Effects of intracellular pH on ATP-sensitive K+ channels in mouse pancreatic beta-cells

KATP channel activity and slow oscillations in pancreatic beta cells are regulated by mitochondrial ATP production

Pancreatic beta cells secrete insulin in response to plasma glucose. The ATP-sensitive potassium channel (KATP ) links glucose metabolism to islet electrical activity in these cells by responding to increased cytosolic [ATP]/[ADP]. It was recently proposed that pyruvate kinase (PK) in close proximity to beta cell KATP locally produces the ATP that inhibits KATP activity. This proposal was largely based on the observation that applying phosphoenolpyruvate (PEP) and ADP to the cytoplasmic side of excised inside-out patches inhibited KATP . To test the relative contributions of local vs. mitochondrial ATP production, we recorded KATP activity using mouse beta cells and INS-1 832/13 cells. In contrast to prior reports, we could not replicate inhibition of KATP activity by PEP + ADP. However, when the pH of the PEP solutions was not corrected for the addition of PEP, strong channel inhibition was observed as a result of the well-known action of protons to inhibit KATP . In cell-attached recordings, perifusing either a PK activator or an inhibitor had little or no effect on KATP channel closure by glucose, further suggesting that PK is not an important regulator of KATP . In contrast, addition of mitochondrial inhibitors robustly increased KATP activity. Finally, by measuring the [ATP]/[ADP] responses to imposed calcium oscillations in mouse beta cells, we found that oxidative phosphorylation could raise [ATP]/[ADP] even when ADP was at its nadir during the burst silent phase, in agreement with our mathematical model. These results indicate that ATP produced by mitochondrial oxidative phosphorylation is the primary controller of KATP in pancreatic beta cells. KEY POINTS: Phosphoenolpyruvate (PEP) plus adenosine diphosphate does not inhibit KATP activity in excised patches. PEP solutions only inhibit KATP activity if the pH is unbalanced. Modulating pyruvate kinase has minimal effects on KATP activity. Mitochondrial inhibition, in contrast, robustly potentiates KATP activity in cell-attached patches. Although the ADP level falls during the silent phase of calcium oscillations, mitochondria can still produce enough ATP via oxidative phosphorylation to close KATP . Mitochondrial oxidative phosphorylation is therefore the main source of the ATP that inhibits the KATP activity of pancreatic beta cells.

Running out of time: the decline of channel activity and nucleotide activation in adenosine triphosphate-sensitive K-channels

K_ATP channels act as key regulators of electrical excitability by coupling metabolic cues—mainly intracellular adenine nucleotide concentrations—to cellular potassium ion efflux. However, their study has been hindered by their rapid loss of activity in excised membrane patches (rundown), and by a second phenomenon, the decline of activation by Mg-nucleotides (DAMN). Degradation of PI(4,5)P₂ and other phosphoinositides is the strongest candidate for the molecular cause of rundown. Broad evidence indicates that most other determinants of rundown (e.g. phosphorylation, intracellular calcium, channel mutations that affect rundown) also act by influencing K_ATP channel regulation by phosphoinositides. Unfortunately, experimental conditions that reproducibly prevent rundown have remained elusive, necessitating post hoc data compensation. Rundown is clearly distinct from DAMN. While the former is associated with pore-forming Kir6.2 subunits, DAMN is generally a slower process involving the regulatory sulfonylurea receptor (SUR) subunits. We speculate that it arises when SUR subunits enter non-physiological conformational states associated with the loss of SUR nucleotide-binding domain dimerization following prolonged exposure to nucleotide-free conditions. This review presents new information on both rundown and DAMN, summarizes our current understanding of these processes and considers their physiological roles.

This article is part of the themed issue ‘Evolution brings Ca²⁺ and ATP together to control life and death’.

Impaired metabolism-secretion coupling in pancreatic beta-cells: role of determinants of mitochondrial ATP production

Glucose-induced insulin secretion from beta-cells is often impaired in diabetic condition and by exposure to diabetogenic pharmacological agents. In pancreatic beta-cells, intracellular glucose metabolism regulates exocytosis of insulin granules, according to metabolism-secretion coupling in which glucose-induced mitochondrial ATP production plays an essential role. Impaired glucose-induced insulin secretion often results from impaired glucose-induced ATP elevation in beta-cells. Mitochondrial ATP production is driven by the proton-motive force including mitochondrial membrane potential (DeltaPsi(m)) generated by the electron transport chain. These electrons are derived from reducing equivalents, generated in the Krebs cycle and transferred from cytosol by the shuttles. Here, roles of the determinants of mitochondrial ATP production in impaired glucose-induced insulin secretion are discussed. Cytosolic alkalization, H(+) leak in the inner membrane by uncoupler (e.g. free fatty acid exposure), decrease in the supply of electron donors including NADH and FADH(2) to the respiratory chain, and endogenous mitochondrial ROS (e.g. Na(+)/K(+)-ATPase inhibition) all reduce hyperpolarlization of DeltaPsi(m) and ATP production, causing decresed glucose-induced insulin release. The decrease in the supply of NADH and FADH(2) to the respiratory chain derives from impairments in glucose metabolism including glycolysis (e.g. MODY2 and exposure to NO) and the shuttles (e.g. diabetic state and exposure to ketone body).

Diphenylhydantoin suppresses glucose-induced insulin release by decreasing cytoplasmic H+ concentration in pancreatic islets

Diphenylhydantoin (DPH), which is clinically used in the treatment of epilepsy, inhibits glucose-induced insulin release from pancreatic islets by a mechanism that remains unknown. In the present study, DPH is shown to suppress glucose-induced insulin release concentration-dependently. In dynamic experiments, 20 microm DPH suppressed 16.7 mm glucose-induced biphasic insulin release. DPH also suppressed insulin release in the presence of 16.7 mm glucose, 200 microm diazoxide, and 30 mm K+ without affecting the intracellular Ca2+ concentration. DPH suppressed ATP content and mitochondrial membrane hyperpolarization in the presence of 16.7 mm glucose without affecting glucose utilization, glucose oxidation, and reduced nicotinamide adenine dinucleotide phosphate fluorescence. DPH increased cytoplasmic pH in the presence of high glucose, but the increase was abolished under Na+ -deprived conditions and HCO3- -deprived conditions, suggesting that Na+ and HCO3- transport across the plasma membrane are involved in the increase in cytoplasmic pH by DPH. Alkalization by adding NH4+ to the extracellular medium also suppressed insulin release, ATP content, and mitochondrial membrane hyperpolarization. Because ATP production from the mitochondrial fraction in the presence of substrates was decreased by increased pH in the medium, DPH suppresses mitochondrial ATP production by reducing the H+ gradient across mitochondrial membrane. Using permeabilized islets, the increase in pH was shown to decrease Ca2+ efficacy at a clamped concentration of ATP in the exocytotic system. Taken together, DPH inhibits glucose-induced insulin secretion not only by inhibiting mitochondrial ATP production, but also by reducing Ca2+ efficacy in the exocytotic system through its alkalizing effect on cytoplasm.

Intracellular pH and KATP channel activity in dorsal vagal neurons of juvenile rats in situ during metabolic disturbances

Intracellular pH (pH(i)) is an important factor for understanding cellular processes associated with the response of central neurons to metabolic disturbances such as anoxia or ischemia. In the present study, pH(i) was fluorometrically measured in 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescin (BCECF)-filled, voltage-clamped dorsal vagal neurons (DVN) of brainstem slices from rats during metabolic disturbances activating ATP-sensitive K(+) (K(ATP)) channels. Chemical anoxia induced by cyanide, rotenone or p-trifluoromethoxy-phenylhydrazone (FCCP) decreased pH(i) by >0.4 pH units. Untreated neurons with normal pH(i) baseline (7.2) responded to glucose-free superfusate after a delay of 7-16 min with a progressive fall of pH(i). In contrast, pH(i) increased by >0.2 pH units after approximately 10 min in cells that had a mean pH(i) of 6.8 due to incomplete recovery from a CN(-)induced acid load prior to glucose depletion. Metabolic arrest, induced by cyanide in glucose-free solution after 30 min preincubation in glucose-free saline, caused a progressive glutamate-mediated inward current with no change of pH(i). Upon metabolic arrest, depolarization-evoked pH(i) decreases ( approximately 0.2 pH units) were abolished, whereas glucose-free superfusate slightly delayed their recovery without major effects on amplitude. The glucose-dependent pH(i) fall coincided with activation of the K(ATP) channel-mediated outward current, while K(ATP) currents due to anoxia or metabolic arrest could reach their maximum in the absence of a major pH(i) change. The results indicate that the anoxic pH(i) decrease is due to enhanced glycolysis and lactate formation with often no obvious effect on K(ATP) channel activity. The origin of glucose-dependent acidosis and its relation to K(ATP) channel activity remain to be determined.

Allosteric modulation of the mouse Kir6.2 channel by intracellular H+ and ATP

The ATP-sensitive K+ (K(ATP)) channels are regulated by intracellular H+ in addition to ATP, ADP, and phospholipids. Here we show evidence for the interaction of H+ with ATP in regulating a cloned K(ATP) channel, i.e. Kir6.2 expressed with and without the SUR1 subunit. Channel sensitivity to ATP decreases at acidic pH, while the pH sensitivity also drops in the presence of ATP. These effects are more evident in the presence of the SUR1 subunit. In the Kir6.2 + SUR1, the pH sensitivity is reduced by about 0.4 pH units with 100 microM ATP and 0.6 pH units with 1 mM ATP, while a decrease in pH from 7.4 to 6.8 lowers the ATP sensitivity by about fourfold. The Kir6.2 + SUR1 currents are strongly activated at pH 5.9-6.5 even in the presence of 1 mM ATP. The modulations appear to take place at His175 and Lys185 that are involved in proton and ATP sensing, respectively. Mutation of His175 completely eliminates the pH effect on the ATP sensitivity. Similarly, the K185E mutant-channel loses the ATP-dependent modulation of the pH sensitivity. Thus, allosteric modulations of the cloned K(ATP) channel by ATP and H+ are demonstrated. Such a regulation allows protons to activate directly the K(ATP) channels and release channel inhibition by intracellular ATP; the pH effect is further enhanced with a decrease in ATP concentration as seen in several pathophysiological conditions.

Distinct histidine residues control the acid-induced activation and inhibition of the cloned K(ATP) channel

The modulation of K(ATP) channels during acidosis has an impact on vascular tone, myocardial rhythmicity, insulin secretion, and neuronal excitability. Our previous studies have shown that the cloned Kir6.2 is activated with mild acidification but inhibited with high acidity. The activation relies on His-175, whereas the molecular basis for the inhibition remains unclear. To elucidate whether the His-175 is indeed the protonation site and what other structures are responsible for the pH-induced inhibition, we performed these studies. Our data showed that the His-175 is the only proton sensor whose protonation is required for the channel activation by acidic pH. In contrast, the channel inhibition at extremely low pH depended on several other histidine residues including His-186, His-193, and His-216. Thus, proton has both stimulatory and inhibitory effects on the Kir6.2 channels, which attribute to two sets of histidine residues in the C terminus.

Requirement of multiple protein domains and residues for gating K(ATP) channels by intracellular pH

ATP-sensitive K(+) channels (K(ATP)) are regulated by pH in addition to ATP, ADP, and phospholipids. In the study we found evidence for the molecular basis of gating the cloned K(ATP) by intracellular protons. Systematic constructions of chimerical Kir6.2-Kir1.1 channels indicated that full pH sensitivity required the N terminus, C terminus, and M2 region. Three amino acid residues were identified in these protein domains, which are Thr-71 in the N terminus, Cys-166 in the M2 region, and His-175 in the C terminus. Mutation of any of them to their counterpart residues in Kir1.1 was sufficient to completely eliminate the pH sensitivity. Creation of these residues rendered the mutant channels clear pH-dependent activation. Thus, critical players in gating K(ATP) by protons are demonstrated. The pH sensitivity enables the K(ATP) to regulate cell excitability in a number of physiological and pathophysiological conditions when pH is low but ATP concentration is normal.

The effects of flecainide on ATP-sensitive K(+) channels in pig urethral myocytes

The effects of the antiarrhythmic drug flecainide on levcromakalim-induced hyperpolarization, macroscopic and unitary K(+) currents in pig urethra were investigated using patch-clamp techniques. The effects of flecainide were also examined on currents in inside-out patches of COS7 cells expressing carboxy terminus truncated inwardly rectifying K(+) channel (Kir6.2) subunits (i.e. Kir6.2DeltaC36) which form ATP-sensitive K(+) channels (K(ATP) channels). In current-clamp mode, application of flecainide (> or =100 microM) caused a significant depolarization after the membrane potential had been hyperpolarized by levcromakalim. In voltage-clamp experiments, the levcromakalim-induced outward current was suppressed by 300 microM flecainide in quasi-physiological K(+) conditions (K(i)=51 microM). In contrast, approximately 20% of the levcromakalim-induced inward current still remained even after application of 300 microM flecainide in symmetrical 140 mM K(+) conditions (K(i)=51 microM). In contrast, approximately 20% of the levcromakalim-induced inwar=126 microM). In cell-attached configuration, the channel activity of the levcromakalim-induced K(ATP) channels was reversibly inhibited by flecainide (> or =30 microM) at -50 mV. Their activity was also suppressed by either disopyramide or cibenzoline. Flecainide reversibly inhibited the channel activity of Kir6.2DeltaC36 expressed in COS7 cells using inside-out configuration. Inhibitory effects of flecainide on the levcromakalim-induced currents became more potent when the value of external pH increased, although this slightly reduced the proportion of drug molecules carrying a positive charge. These results suggest that flecainide inhibits channel activity through blocking the pore site of the K(ATP) channel in pig urethra.

Direct activation of cloned K(atp) channels by intracellular acidosis

ATP-sensitive K(+) (K(ATP)) channels may be regulated by protons in addition to ATP, phospholipids, and other nucleotides. Such regulation allows a control of cellular excitability in conditions when pH is low but ATP concentration is normal. However, whether the K(ATP) changes its activity with pH alterations remains uncertain. In this study we showed that the reconstituted K(ATP) was strongly activated during hypercapnia and intracellular acidosis using whole-cell recordings. Further characterizations in excised patches indicated that channel activity increased with a moderate drop in intracellular pH and decreased with strong acidification. The channel activation was produced by a direct action of protons on the Kir6 subunit and relied on a histidine residue that is conserved in all K(ATP). The inhibition appeared to be a result of channel rundown and was not seen in whole-cell recordings. The biphasic response may explain the contradictory pH sensitivity observed in cell-endogenous K(ATP) in excised patches. Site-specific mutations of two residues showed that pH and ATP sensitivities were independent of each other. Thus, these results demonstrate that the proton is a potent activator of the K(ATP). The pH-dependent activation may enable the K(ATP) to control vascular tones, insulin secretion, and neuronal excitability in several pathophysiologic conditions.

Positive cooperativity without domains or subunits in a monomeric membrane channel

The monomeric VDAC channel shows an accelerated pH titration of its transport properties with a Hill coefficient of about 2. This manifests itself as a sharp peak in conductance noise as well as a fast change in channel selectivity with pH. On the basis of the known structure of this channel, we propose that this cooperativity arises from a mechanically linked mobile pair of ionizable groups. Concerted movement of these groups between two states changes the distance from nearby electrostatic charge to influence the pK of the groups. This model of pH-dependent motion produces positive cooperative behavior that fits the observations without need for subunits or identifiable domains within the protein. The mathematical formalism has never required such domains, but these are generally considered an essential part of cooperative behavior in proteins. The present proposal reduces the size of a cooperative unit to a minimum, extending the limits of what is perceived to be possible. Together with large-scale conformational transitions, these subtle cooperative structural changes may allow proteins to adapt, with high sensitivity, to changes in their environment. They might also be relatively easy to engineer into a protein.

Disopyramide block of K(ATP) channels is mediated by the pore-forming subunit

The class Ia antiarrhythmic agent disopyramide blocks native ATP-sensitive K+ (K(ATP)) channels at micromolar concentrations. The K(ATP) channel is a complex of a pore-forming inwardly rectifying K+ channel (Kir6.2) and a sulfonylurea receptor (SUR). The aim of the present study was to further localize the site of action of disopyramide. We have used a C-terminal truncated form of Kir6.2 (Kir6.2delta26), which--in contrast to Kir6.2--expresses independently of SUR. Kir6.2delta26 channels were expressed in African green monkey kidney COS-7 cells, and enhanced green fluorescent protein (EGFP) cDNA was used as a reporter gene. EGFP fluorescence was visualized by a laser scanning confocal microscope. Disopyramide applied to the cytoplasmic membrane surface of inside-out patches inhibited Kir6.2delta26 channels half-maximally at 7.1 microM (at pH 7.15). Lowering the intracellular pH to 6.5 potentiated the inhibition of Kir6.2delta26 channels by disopyramide. These observations suggest that disopyramide directly blocks the pore-forming Kir6.2 subunit, in particular at reduced intracellular pH values that occur under cardiac ischaemia.

Inward rectification in KATP channels: a pH switch in the pore

Inward-rectifier potassium channels (Kir channels) stabilize the resting membrane potential and set a threshold for excitation in many types of cell. This function arises from voltage-dependent rectification of these channels due to blockage by intracellular polyamines. In all Kir channels studied to date, the voltage-dependence of rectification is either strong or weak. Here we show that in cardiac as well as in cloned KATP channels (Kir6.2 + sulfonylurea receptor) polyamine-mediated rectification is not fixed but changes with intracellular pH in the physiological range: inward-rectification is prominent at basic pH, while at acidic pH rectification is very weak. The pH-dependence of polyamine block is specific for KATP as shown in experiments with other Kir channels. Systematic mutagenesis revealed a titratable C-terminal histidine residue (H216) in Kir6.2 to be the structural determinant, and electrostatic interaction between this residue and polyamines was shown to be the molecular mechanism underlying pH-dependent rectification. This pH-dependent block of KATP channels may represent a novel and direct link between excitation and intracellular pH.

pH-dependent modulation of the cloned renal K+ channel, ROMK

pH is an important modulator of the low-conductance ATP-sensitive K+ channel of the distal nephron. To examine the mechanism of interaction of protons with the channel-forming protein, we expressed the cloned renal K channel, ROMK (Kir1.x), in Xenopus oocytes and examined the response to varied concentrations of protons both in the presence and in the absence of ATP. Initial experiments were performed on inside-out patches in the absence of ATP in Mg2+-free solution, which prevents channel rundown. A steep sigmoidal relationship was shown between bath pH and ROMK1 or ROMK2 channel function with intracellular acidification reducing channel activity. We calculated values for pK = 7.18 and 7.04 and Hill coefficients = 3.1 and 3.3, for ROMK1 and ROMK2, respectively. Intracellular acidification (pH 7.2) also increased the Mg-ATP binding affinity of ROMK2, resulting in a leftward shift of the relationship between ATP concentration and the reduction in channel activity. The K1/2 for Mg-ATP decreased from 2.4 mM at pH 7.4 to approximately 0.5 mM at pH 7.2. Mutation of lysine-61 to methionine in ROMK2, which abolishes pH sensitivity, modulated but did not eliminate the effect of pH on ATP inhibition of channel activity. We previously demonstrated that the putative phosphate loop in the carboxy terminus of ROMK2 is involved in ATP binding and channel inhibition [C. M. McNicholas, Y. Yang, G. Giebisch, and S. C. Hebert. Am. J. Physiol. 271 (Renal Fluid Electrolyte Physiol. 40): F275-F285, 1996]. Conceivably, therefore, protonation of the histidine residue within this region could alter net charge (i.e., positive shift) and increase affinity for the negatively charged nucleotide.

Mechanism of cloned ATP-sensitive potassium channel activation by oleoyl-CoA

Insulin secretion from pancreatic beta cells is coupled to cell metabolism through closure of ATP-sensitive potassium (KATP) channels, which comprise Kir6.2 and sulfonylurea receptor (SUR1) subunits. Although metabolic regulation of KATP channel activity is believed to be mediated principally by the adenine nucleotides, other metabolic intermediates, including long chain acyl-CoA esters, may also be involved. We recorded macroscopic and single-channel currents from Xenopus oocytes expressing either Kir6.2/SUR1 or Kir6. 2DeltaC36 (which forms channels in the absence of SUR1). Oleoyl-CoA (1 microM) activated both wild-type Kir6.2/SUR1 and Kir6.2DeltaC36 macroscopic currents, approximately 2-fold, by increasing the number and open probability of Kir6.2/SUR1 and Kir6.2DeltaC36 channels. It was ineffective on the related Kir subunit Kir1.1a. Oleoyl-CoA also impaired channel inhibition by ATP, increasing the Ki values for both Kir6.2/SUR1 and Kir6.2DeltaC36 currents by approximately 3-fold. Our results indicate that activation of KATP channels by oleoyl-CoA results from an interaction with the Kir6.2 subunit, unlike the stimulatory effects of MgADP and diazoxide which are mediated through SUR1. The increased activity and reduced ATP sensitivity of KATP channels by oleoyl-CoA might contribute to the impaired insulin secretion observed in non-insulin-dependent diabetes mellitus.

Intracellular acidosis differentially regulates KV channels in coronary and pulmonary vascular muscle

Decreases in intracellular pH (pHi) potently dilate coronary resistance arteries but constrict small pulmonary arteries. To define the ionic mechanisms of these responses, this study investigated whether acute decreases in pHi differentially regulate K+ currents in single vascular smooth muscle (VSM) cells isolated from rat coronary and pulmonary resistance arteries. In patch-clamp studies, whole cell K+ currents were elicited by 10-mV depolarizing steps between -60 and 0 mV in VSM cells obtained from 50- to 150-micrometers-OD arterial branches, and pHi was lowered by altering the NH4Cl gradient across the cell membrane. Progressively lowering pHi from calculated values of 7.0 to 6.7 and 6.4 increased the peak amplitude of K+ current in coronary VSM cells by 15 +/- 5 and 23 +/- 3% but reduced K+ current in pulmonary VSM cells by 18 +/- 3 and 21 +/- 3%, respectively. These changes were reversed by returning cells to the control pHi of 7.0 and were eliminated by dialyzing cells with pipette solution containing 50 mmol/l HEPES to buffer NH4Cl-induced changes in pHi. Pharmacological block of ATP-sensitive K+ channels and Ca2+-activated K+ channels by 1 micromol/l glibenclamide and 100 nmol/l iberiotoxin, respectively, did not prevent changes in K+ current levels induced by acidotic pHi. However, block of voltage-gated K+ channels by 3 mmol/l 4-aminopyridine abolished acidosis-induced changes in K+ current amplitudes in both VSM cell types. Interestingly, alpha-dendrotoxin (100 nmol/l), which blocks only select subtypes of voltage-gated K+ channels, abolished the acidosis-induced decrease in K+ current in pulmonary VSM cells but did not affect the acidosis-induced increase in K+ current observed in coronary VSM cells. These findings suggest that opposing, tissue-specific effects of pHi on distinct subtypes of voltage-gated K+ channels in coronary and pulmonary VSM membranes may differentially regulate vascular reactivity in these two circulations under conditions of acidotic stress.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Metabolic inhibition impairs ATP-sensitive K+ channel block by sulfonylurea in pancreatic beta-cells

The effect of metabolic inhibition on the blocking of beta-cell ATP-sensitive K+ channels (KATP channels) by glibenclamide was investigated using a patch-clamp technique. Inhibition of KATP channels by glibenclamide was attenuated in the cell-attached mode under metabolic inhibition induced by 2,4-dinitrophenol. Under a low concentration (0.1 microM) of ATP applied in the inside-out mode, KATP channel activity was not fully abolished, even when a high dose of glibenclamide was applied, in contrast to the dose-dependent and complete KATP channel inhibition under 10 microM ATP. On the other hand, cibenzoline, a class Ia antiarrhythmic agent, inhibits KATP channel activity in a dose-dependent manner and completely blocks it, even under metabolic inhibition. In sulfonylurea receptor (SUR1)- and inward rectifier K+ channel (Kir6.2)-expressed proteins, cibenzoline binds directly to Kir6.2, unlike glibenclamide. Thus, KATP channel inhibition by glibenclamide is impaired under the condition of decreased intracellular ATP in pancreatic beta-cells, probably because of a defect in signal transmission between SUR1 and Kir6.2 downstream of the site of sulfonylurea binding to SUR1.

Effects of pH on the potassium current in rabbit corneal epithelial cells

The effects of pH on K+ conductance were measured using the amphotericin perforated-patch whole cell voltage-clamp technique in freshly dispersed rabbit corneal epithelial cells. Bath perfusion with pH 6.00 Ringer solution after standard Ringer solution (pH 7.35) increased outward K+-selective current (I(K)) from 120 +/- 29 to 312 +/- 64 pA during a step depolarization to +50 mV and hyperpolarized the resting membrane potential (Em) from -52 +/- 5 to -62 +/- 3 mV (n = 15, P < or = 0.05). Increasing bath pH to 8.5 decreased I(K) from 183 +/- 40 to 114 +/- 35 pA (n = 6, P < or = 0.05) and depolarized Em from -63 +/- 6 to -53 +/- 5 mV (n = 6, P < or = 0.05). Intracellular acidification using the weak electrolyte (NH4)2SO4 also increased I(K) from 83 +/- 15 to 183 +/- 20 pA (n = 4, P = 0.01) and hyperpolarized Em from -51 +/- 8 to -68 +/- 6 mV (P = 0.002). Intracellular alkalinization reduced I(K) to 66 +/- 10 pA and depolarized Em to -36 +/- 8 mV (P = 0.009). Single channel studies in perforated outside-out vesicles showed that a decrease in bath pH from 7.35 to 6.00 was accompanied by an increase in the single channel open probability (NPo) from 0.43 to 0.64 at an Em of 15 mV. NPo was also increased in cell-attached patches. The unitary conductance, measured from -100 to +100 mV, was not changed. These results indicate that pH modulates I(K) in rabbit corneal epithelial cells by changes in NPo.

ATP-sensitive K+ channels in the kidney

ATP-sensitive K+ channels (KATP channels) form a link between the metabolic state of the cell and the permeability of the cell membrane for K+ which, in turn, is a major determinant of cell membrane potential. KATP channels are found in many different cell types. Their regulation by ATP and other nucleotides and their modulation by other cellular factors such as pH and kinase activity varies widely and is fine-tuned for the function that these channels have to fulfill. In most excitable tissues they are closed and open when cell metabolism is impaired; thereby the cell is clamped in the resting state which saves ATP and helps to preserve the structural integrity of the cell. There are, however, notable exceptions from this rule; in pancreatic beta-cells, certain neurons and some vascular beds, these channels are open during the normal functioning of the cell. In the renal tubular system, KATP channels are found in the proximal tubule, the thick ascending limb of Henle's loop and the cortical collecting duct. Under physiological conditions, these channels have a high open probability and play an important role in the reabsorption of electrolytes and solutes as well as in K+ homeostasis. The physiological role of their nucleotide sensitivity is not entirely clear; one consequence is the coupling of channel activity to the activity of the Na-K-ATPase (pump-leak coupling), resulting in coordinated vectorial transport. In ischemia, however, the reduced ATP/ADP ratio would increase the open probability of the KATP channels independently from pump activity; this is particularly dangerous in the proximal tubule, where 60 to 70% of the glomerular ultrafiltrate is reabsorbed. The pharmacology of KATP channels is well developed including the sulphonylureas as standard blockers and the structurally heterogeneous family of channel openers. Blockers and openers, exemplified by glibenclamide and levcromakalim, show a wide spectrum of affinities towards the different types of KATP channels. Recent cloning efforts have solved the mystery about the structure of the channel: the KATP channels in the pancreatic beta-cell and in the principal cell of the renal cortical collecting duct are heteromultimers, composed of an inwardly rectifying K+ channel and sulphonylurea binding subunit(s) with unknown stoichiometry. The proteins making up the KATP channel in these two cell types are different (though homologous), explaining the physiological and pharmacological differences between these channel subtypes.

Multiphasic action of glucose and alpha-ketoisocaproic acid on the cytosolic pH of pancreatic beta-cells. Evidence for an acidification pathway linked to the stimulation of Ca2+ influx

Glucose stimulation raises the pHi of pancreatic beta-cells, but the underlying mechanisms are not well understood. We have now investigated the acute effects of metabolizable (glucose and the mitochondrial substrate alpha-ketoisocaproic acid, KIC) and nonmetabolizable (high K+ and the K-ATP channel blocker tolbutamide) insulin secretagogues on the pHi of pancreatic beta-cells isolated from normal mice, as assessed by BCECF fluorescence from single cells or islets in the presence of external bicarbonate. The typical acute effect of glucose (22-30 mM) on the pHi was a fast alkalinization of approximately 0.11 unit, followed by a slower acidification. The relative expression of the alkalinizing and acidifying components was variable, with some cells and islets displaying a predominant alkalinization, others a predominant acidification, and others yet a mixed combination of the two. The initial alkalinization preceded the [Ca2+]i rise associated with the activation of voltage-sensitive Ca2+ channels. There was a significant overlap between the glucose-evoked [Ca2+]i rise and the development of the secondary acidification. Depolarization with 30 mM K+ and tolbutamide evoked pronounced [Ca2+]i rises and concomitant cytosolic acidifications. Blocking glucose-induced Ca2+ influx (with 0 Ca2+, nifedipine, or the K-ATP channel agonist diazoxide) suppressed the secondary acidification while having variable effects (potentiation or slight attenuation) on the initial alkalinization. KIC exerted glucose-like effects on the pHi and [Ca2+]i, but the amplitude of the initial alkalinization was about twice as large for KIC relative to glucose. It is concluded that the acute effect of glucose on the pHi of pancreatic beta-cells is biphasic. While the initial cytosolic alkalinization is an immediate consequence of the activation of H+-consuming metabolic steps in the mitochondria, the secondary acidification appears to originate from enhanced Ca2+ turnover in the cytoplasm. The degree of coupling between glucose metabolism and Ca2+ influx as well as the relative efficacies of these processes determines whether the acute pHi response of a beta-cell (or of a tightly coupled multicellular system such as an islet of Langerhans) is predominantly an alkalinization, an acidification, or a mixed proportion of the two.

Modification by protons of frog skeletal muscle KATP channels: effects on ion conduction and nucleotide inhibition

1. The molecular mechanisms underlying pH regulation of skeletal muscle ATP-sensitive K+ (KATP) channels were studied using the patch clamp technique in the inside-out configuration. Two effects of intracellular protons were studied in detail: the decrease in magnitude of single-channel currents and the increase in open probability (Po) of nucleotide-inhibited channels. 2. The pH dependence of inward unit currents under different ionic conditions was in poor agreement with either a direct block of the pore by protons or an indirect proton-induced conformational change, but was compatible with the protonation of surface charges located near the cytoplasmic entrance of the pore. This latter electrostatic mechanism was modelled using Gouy-Chapman-Stern theory, which predicted the data accurately with a surface charge density of about 0.1 negative elementary charges per square nanometre and a pK (pH value for 50% effect) value for protonation of these charges of 6.25. The same mechanism, i.e. neutralization of negative surface charges by cation binding, could also account for the previously reported reduction of inward unit currents by Mg2+. 3. Intracellular alkalization did not affect Po of the KATP channels. Acidification increased Po. In the presence of 0.1 mM ATP (no Mg2+), the channel activation vs. pH relationship could be fitted with a sigmoid curve with a Hill coefficient slightly above 2 and a pK value of 6. This latter value was dependent on the ATP concentration, decreasing from 6.3 in 30 microM ATP to 5.3 in 1 microM ATP. 4. Conversely, the channel inhibition vs. ATP concentration curve was shifted to the right when the pH was lowered. At pH 7.1, the ATP concentration causing half-maximal inhibition was about 10 microM. At pH 5.4, it was about 400 microM. The Hill coefficient values remained slightly below 2. Similar effects were observed when ADP was used as the inhibitory nucleotide. 5. These results confirm that a reciprocal competitive link exists between proton and nucleotide binding sites. Quantitatively, they are in full agreement with a steady-state model of a KATP channel possessing four identical protonation sites (microscopic pK, 6) allosterically connected to the channel open state and two identical nucleotide sites (microscopic ATP dissociation constant, approximately 30 microM) connected to the closed state.

Activation of ATP-dependent K+ channels by metabolic poisoning in adult mouse skeletal muscle: role of intracellular Mg(2+) and pH

1. The effects of metabolic poisoning, intracellular Mg(2+) and pH on ATP-dependent K+ (K+ATP) channels were examined in adult mouse isolated skeletal muscle fibres using the patch clamp technique. 2. In cell-attached membrane patches, while openings of one kind of channel could only rarely be detected under control conditions, cell poisoning with fluorodinitrobenzene (FDNB), dinitrophenol (DNP) and cyanide (CN) induced a strong and partially reversible increase in channel activity. 3. Slope conductance and glibenclamide sensitivity of this outward current indicated that the channel activated during poisoning was the K+ATP channel. 4. Single channel current amplitude was reduced during poisoning, but remained unchanged when activation of the K+ATP channel was induced by cromakalim. 5. In inside-out membrane patches, in the absence of intracellular ATP, intracellular application of Mg2+ decreased channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 6. In the absence of intracellular ATP, a decrease in intracellular pH induced a reduction in channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 7. The reduction of single channel current amplitude during poisoning was attributed to an increase in intracellular Mg2+ concentration caused by a fall in intracellular ATP concentration. These results also show that metabolic poisoning causes direct activation of K+ATP channels in skeletal muscle, and that is activation is at least partially mediated through an increase in intracellular Mg(2+) concentration and a decrease in intracellular pH.

Effects of internal chloride on ATP-sensitive K-channels in mouse pancreatic beta-cells

The effect of internal Cl- on the K-ATP channel of pancreatic beta-cells was examined. Reducing Cl- from 140mM to 14mM potentiated channel activity (NPO) approximately 4.5 fold in 60% of patches but was without effect in 40% of patches. The Ki for channel inhibition by ATP was not changed. The inhibitory effect of Cl- interacted with the stimulatory action of MgGDP. In 140mM Cl-, 1mM MgGDP increased NPO in Cl(-)-sensitive patches only: when NPO was potentiated by 14mM Cl-, no further increase was produced by MgGDP. These observations suggest that MgGDP and low Cl- solution may increase channel activity via a similar mechanism.