mM ATP, diazoxide activated KATP-channels half-maximally at 38 microM. 6. When KATP-channel activity was inhibited by 0.1 mM ATP, (-)-pinacidil (0.5 mM) elicited a slight activation of KATP-channels in caudate nucleus, whereas (+)-pinacidil (0.5 mM) and lemakalim (0.3 mM) were ineffective. 7. Since our data indicate similar control by drugs and nucleotides of KATP-channels in the somata of spiny neurones and pancreatic beta-cells, we conclude that the high affinity sulphonylurea receptors of these tissues are probably closely related.

The essential role of the Walker A motifs of SUR1 in K-ATP channel activation by Mg-ADP and diazoxide

The ATP-sensitive K-channel (K-ATP channel) plays a key role in insulin secretion from pancreatic beta-cells. It is closed by glucose metabolism, which stimulates insulin secretion, and opened by the drug diazoxide, which inhibits insulin release. Metabolic regulation is mediated by changes in ATP and Mg-ADP, which inhibit and potentiate channel activity, respectively. The beta-cell K-ATP channel consists of a pore-forming subunit, Kir6.2, and a regulatory subunit, SUR1. We have mutated (independently or together) two lysine residues in the Walker A (W(A)) motifs of the first (K719A) and second (K1384M) nucleotide-binding domains (NBDs) of SUR1. These mutations are expected to inhibit nucleotide hydrolysis. Our results indicate that the W(A) lysine of NBD1 (but not NBD2) is essential for activation of K-ATP currents by diazoxide. The potentiatory effects of Mg-ADP required the presence of the W(A) lysines in both NBDs. Mutant currents were slightly more sensitive to ATP than wild-type currents. Metabolic inhibition led to activation of wild-type and K1384M currents, but not K719A or K719A/K1384M currents, suggesting that there may be a factor in addition to ATP and ADP which regulates K-ATP channel activity.

Anionic phospholipids activate ATP-sensitive potassium channels

The ATP-sensitive potassium channel (KATP) controls insulin release in pancreatic beta-cells and also modulates important functions in other cell types. In this study we report that anionic phospholipids activated KATP in pancreatic beta-cells, cardiac myocytes, skeletal muscle cells, and a cloned KATP composed of two subunits (SUR/Kir6. 2) stably expressed in a mammalian cell line. The effectiveness was proportional to the number of negative charges on the head group of the anionic phospholipid. Screening negative charges with polyvalent cations antagonized the effect. Enzymatic treatment with phospholipases that reduced charge on the lipids also reduced or eliminated the effect. These results suggest that intact phospholipids with negative charges are the critical requirement for activation of KATP, in distinction from the usual cell signaling pathway through phospholipids that requires cleavage. Mutations of two positively charged amino acid residues at the C terminus of Kir6. 2 accelerated loss of channel activity and reduced the activating effects of phospholipids, suggesting involvement of this region in the activation. Metabolism of anionic phospholipids in plasmalemmal membrane may be a novel and general mechanism for regulation of KATP and perhaps other ion channels in the family of inward rectifiers.

Developmental and tissue-specific sulfonylurea receptor gene expression

We have studied the developmental regulation of mouse sulfonylurea receptor (SUR) gene expression throughout several embryonic stages as well as in the adult mouse. To this end we used a 229-bp mouse complementary DNA corresponding to the 3'-end of the SUR gene for in situ hybridization and solution hybridization/ribonuclease protection assays. We found that the SUR gene was expressed as early as embryonic day 12 in the developing pancreas, heart, and central nervous system. These tissues maintained significant levels of SUR messenger RNA (mRNA) throughout development. In addition, SUR mRNA was detected in the submandibular gland, anterior duodenum, dorsal root ganglia, lens, retina, and vibrissae by late developmental stages. SUR mRNA is widely distributed in adult mouse tissues, with the exception of the liver. In the adult pancreas, the SUR gene was expressed exclusively in endocrine tissue. Although significant levels of SUR mRNA were broadly seen throughout the brain, neurons of the cerebellum, hippocampus, and thalamus had especially high levels of SUR mRNA. These findings support the idea that the SUR has important functions in many other tissues in addition to the islets of the pancreas.

Inward rectifier potassium channels

The past three years have seen remarkable progress in research on the molecular basis of inward rectification, with significant implications for basic understanding and pharmacological manipulation of cellular excitability. Expression cloning of the first inward rectifier K channel (Kir) genes provided the necessary break-through that has led to isolation of a family of related clones encoding channels with the essential functional properties of classical inward rectifiers, ATP-sensitive K channels, and muscarinic receptor-activated K channels. High-level expression of cloned channels led to the discovery that classical inward so-called anomalous rectification is caused by voltage-dependent block of the channel by polyamines and Mg2+ ions, and it is now clear that a similar mechanism results in inward rectification of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate receptor channels. Knowledge of the primary structures of Kir channels and the ability to mutate them also has led to the determination of many of the structural requirements of inward rectification.

Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers

Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.

Reversal of the ATP-liganded state of ATP-sensitive K+ channels by adenylate kinase activity

The mechanism that promotes transition from the ATP- to the ADP-liganded state of ATP-sensitive K+ (KATP) channels and consequent channel opening in a cytosolic environment of high ATP concentration has yet to be understood. A mechanism examined here that could reverse the ATP-inhibited state is based on the action of adenylate kinase to catalyze phosphoryl transfer between ATP and AMP, resulting in transformation of ATP into ADP. In membrane patches excised from guinea pig cardiomyocytes, AMP alone did not affect channel behavior but increased the open probability of ATP-inhibited KATP channels. This required MgCl2 and a hydrolyzable form of ATP and was prevented by P1,P5-di-adenosine-5'-pentaphosphate, an inhibitor of adenylate kinase. The single channel amplitude and kinetics of channel openings induced by the ADP-generating substrates of adenylate kinase, AMP and MgATP, were indistinguishable from the biophysical properties of the KATP channel exhibited after addition of MgADP. In whole cell voltage-clamped cardiomyocytes, introduction of exogenous adenylate kinase along with millimolar MgATP and AMP induced a K+ current that was suppressed by a sulfonylurea blocker of KATP channels. Enriched sarcolemmal membrane preparations were found to possess ATP.AMP phosphotransferase activity with properties attributable to an extramitochondrial isoform of adenylate kinase. These results indicate that adenylate kinase is a naturally occurring component of sarcolemmal membranes that could provide dynamic governance of KATP channel opening through its phosphoryl transfer catalytic action in the microenvironment of the channel.

Loss of functional KATP channels in pancreatic beta-cells causes persistent hyperinsulinemic hypoglycemia of infancy

Persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is a disorder of childhood associated with inappropriate hypersecretion of insulin by the pancreas. The pathogenesis of the condition has hitherto remained controversial. We show here that insulin-secreting cells from a homogeneous group of five infants with PHHI lack ATP-sensitive K+ channel (KATP) activity. As a consequence, PHHI beta-cells are spontaneously electrically active with high basal cytosolic Ca2+ concentrations due to Ca2+ influx. Our findings define the pathogenesis of this disease as a novel K+ channel disorder.