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

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.

A mechano- and heat-gated two-pore domain K+ channel controls excitability in adult zebrafish skeletal muscle

TRAAK channels are mechano-gated two-pore-domain K+ channels. Up to now, activity of these channels has been reported in neurons but not in skeletal muscle, yet an archetype of tissue challenged by mechanical stress. Using patch clamp methods on isolated skeletal muscle fibers from adult zebrafish, we show here that single channels sharing properties of TRAAK channels, i.e., selective to K+ ions, of 56 pS unitary conductance in the presence of 5 mM external K+, activated by membrane stretch, heat, arachidonic acid, and internal alkaline pH, are present in enzymatically isolated fast skeletal muscle fibers from adult zebrafish. The kcnk4b transcript encoding for TRAAK channels was cloned and found, concomitantly with activity of mechano-gated K+ channels, to be absent in zebrafish fast skeletal muscles at the larval stage but arising around 1 mo of age. The transfer of the kcnk4b gene in HEK cells and in the adult mouse muscle, that do not express functional TRAAK channels, led to expression and activity of mechano-gated K+ channels displaying properties comparable to native zebrafish TRAAK channels. In whole-cell voltage-clamp and current-clamp conditions, membrane stretch and heat led to activation of macroscopic K+ currents and to acceleration of the repolarization phase of action potentials respectively, suggesting that heat production and membrane deformation associated with skeletal muscle activity can control muscle excitability through TRAAK channel activation. TRAAK channels may represent a teleost-specific evolutionary product contributing to improve swimming performance for escaping predators and capturing prey at a critical stage of development.

Exercise and fatigue: integrating the role of K+, Na+ and Cl- in the regulation of sarcolemmal excitability of skeletal muscle

Perturbations in K+ have long been considered a key factor in skeletal muscle fatigue. However, the exercise-induced changes in K+ intra-to-extracellular gradient is by itself insufficiently large to be a major cause for the force decrease during fatigue unless combined to other ion gradient changes such as for Na+. Whilst several studies described K+-induced force depression at high extracellular [K+] ([K+]e), others reported that small increases in [K+]e induced potentiation during submaximal activation frequencies, a finding that has mostly been ignored. There is evidence for decreased Cl- ClC-1 channel activity at muscle activity onset, which may limit K+-induced force depression, and large increases in ClC-1 channel activity during metabolic stress that may enhance K+ induced force depression. The ATP-sensitive K+ channel (KATP channel) is also activated during metabolic stress to lower sarcolemmal excitability. Taking into account all these findings, we propose a revised concept in which K+ has two physiological roles: (1) K+-induced potentiation and (2) K+-induced force depression. During low-moderate intensity muscle contractions, the K+-induced force depression associated with increased [K+]e is prevented by concomitant decreased ClC-1 channel activity, allowing K+-induced potentiation of sub-maximal tetanic contractions to dominate, thereby optimizing muscle performance. When ATP demand exceeds supply, creating metabolic stress, both KATP and ClC-1 channels are activated. KATP channels contribute to force reductions by lowering sarcolemmal generation of action potentials, whilst ClC-1 channel enhances the force-depressing effects of K+, thereby triggering fatigue. The ultimate function of these changes is to preserve the remaining ATP to prevent damaging ATP depletion.

Chloride Channels Take Center Stage in Acute Regulation of Excitability in Skeletal Muscle: Implications for Fatigue

Initiation and propagation of action potentials in muscle fibers is a key element in the transmission of activating motor input from the central nervous system to their contractile apparatus, and maintenance of excitability is therefore paramount for their endurance during work. Here, we review current knowledge about the acute regulation of ClC-1 channels in active muscles and its importance for muscle excitability, function, and fatigue.

KATP channel deficiency in mouse FDB causes an impairment of energy metabolism during fatigue

The skeletal muscle ATP-sensitive K⁺ (K_ATP) channel is crucial in preventing fiber damage and contractile dysfunction, possibly by preventing damaging ATP depletion. The objective of this study was to investigate changes in energy metabolism during fatigue in wild-type and inwardly rectifying K⁺ channel (Kir6.2)-deficient (Kir6.2^-/-) flexor digitorum brevis (FDB), a muscle that lacks functional K_ATP channels. Fatigue was elicited with one tetanic contraction every second. Decreases in ATP and total adenylate levels were significantly greater in wild-type than Kir6.2^-/- FDB during the last 2 min of the fatigue period. Glycogen depletion was greater in Kir6.2^-/- FDB for the first 60 s, but not by the end of the fatigue period, while there was no difference in glucose uptake. The total amount of glucosyl units entering glycolysis was the same in wild-type and Kir6.2^-/- FDB. During the first 60 s, Kir6.2^-/- FDB generated less lactate and more CO₂; in the last 120 s, Kir6.2^-/- FDB stopped generating CO₂ and produced more lactate. The ATP generated during fatigue from phosphocreatine, glycolysis (lactate), and oxidative phosphorylation (CO₂) was 3.3-fold greater in Kir6.2^-/- than wild-type FDB. Because ATP and total adenylate were significantly less in Kir6.2^-/- FDB, it is suggested that Kir6.2^-/- FDB has a greater energy deficit, despite a greater ATP production, which is further supported by greater glucose uptake and lactate and CO₂ production in Kir6.2^-/- FDB during the recovery period. It is thus concluded that a lack of functional K_ATP channels results in an impairment of energy metabolism.

Adenosine triphosphate depletion by cyanide results in a Na(+)-dependent Mg(2+) extrusion from liver cells

Addition of NaCN to isolated hepatocytes results in a marked and rapid decrease in cellular adenosine triphosphate (ATP) content, and in the extrusion of a sizable amount of cellular Mg(2+). This extrusion starts after a 10-minute lag phase and reaches a maximum of 35 to 40 nmol Mg(2+) per milligram protein within 60 minutes from the addition of CN(-). A quantitatively similar Mg(2+) extrusion is also observed after the addition of the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone but not that of the glycolysis inhibitor iodoacetate. The Mg(2+) extrusion is completely inhibited by the removal of extracellular Na(+) or the addition of imipramine, quinidine, or glibenclamide, whereas it persists after the removal of extracellular Ca(2+) or K(+), or the addition of amiloride. An acidic extracellular pH or the removal of extracellular HCO₃⁻ inhibits the cyanide-induced Mg(2+) extrusion by at least 80%. Taken together, these data suggest that the decrease in cellular adenosine triphosphate content removes a major Mg(2+) complexing agent from the hepatocyte and results in an extrusion of hepatic Mg(2+) exclusively through a Na(+)-dependent exchange mechanism modulated by acidic changes in extracellular pH.

K(ATP) channels process nucleotide signals in muscle thermogenic response

Uniquely gated by intracellular adenine nucleotides, sarcolemmal ATP-sensitive K(+) (K(ATP)) channels have been typically assigned to protective cellular responses under severe energy insults. More recently, K(ATP) channels have been instituted in the continuous control of muscle energy expenditure under non-stressed, physiological states. These advances raised the question of how K(ATP) channels can process trends in cellular energetics within a milieu where each metabolic system is set to buffer nucleotide pools. Unveiling the mechanistic basis of the K(ATP) channel-driven thermogenic response in muscles thus invites the concepts of intracellular compartmentalization of energy and proteins, along with nucleotide signaling over diffusion barriers. Furthermore, it requires gaining insight into the properties of reversibility of intrinsic ATPase activity associated with K(ATP) channel complexes. Notwithstanding the operational paradigm, the homeostatic role of sarcolemmal K(ATP) channels can be now broadened to a wider range of environmental cues affecting metabolic well-being. In this way, under conditions of energy deficit such as ischemic insult or adrenergic stress, the operation of K(ATP) channel complexes would result in protective energy saving, safeguarding muscle performance and integrity. Under energy surplus, downregulation of K(ATP) channel function may find potential implications in conditions of energy imbalance linked to obesity, cold intolerance and associated metabolic disorders.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Abstract Potassium (K(+)) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K(+) concentration ([K(+)](e)) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K(+)](e) increases progressively with increasing work intensity, reaching values higher than 10 mm. This increase in [K(+)](e) is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K(+) (K(v)) channels that generate the action potential (AP) it is suggested that the big-conductance Ca(2+)-dependent K(+) (K(Ca)1.1) channel contributes significantly to the K(+) release into the T-tubules. Also the ATP-dependent K(+) (K(ATP)) channel participates, but is suggested primarily to participate in K(+) release to the interstitium. Because there is restricted diffusion of K(+) to the interstitium, K(+) released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K(+) reuptake in the T-tubules is the Na(+),K(+)-ATPase alpha(2) dimers, but a significant contribution of the strong inward rectifier K(+) (Kir2.1) channel is also suggested. The Na(+), K(+), 2Cl(-) 1 (NKCC1) cotransporter also participates in K(+) reuptake but probably mainly from the interstitium. The relative content of the different K(+)-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K(+)](e) tolerance observed.

Regulation of ClC-1 and KATP channels in action potential-firing fast-twitch muscle fibers

Action potential (AP) excitation requires a transient dominance of depolarizing membrane currents over the repolarizing membrane currents that stabilize the resting membrane potential. Such stabilizing currents, in turn, depend on passive membrane conductance (G(m)), which in skeletal muscle fibers covers membrane conductances for K(+) (G(K)) and Cl(-) (G(Cl)). Myotonic disorders and studies with metabolically poisoned muscle have revealed capacities of G(K) and G(Cl) to inversely interfere with muscle excitability. However, whether regulation of G(K) and G(Cl) occur in AP-firing muscle under normal physiological conditions is unknown. This study establishes a technique that allows the determination of G(Cl) and G(K) with a temporal resolution of seconds in AP-firing muscle fibers. With this approach, we have identified and quantified a biphasic regulation of G(m) in active fast-twitch extensor digitorum longus fibers of the rat. Thus, at the onset of AP firing, a reduction in G(Cl) of approximately 70% caused G(m) to decline by approximately 55% in a manner that is well described by a single exponential function characterized by a time constant of approximately 200 APs (phase 1). When stimulation was continued beyond approximately 1,800 APs, synchronized elevations in G(K) ( approximately 14-fold) and G(Cl) ( approximately 3-fold) caused G(m) to rise sigmoidally to approximately 400% of its level before AP firing (phase 2). Phase 2 was often associated with a failure to excite APs. When AP firing was ceased during phase 2, G(m) recovered to its level before AP firing in approximately 1 min. Experiments with glibenclamide (K(ATP) channel inhibitor) and 9-anthracene carboxylic acid (ClC-1 Cl(-) channel inhibitor) revealed that the decreased G(m) during phase 1 reflected ClC-1 channel inhibition, whereas the massively elevated G(m) during phase 2 reflected synchronized openings of ClC-1 and K(ATP) channels. In conclusion, G(Cl) and G(K) are acutely regulated in AP-firing fast-twitch muscle fibers. Such regulation may contribute to the physiological control of excitability in active muscle.

Contractile dysfunctions in ATP-dependent K+ channel-deficient mouse muscle during fatigue involve excessive depolarization and Ca2+ influx through L-type Ca2+ channels

Muscles deficient in ATP-dependent potassium (KATP) channels develop contractile dysfunctions during fatigue that may explain their apparently faster rate of fatigue compared with wild-type muscles. The objectives of this study were to determine: (1) whether the contractile dysfunctions, namely unstimulated force and depressed force recovery, result from excessive membrane depolarization and Ca2+ influx through L-type Ca2+ channels; and (2) whether reducing the magnitude of these two contractile dysfunctions reduces the rate of fatigue in KATP channel-deficient muscles. To reduce Ca2+ influx, we lowered the extracellular Ca2+ concentration ([Ca2+]o) from 2.4 to 0.6 mM or added 1 microM verapamil, an L-type Ca2+ channel blocker. Flexor digitorum brevis (FDB) muscles deficient in KATP channels were obtained by exposing wild-type muscles to 10 microM glibenclamide or by using FDB from Kir6.2-/- mice. Fatigue was elicited with one contraction per second for 3 min at 37 degrees C. In wild-type FDB, lowered [Ca2+]o or verapamil did not affect the decrease in peak tetanic force and unstimulated force during fatigue and force recovery following fatigue. In KATP channel-deficient FDB, lowered [Ca2+]o or verapamil slowed down the decrease in peak tetanic force recovery, reduced unstimulated force and improved force recovery. In Kir6.2-/- FDB, the rate of fatigue became slower than in wild-type FDB in the presence of verapamil. The cell membrane depolarized from -83 to -57 mV in normal wild-type FDB. The depolarizations in some glibenclamide-exposed fibres were similar to those of normal FDB, while in other fibres the cell membrane depolarized to -31 mV in 80 s, which was also the time when these fibres supercontracted. It is concluded that: (1) KATP channels are crucial in preventing excessive membrane depolarization and Ca2+ influx through L-type Ca2+ channels; and (2) they contribute to the decrease in force during fatigue.

Effects of extracellular HCO3(-) on fatigue, pHi, and K+ efflux in rat skeletal muscles

Elevated plasma HCO(3)(-) can improve exercise endurance in humans. This effect has been related to attenuation of the work-induced reduction in muscle pH, which is suggested to improve performance via at least two mechanisms: 1) less inhibition of muscle enzymes and 2) reduced opening of muscle K(ATP) channels with less ensuing reduction in excitability. Aiming at determining whether the ergogenic effect of HCO(3)(-) is related to effects on muscles, we examined the effect of elevating extracellular HCO(3)(-) from 25 to 40 mM (pH from 7.4 to 7.6) on fatigue, intracellular pH (pH(i)), and K(+) efflux in isolated rat skeletal muscles contracting isometrically. Fatigue induced by 30-Hz stimulation at 30 and 37 degrees C was similar between soleus muscles incubated in high and normal HCO(3)(-) concentrations. In extensor digitorum longus muscles stimulated at 60 Hz, elevated HCO(3)(-) did not affect fatigue at 30 degrees C. In soleus muscles, 30-Hz stimulation induced a approximately 0.2 unit reduction in pH(i), as determined by using the pH-sensitive probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. This reduction in pH(i) was not affected by elevated HCO(3)(-). Estimation of K(+) efflux using (86)Rb(+) showed that elevated HCO(3)(-) did not affect K(+) efflux at rest or during contractions. Similarly, other modifications of the intra- and extracellular pH had little effect on K(+) efflux during contraction. In conclusion, elevated extracellular HCO(3)(-) had no significant effect on muscle fatigue, pH(i), and K(+) efflux. These findings indicate that alternative mechanisms must be considered for the ergogenic effect of HCO(3)(-) observed in integral exercise studies.

Sarcolemmal ion channels in dystrophin-deficient skeletal muscle fibres

Duchenne muscular dystrophy (DMD) is a genetic disease caused by mutations in the dystrophin gene and characterized by progressive skeletal muscle degeneration. A current hypothesis suggests that degeneration of dystrophin-deficient skeletal muscle results from a chronic intracellular Ca2+ overload. Ca2+ handling in skeletal muscle is tightly controlled by the membrane potential which is set by sarcolemmal ion channels activity. Also, with regard to the subsarcolemmal localization of dystrophin, it is reasonable to enquire if the distribution and function of ion channels might be affected by the absence of dystrophin. This paper briefly summarizes the current knowledge of the properties of sarcolemmal ion channels in fully differentiated dystrophin-deficient skeletal muscle fibres.

Syntaxin-1A actions on sulfonylurea receptor 2A can block acidic pH-induced cardiac K(ATP) channel activation

During cardiac ischemia, ATP stores are depleted, and cardiomyocyte intracellular pH lowers to <7.0. The acidic pH acts on the Kir6.2 subunit of K(ATP) channels to reduce its sensitivity to ATP, causing channel opening. We recently reported that syntaxin-1A (Syn-1A) binds nucleotide binding folds (NBF)-1 and NBF2 of sulfonylurea receptor 2A (SUR2A) to inhibit channel activity (Kang, Y., Leung, Y. M., Manning-Fox, J. E., Xia, F., Xie, H., Sheu, L., Tsushima, R. G., Light, P. E., and Gaisano, H. Y. (2004) J. Biol. Chem. 279, 47125-47131). Here, we examined Syn-1A actions on SUR2A to influence the pH regulation of cardiac K(ATP) channels. K(ATP) channel currents from inside-out patches excised from Kir6.2/SUR2A expressing HEK293 cells and freshly isolated cardiac myocytes were increased by reducing intracellular pH from 7.4 to 6.8, which could be blocked by increasing concentrations of Syn-1A added to the cytoplasmic surface. Syn-1A had no effect on C-terminal truncated Kir6.2 (Kir6.2-deltaC26) channels expressed in TSA cells without the SUR subunit. In vitro binding and co-immunoprecipitation studies show that Syn-1A binding to SUR2A or its NBF-1 and NBF-2 domain proteins increased progressively as pH was reduced from 7.4 to 6.0. The enhancement of Syn-1A binding to SUR2A by acidic pH was further regulated by Mg2+ and ATP. Therefore, pH regulates Kir.6.2/SUR2A channels not only by its direct actions on the Kir6.2 subunit but also by modulation of Syn-1A binding to SUR2A. The increased Syn-1A binding to the SUR2A at acidic pH would assert some inhibition of the K(ATP) channels, which may serve as a "brake" to temper the fluctuation of low pH-induced K(ATP) channel opening that could induce fatal reentrant arrhythmias.

Treadmill running causes significant fiber damage in skeletal muscle of KATP channel-deficient mice

Although it has been suggested that the ATP-sensitive K+ (KATP) channel protects muscle against function impairment, most studies have so far given little evidence for significant perturbation in the integrity and function of skeletal muscle fibers from inactive mice that lack KATP channel activity in their cell membrane. The objective was, therefore, to test the hypothesis that KATP channel-deficient skeletal muscle fibers become damaged when mice are subjected to stress. Wild-type and KATP channel-deficient mice (Kir6.2−/− mice) were subjected to 4–5 wk of treadmill running at either 20 m/min with 0° inclination or at 24 m/min with 20° uphill inclination. Muscles of all wild-type mice and of nonexercised Kir6.2−/− mice had very few fibers with internal nuclei. After 4–5 wk of treadmill running, there was little evidence for connective tissues and mononucleated cells in Kir6.2−/− hindlimb muscles, whereas the number of fibers with internal nuclei, which appear when damaged fibers are regenerated by satellite cells, was significantly higher in Kir6.2−/− than wild-type mice. Between 5% and 25% of the total number of fibers in Kir6.2−/− extensor digitum longus, plantaris, and tibialis muscles had internal nuclei, and most of such fibers were type IIB fibers. Contrary to hindlimb muscles, diaphragms of Kir6.2−/− mice that had run at 24 m/min had few fibers with internal nuclei, but mild to severe fiber damage was observed. In conclusion, the study provides for the first time evidence 1) that the KATP channels of skeletal muscle are essential to prevent fiber damage, and thus muscle dysfunction; and 2) that the extent of fiber damage is greater and the capacity of fiber regeneration is less in Kir6.2−/− diaphragm muscles compared with hindlimb muscles.

Role of adenosine A1 and A3 receptors in regulation of cardiomyocyte homeostasis after mitochondrial respiratory chain injury

Activation of either the A(1) or the A(3) adenosine receptor (A(1)R or A(3)R, respectively) elicits delayed cardioprotection against infarction, ischemia, and hypoxia. Mitochondrial contribution to the progression of cardiomyocyte injury is well known; however, the protective effects of adenosine receptor activation in cardiac cells with a respiratory chain deficiency are poorly elucidated. The aim of our study was to further define the role of A(1)R and A(3)R activation on functional tolerance after inhibition of the terminal link of the mitochondrial respiratory chain with sodium azide, in a state of normoxia or hypoxia, compared with the effects of the mitochondrial ATP-sensitive K(+) channel opener diazoxide. Treatment with 10 mM sodium azide for 2 h in normoxia caused a considerable decrease in the total ATP level; however, activation of adenosine receptors significantly attenuated this decrease. Diazoxide (100 muM) was less effective in protection. During treatment of cultured cardiomyocytes with hypoxia in the presence of 1 mM sodium azide, the A(1)R agonist 2-chloro-N(6)-cyclopentyladenosine was ineffective, whereas the A(3)R agonist 2-chloro-N(6)-iodobenzyl-5'-N-methylcarboxamidoadenosine (Cl-IB-MECA) attenuated the decrease in ATP level and prevented cell injury. Cl-IB-MECA delayed the dissipation in the mitochondrial membrane potential during hypoxia in cells impaired in the mitochondrial respiratory chain. In cells with elevated intracellular Ca(2+) concentration after hypoxia and treatment with NaN(3) or after application of high doses of NaN(3), Cl-IB-MECA immediately decreased the elevated intracellular Ca(2+) concentration toward the diastolic control level. The A(1)R agonist was ineffective. This may be especially important for the development of effective pharmacological agents, because mitochondrial dysfunction is a leading factor in the pathophysiological cascade of heart disease.

Localization and function of ATP-sensitive potassium channels in human skeletal muscle

The present study investigated the localization of ATP-sensitive K+ (KATP) channels in human skeletal muscle and the functional importance of these channels for human muscle K+ distribution at rest and during muscle activity. Membrane fractionation based on the giant vesicle technique or the sucrose-gradient technique in combination with Western blotting demonstrated that the KATP channels are mainly located in the sarcolemma. This localization was confirmed by immunohistochemical measurements. With the microdialysis technique, it was demonstrated that local application of the KATP channel inhibitor glibenclamide reduced (P < 0.05) interstitial K+ at rest from approximately 4.5 to 4.0 mM, whereas the concentration in the control leg remained constant. Glibenclamide had no effect on the interstitial K+ accumulation during knee-extensor exercise at a power output of 60 W. In contrast to in vitro conditions, the present study demonstrated that under in vivo conditions the KATP channels are active at rest and contribute to the accumulation of interstitial K+.

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.

Denervation enhances the physiological effects of the K(ATP) channel during fatigue in EDL and soleus muscle

The objective was to determine whether denervation reduces or enhances the physiological effects of the K(ATP) channel during fatigue in mouse extensor digitorum longus (EDL) and soleus muscle. For this, we measured the effects of 100 microM of pinacidil, a channel opener, and of 10 microM of glibenclamide, a channel blocker, in denervated muscles and compared the data to those observed in innervated muscles from the study of Matar et al. (Matar W, Nosek TM, Wong D, and Renaud JM. Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during fatigue in skeletal muscle. Am J Physiol Cell Physiol 278: C404-C416, 2000). Pinacidil increased the (86)Rb(+) fractional loss during fatigue, and this effect was 2.6- to 3.4-fold greater in denervated than innervated muscle. Pinacidil also increased the rate of fatigue; for EDL the effect was 2.5-fold greater in denervated than innervated muscle, whereas for soleus the difference was 8.6-fold. A major effect of glibenclamide was an increase in resting tension during fatigue, which was for the EDL and soleus muscle 2.7- and 1.9-fold greater, respectively, in denervated than innervated muscle. A second major effect of glibenclamide was a reduced capacity to recover force after fatigue, an effect observed only in denervated muscle. We therefore suggest that the physiological effects of the K(ATP) channel are enhanced after denervation.

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.

Stretch-induced activation of Ca(2+)-activated K(+) channels in mouse skeletal muscle fibers

High-conductance Ca(2+)-activated K(+) (K(Ca)) channels were studied in mouse skeletal muscle fibers using the patch-clamp technique. In inside-out patches, application of negative pressure to the patch induced a dose-dependent and reversible activation of K(Ca) channels. Stretch-induced increase in channel activity was found to be of the same magnitude in the presence and in the absence of Ca(2+) in the pipette. The dose-response relationships between K(Ca) channel activity and intracellular Ca(2+) and between K(Ca) channel activity and membrane potential revealed that voltage and Ca(2+) sensitivity were not altered by membrane stretch. In cell-attached patches, in the presence of high external Ca(2+) concentration, stretch-induced activation was also observed. We conclude that membrane stretch is a potential mode of regulation of skeletal muscle K(Ca) channel activity and could be involved in the regulation of muscle excitability during contraction-relaxation cycles.

Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during muscle fatigue

The effects of 10 microM glibenclamide, an ATP-sensitive K(+) (K(ATP)) channel blocker, and 100 microM pinacidil, a channel opener, were studied to determine how the K(ATP) channel affects mouse extensor digitorum longus (EDL) and soleus muscle during fatigue. Fatigue was elicited with 200-ms-long tetanic contractions every second. Glibenclamide did not affect rate and extent of fatigue, force recovery, or (86)Rb(+) fractional loss. The only effects of glibenclamide during fatigue were: an increase in resting tension (EDL and soleus), a depolarization of the cell membrane, a prolongation of the repolarization phase of action potential, and a greater ATP depletion in soleus. Pinacidil, on the other hand, increased the rate but not the extent of fatigue, abolished the normal increase in resting tension during fatigue, enhanced force recovery, and increased (86)Rb(+) fractional loss in both the EDL and soleus. During fatigue, the decreases in ATP and phosphocreatine of soleus muscle were less in the presence of pinacidil. The glibenclamide effects suggest that fatigue, elicited with intermittent contractions, activates few K(ATP) channels that affect resting tension and membrane potentials but not tetanic force, whereas opening the channel with pinacidil causes a faster decrease in tetanic force, improves force recovery, and helps in preserving energy.

Intracellular signalling pathways modulate K(ATP) channels in inspiratory brainstem neurones and their hypoxic activation: involvement of metabotropic receptors, G-proteins and cytoskeleton

K(ATP) channels regulate the neuronal excitability and their activation during hypoxia/ischemia protect neurons. The activation of K(ATP) channels during hypoxia is assumed to occur mainly due to the fall in intracellular ATP levels, but other intracellular signalling pathways can be also involved. We measured single K(ATP) channel currents in inspiratory brainstem neurones of neonatal mice. The activity of K(ATP) channels was enhanced in hypoosmotic bath solutions, or after applying negative pressure to the recording pipette. Cytochalasin B activated K(ATP) channels and prevented the effects of osmo-mechanical stress, indicating that cytoskeleton rearrangements, which occur during hypoxia, contribute to the activation of K(ATP) channels. During hypoxia, extracellular levels of many neurotransmitters increase, leading to activation of corresponding metabotropic receptors that can modulate K(ATP) channels. K(ATP) channels were activated by GABA(B) agonist, baclofen, by mGLUR2/3 agonists and were inhibited by mGLUR1/5 agonists. K(ATP) channels were activated by phorbol esters and were inhibited by staurosporine. These treatments did not occlude the modulating actions of mGLUR agonists, indicating that they are not mediated by protein kinase C. Activator of alpha-subunits of G-proteins Mas 7 increased and their inhibitor GPant-2 decreased the activity of K(ATP) channels. In the presence of either agent, the modulatory actions of baclofen and mGLUR agonists were not observed. We conclude that K(ATP) channels are modulated by G-proteins that are activated by metabotropic receptors for GABA and glutamate and their release during hypoxia complements activation of channels by osmo-mechanical stress and [ATP](i) depletion.

Molecular aspects of ATP-sensitive K+ channels in the cardiovascular system and K+ channel openers

ATP-sensitive K+ (K(ATP)) channels are inhibited by intracellular ATP (ATPi) and activated by intracellular nucleoside diphosphates and thus, provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions, and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective, as well as proarrhythmic, effects. These channels are clearly heterogeneous. The cardiac K(ATP) channel is the prototype of K(ATP) channels possessing approximately 80 pS of single-channel conductance in the presence of approximately 150 mM extracellular K+ and opens spontaneously in the absence of ATPi. A vascular K(ATP) channel called a nucleoside diphosphate-dependent K+ (K(NDP)) channel exhibits properties significantly different from those of the cardiac K(ATP) channel. The K(NDP) channel has the single-channel conductance of approximately 30-40 pS in the presence of approximately 150 mM extracellular K+, is closed in the absence of ATPi, and requires intracellular nucleoside di- or triphosphates, including ATPi to open. Nevertheless, K(ATP) and K(NDP) channels are both activated by K+ channel openers, including pinacidil and nicorandil, and inhibited by sulfonylurea derivatives such as glibenclamide. It recently was found that the cardiac K(ATP) channel is composed of a sulfonylurea receptor (SUR)2A and a two-transmembrane-type K+ channel subunit Kir6.2, while the vascular K(NDP) channel may be the complex of SUR2B and Kir6.1. By precisely comparing the functional properties of the SUR2A/Kir6.2 and the SUR2B/Kir6.1 channels, we shall show that the single-channel characteristics and pharmacological properties of SUR/Kir6.0 channels are determined by Kir and SUR subunits, respectively, while responses to intracellular nucleotides are determined by both SUR and Kir subunits.

ATP-sensitive potassium channels: a model of heteromultimeric potassium channel/receptor assemblies

ATP-sensitive K+ channels (KATP channels) play important roles in many cellular functions by coupling cell metabolism to electrical activity. By cloning members of the novel inwardly rectifying K+ channel subfamily Kir6.0 (Kir6.1 and Kir6.2) and the receptors for sulfonylureas (SUR1 and SUR2), researchers have clarified the molecular structure of KATP channels. KATP channels comprise two subunits: a Kir6.0 subfamily subunit, which is a member of the inwardly rectifying K+ channel family; and a SUR subunit, which is a member of the ATP-binding cassette (ABC) protein superfamily. KATP channels are the first example of a heteromultimeric complex assembled with a K+ channel and a receptor that are structurally unrelated to each other. Since 1995, molecular biological and molecular genetic studies of KATP channels have provided insights into the structure-function relationships, molecular regulation, and pathophysiological roles of KATP channels.

Dual actions of the metabolic inhibitor, sodium azide on K(ATP) channel currents in the rat CRI-G1 insulinoma cell line

1. The effects of various inhibitors of the mitochondrial electron transport chain on the activity of ATP-sensitive K+ channels were examined in the Cambridge rat insulinoma G1 (CRI-G1) cell line using a combination of whole cell and single channel recording techniques. 2. Whole cell current clamp recordings, with 5 mM ATP in the pipette, demonstrate that the mitochondrial uncoupler sodium azide (3 mM) rapidly hyperpolarizes CRI-G1 cells with a concomitant increase in K+ conductance. This is due to activation of K(ATP) channels as the sulphonylurea tolbutamide (100 microM) completely reversed the actions of azide. Other inhibitors of the mitochondrial electron transport chain, rotenone (10 microM) or oligomycin (2 microM) did not hyperpolarize CRI-G1 cells or increase K+ conductance. 3. In cell-attached recordings, bath application of 3 mM sodium azide (in the absence of glucose) resulted in a rapid increase in K(ATP) channel activity, an action readily reversible by tolbutamide (100 microM). Application of sodium azide (3 mM), in the presence of Mg-ATP, to the intracellular surface of excised inside-out patches also increased K(ATP) channel activity, in a reversible manner. 4. In contrast, rotenone (10 microM) or oligomycin (2 microM) did not increase K(ATP) channel activity in either cell-attached, in the absence of glucose, or inside-out membrane patch recordings. 5. Addition of sodium azide (3 mM) to the intracellular surface of inside-out membrane patches in the presence of Mg-free ATP or the non-hydrolysable analogue 5'-adenylylimidodiphosphate (AMP-PNP) inhibited, rather than increased, K(ATP) channel activity. 6. In conclusion, sodium azide, but not rotenone or oligomycin, directly activates K(ATP) channels in CRI-G1 insulin secreting cells. This action of azide is similar to that reported previously for diazoxide.

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.

Recombinant cardiac ATP-sensitive K+ channel subunits confer resistance to chemical hypoxia-reoxygenation injury

BACKGROUND

Opening of cardiac ATP-sensitive K+ (KATP) channels has emerged as a promising but still controversial cardioprotective mechanism. Defining KATP channel function at the level of recombinant channel proteins is a necessary step toward further evaluation of the cardioprotective significance of this ion conductance.

METHODS AND RESULTS

KATP channel deficient COS-7 cells were found to be vulnerable to chemical hypoxia-reoxygenation injury that induced significant cytosolic Ca2+ loading (from 97+/-3 to 236+/-11 nmol/L). In these cells, the potassium channel opener pinacidil (10 micromol/L) did not prevent Ca2+ loading (from 96+/-3 nmol/L before to 233+/-12 nmol/L after reoxygenation) or evoked membrane current. Cotransfection with Kir6.2/SUR2A genes, which encode cardiac KATP channel subunits, resulted in a cellular phenotype that, in the presence of pinacidil (10 micromol/L), expressed K+ current and gained resistance to hypoxia-reoxygenation (Ca2+ concentration from 99+/-7 to 127+/-11 nmol/L; P>0.05). Both properties were abolished by the KATP channel blocker glyburide (1 micromol/L). In COS-7 cells transfected with individual channel subunits Kir6.2 or SUR2A, which alone do not form functional cardiac KATP channels, pinacidil did not protect against hypoxia-reoxygenation.

CONCLUSIONS

The fact that transfer of cardiac KATP channel subunits protected natively KATP channel deficient cells provides direct evidence that the cardiac KATP channel protein complex harbors intrinsic cytoprotective properties. These findings validate the concept that targeting cardiac KATP channels should be considered a valuable approach to protect the myocardium against injury.

ATP-sensitive K+ channel blocker glibenclamide and diaphragm fatigue during normoxia and hypoxia

The role of ATP-sensitive K+ channels in skeletal muscle contractile performance is controversial: blockers of these channels have been found to not alter, accelerate, or attenuate fatigue. The present study reexamined whether glibenclamide affects contractile performance during repetitive contraction. Experiments systematically assessed the effects of stimulation paradigm, temperature, and presence of hypoxia and in addition compared intertrain with intratrain fatigue. Adult rat diaphragm muscle strips were studied in vitro. At 37 degrees C and normoxia, glibenclamide did not significantly affect any measure of fatigue during continuous 5- or 100-Hz or intermittent 20-Hz stimulation but progressively prolonged relaxation time during 20-Hz stimulation. At 20 degrees C and normoxia, neither force nor relaxation rate was affected significantly by glibenclamide during 20-Hz stimulation. At 37 degrees C and hypoxia, glibenclamide did not significantly affect fatigue at 5-Hz or intertrain fatigue during 20-Hz stimulation but reduced intratrain fatigue and prolonged relaxation time during 20-Hz stimulation. These findings indicate that, although ATP-sensitive K+ channels may be activated during repetitive contraction, their activation has only a modest effect on the rate of fatigue development.

Hypoxia activates ATP-dependent potassium channels in inspiratory neurones of neonatal mice

1. The respiratory centre of neonatal mice (4 to 12 days old) was isolated in 700 micro(m) thick brainstem slices. Whole-cell K+ currents and single ATP-dependent potassium (KATP) channels were analysed in inspiratory neurones. 2. In cell-attached patches, KATP channels had a conductance of 75 pS and showed inward rectification. Their gating was voltage dependent and channel activity decreased with membrane hyperpolarization. Using Ca2+-containing pipette solutions the measured conductance was lower (50 pS at 1.5 mM Ca2+), indicating tonic inhibition by extracellular Ca2+. 3. KATP channel activity was reversibly potentiated during hypoxia. Maximal effects were attained 3-4 min after oxygen removal from the bath. Hypoxic potentiation of open probability was due to an increase in channel open times and a decrease in channel closed times. 4. In inside-out patches and symmetrical K+ concentrations, channel currents reversed at about 0 mV. Channel activity was blocked by ATP (300-600 microM), glibenclamide (10-70 microM) and tolbutamide (100-300 microM). 5. In the presence of diazoxide (10-60 microM), the activity of KATP channels was increased both in inside-out, outside-out and cell-attached patches. In outside-out patches, that remained within the slice after excision, the activity of KATP channels was enhanced by hypoxia, an effect that could be mediated by a release of endogenous neuromodulators. 6. The whole-cell K+ current (IK) was inactivated at negative membrane potentials, which resembled the voltage dependence of KATP channel gating. After 3-4 min of hypoxia, K+ currents at both hyperpolarizing and depolarizing membrane potentials increased. IK was partially blocked by tolbutamide (100-300 microM) and in its presence, hypoxic potentiation of IK was abolished. 7. We conclude that KATP channels are involved in the hypoxic depression of medullary respiratory activity.

Similarity of ATP-dependent K+ channels in skeletal muscle fibres from normal and mutant mdx mice

1. ATP-dependent K+ (KATP) channels were studied in fibres isolated from flexor digitorum brevis and interosseal skeletal muscles of normal and mutant mdx mice using the patch clamp technique in the presence of asymmetrical K+ concentrations (5 mM K+ in the pipette and in vivo intracellular [K+] or 145 mM K+ at the cytoplasmic face). 2. In cell-attached patches from mdx muscle fibres bathed in K(+)-rich solution, cell poisoning with fluorodinitrobenzene induced partially reversible opening of channels carrying an outward current of an amplitude of 1.2 pA at 0 mV. Exposure of fibres to the K+ channel opener cromakalim led to opening of the same type of channel. These channels were assumed to be KATP channels. 3. On excision of inside-out patches from mdx muscle fibres, in the absence of intracellular ATP, KATP channels were active: they carried a unitary outward current of 1.6 pA at 0 mV and were inhibited by intracellular ATP and glibenclamide. The number of KATP channels per patch was not significantly different in muscles from normal and mdx mice. 4. In inside-out patches, in the presence of 1 mM intracellular Mg2+, slope conductances of 21 and 20.3 pS were found for KATP channels in normal and mdx muscle, respectively. In the absence of Mg2+, slope conductances of KATP channels were 31.3 and 32 pS in normal and mdx muscle, respectively and KATP channel activity was augmented in mdx muscle in the same way as in normal muscle. Activity of the same KATP channel was observed in extensor digitorum longus muscle from normal and mdx mice. 5. In inside-out patches held at 0 mV, the relationship between KATP channel activity and intracellular ATP was described by a Hill equation: Ki values were 23 and 21 microM and Hill coefficients were 1.8 and 1.9 in normal and mdx muscle, respectively. 6. These results indicate that the distribution, the conductance properties and ATP sensitivity of KATP channels do not differ in normal and in mdx mouse skeletal muscle.

Cell-type specificity of preconditioning in an in vitro model

We investigated whether preconditioning could protect several cultured cell lines, to determine whether the protection is specific for cells derived from different myogenic and non-myogenic sources. Ischemia was simulated by centrifugation of cells into a pellet, and cell viability was determined by hypotonic trypan blue solution. Preconditioning was produced by brief exposures to either glucose-free solution or metabolic inhibition. Freshly isolated rabbit ventricular myocytes were studied to confirm that preconditioning occurs in this model. We then compared these results to those in several cultured cell lines, including HEK 293 cells derived from human embryonic kidney, HIT-T15 cells from Syrian hamster pancreatic islets, and C2C12 cells from mouse skeletal muscle. In the latter cell line, we also determined whether differentiation alters preconditioning. Preconditioning protected rabbit ventricular myocytes: the percentage of dead cells was decreased from 36.8 +/- 4.7% in the control group to 23.0 +/- 5.2% in the preconditioned group after 60 min and from 50.7 +/- 2.1% in the control group to 25.5 +/- 4.5% in the preconditioned group after 120 min ischemia (p < 0.02). In contrast, there was no protection from preconditioning in HEK 293 cells or HIT-T15 cells. Preconditioning did not protect C2C12 myoblasts either. Interestingly, after C2C12 myoblasts had differentiated into myotubes (induced by exposing the cells to low-serum medium), they could then be protected by preconditioning (46.3 +/- 3.6% in the control group vs 26.0 +/- 2.7% in the preconditioned group after 60 min and 67.4 +/- 3.6% in the control group vs 46.0 +/- 4.6% in the preconditioned group after 120 min ischemia; p < 0.05). In conclusion, protection from preconditioning is cell-type specific. The presence of endogenous KATP channels (which are plentiful in HIT-T15 cells) is insufficient to enable preconditioning of the cell. Among the various cell types studied, only differentiated muscle cells (rabbit ventricular myocytes and C2C12 myotubes) exhibited preconditioning.

Binding of [3H]-P1075, an opener of ATP-sensitive K+ channels, to rat glomerular preparations

ATP-sensitive K+ channels (KATP channels) in the kidney have been found in the tubular system and in the afferent arteriole. In this study we have examined the binding of [3H]-P1075 ([3H]-N-cyano-N'-(1, 1-dimethylpropyl)-N"-3-pyridylguanidine), a selective opener of KATP channels, in rat glomerular preparations. Equilibrium (saturation, competition) and kinetic experiments indicated that [3H]-P1075 binds to a single class of sites with a dissociation constant of about 3 nM and a maximum binding capacity of 10 fmol mg-1 glomerular protein. The association rate constant of the complex was 6,5 x 10(7) M-1 min-1; dissociation occurred with a half-time of 6.2 min. Specific [3H]-P1075 binding was strongly reduced when the metabolic state of the glomerular preparation was impaired during the preparation procedure or the binding assay or when the preparation was subjected to mild collagenase treatment. In different metabolically competent preparations, the amount of specific [3H]-P1075 binding correlated well with the number of vascular endings adherent to the glomeruli; no specific binding was found in mesangial cells in culture. Specific [3H]-P1075 binding was inhibited by representatives of the different classes of KATP channel openers and by sulphonylurea-type blockers with inhibition constants similar to those obtained in rat aortic rings. It is concluded that rat glomerular preparations possess specific binding sites for KATP channel openers with vascular characteristics. The sensitivity of binding to mild collagenase treatment suggests that these sites are located on a membrane protein; in addition, the data suggest that these sites are localized on smooth muscle and/or renin secreting cells of the afferent vascular endings attached to some of the glomeruli. Their estimated density (1,500 microns-2) is much higher than that of KATP channels in smooth muscle.

Dual effect of glyburide, an antagonist of KATP channels, on metabolic inhibition-induced Ca2+ loading in cardiomyocytes

Whether sulfonylurea therapy, which blocks ATP-sensitive K+ (KATP) channels, impedes endogenous cardioprotective mechanisms during cellular metabolic impairment remains controversial. Therefore, the effect of glyburide, a prototype sulphonylurea drug, on cytosolic Ca2+ concentration and KATP channel activity, was measured in 2-4-dinitrophenol-treated guinea-pig cardiomyocytes, using epifluorescent digital-imaging and cell-attached patch-clamp electrophysiology. Dinitrophenol (200 microM), which uncouples oxidative phosphorylation, induced opening of KATP channels and Ca2+ loading. Glyburide (6 microM) which reduced the opening of KATP channels, aggravated Ca2+ loading only when applied to dinitrophenol-pretreated myocytes but not when applied with dinitrophenol treatment. We conclude that a blocker of KATP channels has differential effects upon dinitrophenol-induced intracellular Ca2+ loading, which appear to depend upon the stage of metabolic insult.

Intracellular Ca2+ changes and Ca2+-activated K+ channel activation induced by acetylcholine at the endplate of mouse skeletal muscle fibres

1. Enzymatically isolated skeletal muscle fibres were used to investigate the effects of applying acetylcholine (ACh) onto the endplate area on intracellular free calcium concentration ([Ca2+]i) measured using the indicator indo-1 and single channel activity using the patch clamp technique. 2. Using a Tyrode solution containing 5 microM tetrodotoxin (TTX) as extracellular solution, ACh applications (at 0.1 or 1 mM) onto the endplate induced intracellular free calcium transients the mean maximal amplitude of which was 360 +/- 30 nM from a mean resting value of 72 +/- 7 nM (n = 13). In cells bathed with a K(+)-rich solution (145 mM K+), applications of ACh (0.1 mM) induced transient rises in [Ca2+]i from a mean resting value of 53 +/- 7 nM to a maximum of 222 +/- 24 nM (n = 33). 3. In cell-attached membrane patches at the endplate membrane of muscle fibres bathed in a K(+)-rich external solution, using a pipette filled with Tyrode solution, external application of 0.1 mM ACh could induce a transient burst opening of channels carrying an outward current of an average amplitude of 4.6 +/- 0.2 pA at 0 mV (n = 8). 4. These channels were characterized as Ca2(+)-activated K+ channels. At 0 mV, in inside-out patches excised from the endplate membrane area, they displayed a conductance of 60 and 224 pS in the presence of Tyrode and K(+)-rich solution in the pipette, respectively. Half-maximum activation was found for a [Ca2+]i close to 4 microM. The channels showed a typical voltage dependence. In outside-out patches these channels were shown to be blocked by 100 nM charybdotoxin (CTX). 5. In fibres bathed in a Tyrode solution containing TTX (5 microM), CTX had no clear effect on the change in membrane voltage, recorded near the endplate with a single intracellular microelectrode, in response to the application of ACh. 6. Although the physiological relevance of this ACh-induced K+ channel activation remains unclear, results suggest that, in the presence of a physiological extracellular [Ca2+], Ca2+ entry through the endplate nicotinic receptors can produce a local increase in [Ca2+]i, sufficient to trigger the opening of Ca2+-activated K+ channels in the adjacent surface membrane.

Dose-dependent activation and block by bisG10, a K+ channel blocker, of mouse and frog skeletal muscle KATP channels

The effects of a K+ channel blocker, bisG10, were examined on ATP-sensitive K+ (KATP) channels in membrane patches excised from mammalian and amphibian skeletal muscle fibres using the patch-clamp technique. At micromolar concentrations, bisG10, added on the intracellular side, induced a strong, reversible, flickery block of KATP channels. BisG10, added on the extracellular side, was about 100-fold less potent at inhibiting channel activity. At 10 nM, intracellular bisG10 increased KATP channel activity. This activation was independent of the presence of internal ATP or Mg2+. The inhibitory effect of bisG10 most likely arose from open-channel block whereas activation could result from more complex, indirect interactions.