A K(ATP) channel deficiency affects resting tension, not contractile force, during fatigue in skeletal muscle

Bayliss-Starling Prize Lecture: KATP channel pathophysiology - a whole-body odyssey

First identified 40 years ago in cardiac myocytes, ATP-sensitive potassium (KATP) channels have been found in almost all excitable tissues, with paradigmatic inhibition by ATP and activation by ADP underlying their physiological role in coupling cellular metabolism to electrical activity. Cloning of the underlying genes, 30 years ago, revealed their unique assembly as four Kir6.x pore-forming subunit proteins and four sulfonylurea receptor (SURx) subunit proteins and has since led to discovery of a spectrum of monogenic diseases resulting from gain- (GOF) or loss-of-function (LOF) mutations, in turn leading to recognition of novel physiological roles and pathophysiological consequences throughout the body. With this perspective, this lecture represents a personal view of these discoveries and the potential for future pathophysiological insights.

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.

Trade-Off Between Maximal Power Output and Fatigue Resistance of the Knee Extensors for Older Men

This study investigated associations of fatigue resistance determined by an exercise-induced decrease in neuromuscular power with prefatigue neuromuscular strength and power of the knee extensors in 31 older men (65-88 years). A fatigue task consisted of 50 consecutive maximal effort isotonic knee extensions (resistance: 20% of prefatigue isometric maximal voluntary contraction torque) over a 70° range of motion. The average of the peak power values calculated from the 46th to 50th contractions during the fatigue task was normalized to the prefatigue peak power value, which was defined as neuromuscular fatigue resistance. Neuromuscular fatigue resistance was negatively associated with prefatigue maximal power output (r = -.530) but not with prefatigue maximal voluntary contraction torque (r = -.252). This result highlights a trade-off between prefatigue maximal power output and neuromuscular fatigue resistance, implying that an improvement in maximal power output might have a negative impact on neuromuscular fatigue resistance.

Sexual dimorphism in vascular ATP-sensitive K+ channel function supporting interstitial PO2 via convective and/or diffusive O2 transport

KEY POINTS

Inhibition of pancreatic ATP-sensitive K+ (KATP ) channels is the intended effect of oral sulphonylureas to increase insulin release in diabetes. However, pertinent to off-target effects of sulphonylurea medication, sex differences in cardiac KATP channel function exist, whereas potential sex differences in vascular KATP channel function remain unknown. In the present study, we assessed vascular KATP channel function (topical glibenclamide superfused onto fast-twitch oxidative skeletal muscle) supporting blood flow and interstitial O2 delivery-utilization matching ( P O 2 is) during twitch contractions in male, female during pro-oestrus and ovariectomized female (F+OVX) rats. Glibenclamide decreased blood flow (convective O2 transport) and interstitial P O 2 in male and female, but not F+OVX, rats. Compared to males, females also demonstrated impaired diffusive O2 transport and a faster fall in interstitial P O 2 . Our demonstration, in rats, that sex differences in vascular KATP channel function exist support the tentative hypothesis that oral sulphonylureas may exacerbate exercise intolerance and morbidity, especially in premenopausal females.

ABSTRACT

Vascular ATP-sensitive K+ (KATP ) channels support skeletal muscle blood flow ( Q ̇ m ), interstitial O2 delivery ( Q ̇ O 2 )-utilization ( V ̇ O 2 ) matching (i.e. interstitial-myocyte O2 flux driving pressure; P O 2 is) and exercise tolerance. Potential sex differences in skeletal muscle vascular KATP channel function remain largely unexplored. We hypothesized that local skeletal muscle KATP channel inhibition via glibenclamide superfusion (5 mg kg-1 GLI; sulphonylurea diabetes medication) in anaesthetized female Sprague-Dawley rats, compared to males, would demonstrate greater reductions in contracting (1 Hz, 7 V, 180 s) fast-twitch oxidative mixed gastrocnemius (97% type IIA+IID/X+IIB) Q ̇ m (15 μm microspheres) and P O 2 is (phosphorescence quenching), resulting from more compromised convective ( Q ̇ O 2 ) and diffusive ( D O 2   ) O2 conductances. Furthermore, these GLI-induced reductions in ovary-intact females measured during pro-oestrus would be diminished following ovariectomy (F+OVX). GLI similarly impaired mixed gastrocnemius V ̇ O 2 in both males (↓28%) and females (↓33%, both P < 0.032) via reduced Q ̇ m (male: ↓31%, female: ↓35%, both P < 0.020), Q ̇ O 2 (male: 5.6 ± 0.5 vs. 4.0 ± 0.5, female: 6.4 ± 1.1 vs. 4.2 ± 0.6 mL O2  min-1 100 g tissue-1 , P < 0.022) and the resulting P O 2 is, with females also demonstrating a reduced D O 2   (0.40 ± 0.07 vs. 0.30 ± 0.04 mL O2  min-1 100 g tissue-1 , P < 0.042) and a greater GLI-induced speeding of P O 2 is fall (mean response time: Sex × Drug interaction, P = 0.026). Conversely, GLI did not impair the mixed gastrocnemius of F+OVX rats. Therefore, in patients taking sulphonylureas, these results support the potential for impaired vascular KATP channel function to compromise muscle Q ̇ m and therefore exercise tolerance. Such an effect, if present, would likely contribute to adverse cardiovascular events in premenopausal females more than males.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

Vascular ATP-sensitive K+ channels support maximal aerobic capacity and critical speed via convective and diffusive O2 transport

KEY POINTS

Oral sulphonylureas, widely prescribed for diabetes, inhibit pancreatic ATP-sensitive K+ (KATP ) channels to increase insulin release. However, KATP channels are also located within vascular (endothelium and smooth muscle) and muscle (cardiac and skeletal) tissue. We evaluated left ventricular function at rest, maximal aerobic capacity ( V ̇ O2 max) and submaximal exercise tolerance (i.e. speed-duration relationship) during treadmill running in rats, before and after systemic KATP channel inhibition via glibenclamide. Glibenclamide impaired critical speed proportionally more than V ̇ O2 max but did not alter resting cardiac output. Vascular KATP channel function (topical glibenclamide superfused onto hindlimb skeletal muscle) resolved a decreased blood flow and interstitial PO2 during twitch contractions reflecting impaired O2 delivery-to-utilization matching. Our findings demonstrate that systemic KATP channel inhibition reduces V ̇ O2 max and critical speed during treadmill running in rats due, in part, to impaired convective and diffusive O2 delivery, and thus V ̇ O2 , especially within fast-twitch oxidative skeletal muscle.

ABSTRACT

Vascular ATP-sensitive K+ (KATP ) channels support skeletal muscle blood flow and microvascular oxygen delivery-to-utilization matching during exercise. However, oral sulphonylurea treatment for diabetes inhibits pancreatic KATP channels to enhance insulin release. Herein we tested the hypotheses that: i) systemic KATP channel inhibition via glibenclamide (GLI; 10 mg kg-1 i.p.) would decrease cardiac output at rest (echocardiography), maximal aerobic capacity ( V ̇ O2 max) and the speed-duration relationship (i.e. lower critical speed (CS)) during treadmill running; and ii) local KATP channel inhibition (5 mg kg-1 GLI superfusion) would decrease blood flow (15 µm microspheres), interstitial space oxygen pressures (PO2 is; phosphorescence quenching) and convective and diffusive O2 transport ( Q ̇ O2 and DO2 , respectively; Fick Principle and Law of Diffusion) in contracting fast-twitch oxidative mixed gastrocnemius muscle (MG: 9% type I+IIa fibres). At rest, GLI slowed left ventricular relaxation (2.11 ± 0.59 vs. 1.70 ± 0.23 cm s-1 ) and decreased heart rate (321 ± 23 vs. 304 ± 22 bpm, both P < 0.05) while cardiac output remained unaltered (219 ± 64 vs. 197 ± 39 ml min-1 , P > 0.05). During exercise, GLI reduced V ̇ O2 max (71.5 ± 3.1 vs. 67.9 ± 4.8 ml kg-1 min-1 ) and CS (35.9 ± 2.4 vs. 31.9 ± 3.1 m min-1 , both P < 0.05). Local KATP channel inhibition decreased MG blood flow (52 ± 25 vs. 34 ± 13 ml min-1 100 g tissue-1 ) and PO2 isnadir (5.9 ± 0.9 vs. 4.7 ± 1.1 mmHg) during twitch contractions. Furthermore, MG V ̇ O2 was reduced via impaired Q ̇ O2 and DO2 (P < 0.05 for each). Collectively, these data support that vascular KATP channels help sustain submaximal exercise tolerance in healthy rats. For patients taking sulfonylureas, KATP channel inhibition may exacerbate exercise intolerance.

Sam68 splicing regulation contributes to motor unit establishment in the postnatal skeletal muscle

Sam68 ensures the establishment of neuromuscular junctions (NMJs) and motor unit integrity by orchestrating a neuronal splicing program.

RNA-binding proteins orchestrate the composite life of RNA molecules and impact most physiological processes, thus underlying complex phenotypes. The RNA-binding protein Sam68 regulates differentiation processes by modulating splicing, polyadenylation, and stability of select transcripts. Herein, we found that Sam68^−/− mice display altered regulation of alternative splicing in the spinal cord of key target genes involved in synaptic functions. Analysis of the motor units revealed that Sam68 ablation impairs the establishment of neuromuscular junctions and causes progressive loss of motor neurons in the spinal cord. Importantly, alterations of neuromuscular junction morphology and properties in Sam68^−/− mice correlate with defects in muscle and motor unit integrity. Sam68^−/− muscles display defects in postnatal development, with manifest signs of atrophy. Furthermore, fast-twitch muscles in Sam68^−/− mice show structural features typical of slow-twitch muscles, suggesting alterations in the metabolic and functional properties of myofibers. Collectively, our data identify a key role for Sam68 in muscle development and suggest that proper establishment of motor units requires timely expression of synaptic splice variants.

Structural Determinants of Insulin Release: Disordered N-Terminal Tail of Kir6.2 Affects Potassium Channel Dynamics through Interactions with Sulfonylurea Binding Region in a SUR1 Partner

Inward rectifying potassium ion channels (KATP), sensitive to the ATP/ADP concentration ratio, play an important, control role in pancreatic β cells. The channels close upon the increase of this ratio, which, in turn, triggers insulin release to blood. Numerous mutations in KATP lead to severe and widespread medical conditions such as diabetes. The KATP system consists of a pore made of four Kir6.2 subunits and four accompanying large SUR1 proteins belonging to the ABCC transporters group. How SUR1 affects KATP function is not yet known; therefore, we created simplified models of the Kir6.2 tetramer based on recently determined cryo-EM KATP structures. Using all-atom molecular dynamics (MD) with the CHARMM36 force field, targeted MD, and molecular docking, we revealed functionally important rearrangements in the Kir6.2 pore, induced by the presence of the SUR1 protein. The cytoplasmic domain of Kir6.2 (CTD) is brought closer to the membrane due to interactions with SUR1. Each Kir6.2 subunit has a conserved, functionally important, disordered N-terminal tail. Using molecular docking, we found that the Kir6.2 tail easily docks to the sulfonylurea drug binding region located in the adjacent SUR1 protein. We reveal, for the first time, dynamical behavior of the Kir6.2/SUR1 system, confirming a physiological role of the Kir6.2 disordered tail, and we indicate structural determinants of KATP-dependent insulin release from pancreatic β cells.

Sex difference in fatigability of knee extensor muscles during sustained low-level contractions

This study investigated whether the sex difference in fatigability of the knee extensors (KE) is explained by the sex difference in fatigue-induced changes in the shear modulus of one or more muscles of KE in 18 young men and 23 young women. The shear moduli of the resting rectus femoris and medial and lateral vastus muscles (VL) were measured before and after a sustained contraction at 20% peak torque during a maximal voluntary isometric contraction of KE until the endurance limit, in addition to evoked torque and voluntary activation (VA%). The fatigue-induced decrease in maximal muscle strength was more prominent in men than in women. Only the VL shear modulus for men increased after the fatiguing task, and a sex difference was observed in the percentage change in the VL shear modulus before and after the fatiguing task. The fatigue-induced decreased ratio was greater for men than for women in evoked torque, but not in VA%. These results suggest that although peripheral and central fatigue both influenced the fatigue-induced decrease in maximal muscle strength regardless of sex, the sex difference in KE fatigability is explained by that in peripheral fatigue, particularly the degree of peripheral VL fatigue.

Possible involvement of monoamine neurons in the emotional abnormality in Kir6.2-deficient mice

ATP-sensitive potassium (K_ATP) channels consist of two structurally different subunits: a pore-forming subunit of the Kir6.0-family (Kir6.1 or Kir6.2) and a regulatory sulfonylurea receptor subunit (SUR1, SUR2A or SUR2B). Although Kir6.2 is widely distributed in the brain, the mechanisms that underlie the impact of Kir6.2 on emotional behavior are not yet fully understood. To clarify the role of Kir6.2 in emotional behavior, in the present study, we investigated the behavioral characteristics of Kir6.2-knockout (Kir6.2^-/-) mice. Kir6.2^-/- mice showed impaired general behavior in a locomotor activity test and open field test. In addition, anxiety-like behavior was observed in the open field test, elevated plus-maze test and light-dark test. In particular, excessive anxiety-like behavior was observed in female Kir6.2^-/- mice. Moreover, we investigated whether Kir6.2 is expressed on monoamine neurons in the brain. Immunohistochemical studies showed that Kir6.2 was co-localized with tryptophan hydroxylase (TPH), a marker of serotonergic neurons, in dorsal raphe nuclei. Kir6.2 was also co-localized with tyrosine hydroxylase (TH), a marker of dopaminergic/noradrenergic neurons, in the ventral tegmental area and locus coeruleus. Next, we checked the protein levels of TH and TPH in the midbrain. Interestingly, TPH expression was significantly elevated in female Kir6.2^-/- mice. These results suggest that Kir6.2 in monoamine neurons, especially serotonergic neurons, could play a key role in emotional behavior.

Partial exposure of frog heart to high-potassium solution: an easily reproducible model mimicking ST segment changes

By simply inducing burn injuries on the bullfrog heart, we previously reported a simple model of abnormal ST segment changes observed in human ischemic heart disease. In the present study, instead of inducing burn injuries, we partially exposed the surface of the frog heart to high-potassium (K⁺) solution to create a concentration gradient of the extracellular K⁺ within the myocardium. Dual recordings of ECG and the cardiac action potential demonstrated significant elevation of the ST segment and the resting membrane potential, indicating its usefulness as a simple model of heart injury. Additionally, from our results, Na⁺/K⁺-ATPase activity was thought to be primarily responsible for generating the K⁺ concentration gradient and inducing the ST segment changes in ECG.

Muscle Shear Moduli Changes and Frequency of Alternate Muscle Activity of Plantar Flexor Synergists Induced by Prolonged Low-Level Contraction

During prolonged low-level contractions, synergist muscles are activated in an alternating pattern of activity and silence called as alternate muscle activity. Resting muscle stiffness is considered to increase due to muscle fatigue. Thus, we investigated whether the difference in the extent of fatigue of each plantar flexor synergist corresponded to the difference in the frequency of alternate muscle activity between the synergists using muscle shear modulus as an index of muscle stiffness. Nineteen young men voluntarily participated in this study. The shear moduli of the resting medial and lateral gastrocnemius muscles (MG and LG) and soleus muscle (SOL) were measured using shear wave ultrasound elastography before and after a 1-h sustained contraction at 10% peak torque during maximal voluntary contraction of isometric plantar flexion. One subject did not accomplish the task and the alternate muscle activity for MG was not found in 2 subjects; therefore, data for 16 subjects were used for further analyses. The magnitude of muscle activation during the fatiguing task was similar in MG and SOL. The percent change in shear modulus before and after the fatiguing task (MG: 16.7 ± 12.0%, SOL: −4.1 ± 13.9%; mean ± standard deviation) and the alternate muscle activity during the fatiguing task (MG: 33 [20–51] times, SOL: 30 [17–36] times; median [25th–75th percentile]) were significantly higher in MG than in SOL. The contraction-induced change in shear modulus (7.4 ± 20.3%) and the alternate muscle activity (37 [20–45] times) of LG with the lowest magnitude of muscle activation during the fatiguing task among the plantar flexors were not significantly different from those of the other muscles. These results suggest that the degree of increase in muscle shear modulus induced by prolonged contraction corresponds to the frequency of alternate muscle activity between MG and SOL during prolonged contraction. Thus, it is likely that, compared with SOL, the alternate muscle activity of MG occurs more frequently during prolonged contraction due to the greater increase in fatigue of MG induced by the progression of a fatiguing task.

Role of ATP-Sensitive Potassium Channels in Remote Ischemic Preconditioning Induced Tissue Protection

Remote ischemic preconditioning (RIPC) is an innovative treatment strategy that alleviates ischemia-reperfusion injury, whereby short episodes of regional ischemia and reperfusion delivered to remote organs including hind limb, kidney and intestine, and so on provide protection to the heart. The RIPC is known to reduce infarct size, serum levels of cardiac enzymes, and myocardial dysfunction in various animal species as well as in patients. There have been a large number of studies suggesting that the ATP-sensitive potassium channels (KATP channel) play a significant role as a mediator or end effector in RIPC. The present review discusses the role of KATP channels and possible mechanisms in RIPC-induced cardioprotection.

Non-cell autonomous cues for enhanced functionality of human embryonic stem cell-derived cardiomyocytes via maturation of sarcolemmal and mitochondrial KATP channels

Human embryonic stem cells (hESCs) is a potential unlimited ex vivo source of ventricular (V) cardiomyocytes (CMs), but hESC-VCMs and their engineered tissues display immature traits. In adult VCMs, sarcolemmal (sarc) and mitochondrial (mito) ATP-sensitive potassium (K_ATP) channels play crucial roles in excitability and cardioprotection. In this study, we aim to investigate the biological roles and use of sarcK_ATP and mitoK_ATP in hESC-VCM. We showed that SarcI_{K, ATP} in single hESC-VCMs was dormant under baseline conditions, but became markedly activated by cyanide (CN) or the known opener P1075 with a current density that was ~8-fold smaller than adult; These effects were reversible upon washout or the addition of GLI or HMR1098. Interestingly, sarcI_{K, ATP} displayed a ~3-fold increase after treatment with hypoxia (5% O₂). MitoI_{K, ATP} was absent in hESC-VCMs. However, the thyroid hormone T3 up-regulated mitoI_{K, ATP,} conferring diazoxide protective effect on T3-treated hESC-VCMs. When assessed using a multi-cellular engineered 3D ventricular cardiac micro-tissue (hvCMT) system, T3 substantially enhanced the developed tension by 3-folds. Diazoxide also attenuated the decrease in contractility induced by simulated ischemia (1% O₂). We conclude that hypoxia and T3 enhance the functionality of hESC-VCMs and their engineered tissues by selectively acting on sarc and mitoI_{K, ATP}.

ATP Sensitive Potassium Channels in the Skeletal Muscle Function: Involvement of the KCNJ11(Kir6.2) Gene in the Determination of Mechanical Warner Bratzer Shear Force

The ATP-sensitive K⁺-channels (KATP) are distributed in the tissues coupling metabolism with K⁺ ions efflux. KATP subunits are encoded by KCNJ8 (Kir6.1), KCNJ11 (Kir6.2), ABCC8 (SUR1), and ABCC9 (SUR2) genes, alternative RNA splicing give rise to SUR variants that confer distinct physiological properties on the channel. An high expression/activity of the sarco-KATP channel is observed in various rat fast-twitch muscles, characterized by elevated muscle strength, while a low expression/activity is observed in the slow-twitch muscles characterized by reduced strength and frailty. Down-regulation of the KATP subunits of fast-twitch fibers is found in conditions characterized by weakness and frailty. KCNJ11 gene knockout mice have reduced glycogen, lean phenotype, lower body fat, and weakness. KATP channel is also a sensor of muscle atrophy. The KCNJ11 gene is located on BTA15, close to a QTL for meat tenderness, it has also a role in glycogen storage, a key mechanism of the postmortem transformation of muscle into meat. The role of KCNJ11 gene in muscle function may underlie an effect of KCNJ11 genotypes on meat tenderness, as recently reported. The fiber phenotype and genotype are important in livestock production science. Quantitative traits including meat production and quality are influenced both by environment and genes. Molecular markers can play an important role in the genetic improvement of animals through breeding strategies. Many factors influence the muscle Warner-Bratzler shear force including breed, age, feeding, the biochemical, and functional parameters. The role of KCNJ11gene and related genes on muscle tenderness will be discussed in the present review.

Physiological basis for muscle stiffness and weakness in a knock-in M1592V mouse model of hyperkalemic periodic paralysis

The mechanisms responsible for the onset and progressive worsening of episodic muscle stiffness and weakness in hyperkalemic periodic paralysis (HyperKPP) are not fully understood. Using a knock‐in HyperKPP mouse model harboring the M1592V Na_V1.4 channel mutant, we interrogated changes in physiological defects during the first year, including tetrodotoxin‐sensitive Na⁺ influx, hindlimb electromyographic (EMG) activity and immobility, muscle weakness induced by elevated [K⁺]e, myofiber‐type composition, and myofiber damage. In situ EMG activity was greater in HyperKPP than wild‐type gastrocnemius, whereas spontaneous muscle contractions were observed in vitro. We suggest that both the greater EMG activity and spontaneous contractions are related to periods of hyperexcitability during which fibers generate action potentials by themselves in the absence of any stimulation and that these periods are the cause of the muscle stiffness reported by patients. HyperKPP muscles had a greater sensitivity to the K⁺‐induced force depression than wild‐type muscles. So, an increased interstitial K⁺ concentration locally near subsets of myofibers as a result of the hyperexcitability likely produced partial loss of force rather than complete paralysis. Na_V1.4 channel protein content reached adult level by 3 weeks postnatal in both wild type and HyperKPP and apparent symptoms did not worsen after the first month of age suggesting (i) that the phenotypic behavior of M1592V HyperKPP muscles results from defective function of mutant Na_V1.4 channels rather than other changes in protein expression after the first month and (ii) that the lag in onset during the first decade and the progression of human HyperKPP symptoms during adolescence are a function of Na_V1.4 channel content.

Po2 cycling protects diaphragm function during reoxygenation via ROS, Akt, ERK, and mitochondrial channels

Po2 cycling, often referred to as intermittent hypoxia, involves exposing tissues to brief cycles of low oxygen environments immediately followed by hyperoxic conditions. After experiencing long-term hypoxia, muscle can be damaged during the subsequent reintroduction of oxygen, which leads to muscle dysfunction via reperfusion injury. The protective effect and mechanism behind Po2 cycling in skeletal muscle during reoxygenation have yet to be fully elucidated. We hypothesize that Po2 cycling effectively increases muscle fatigue resistance through reactive oxygen species (ROS), protein kinase B (Akt), extracellular signal-regulated kinase (ERK), and certain mitochondrial channels during reoxygenation. Using a dihydrofluorescein fluorescent probe, we detected the production of ROS in mouse diaphragmatic skeletal muscle in real time under confocal microscopy. Muscles treated with Po2 cycling displayed significantly attenuated ROS levels (n = 5; P < 0.001) as well as enhanced force generation compared with controls during reperfusion (n = 7; P < 0.05). We also used inhibitors for signaling molecules or membrane channels such as ROS, Akt, ERK, as well as chemical stimulators to close mitochondrial ATP-sensitive potassium channel (KATP) or open mitochondrial permeability transition pore (mPTP). All these blockers or stimulators abolished improved muscle function with Po2 cycling treatment. This current investigation has discovered a correlation between KATP and mPTP and the Po2 cycling pathway in diaphragmatic skeletal muscle. Thus we have identified a unique signaling pathway that may involve ROS, Akt, ERK, and mitochondrial channels responsible for Po2 cycling protection during reoxygenation conditions in the diaphragm.

Changes in myoplasmic Ca2+ during fatigue differ between FDB fibers, between glibenclamide-exposed and Kir6.2-/- fibers and are further modulated by verapamil

One objective of this study was to document how individual FDB muscle fibers depend on the myoprotection of KATP channels during fatigue. Verapamil, a CaV1.1 channel blocker, prevents large increases in unstimulated force during fatigue in KATP-channel-deficient muscles. A second objective was to determine if verapamil reduces unstimulated [Ca(2+)]i in KATP-channel-deficient fibers. We measured changes in myoplasmic [Ca(2+)] ([Ca(2+)]i) using two KATP-channel-deficient models: (1) a pharmacological approach exposing fibers to glibenclamide, a channel blocker, and (2) a genetic approach using fibers from null mice for the Kir6.2 gene. Fatigue was elicited with one tetanic contraction every sec for 3 min. For all conditions, large differences in fatigue kinetics were observed from fibers which had greater tetanic [Ca(2+)]i at the end than at the beginning of fatigue to fibers which eventually completely failed to release Ca(2+) upon stimulation. Compared to control conditions, KATP-channel-deficient fibers had a greater proportion of fiber with large decreases in tetanic [Ca(2+)]i, fade and complete failure to release Ca(2+) upon stimulation. There was, however, a group of KATP-channel-deficient fibers that had similar fatigue kinetics to those of the most fatigue-resistant control fibers. For the first time, differences in fatigue kinetics were observed between Kir6.2(-/-) and glibenclamide-exposed muscle fibers. Verapamil significantly reduced unstimulated and tetanic [Ca(2+)]i. It is concluded that not all fibers are dependent on the myoprotection of KATP channels and that the decrease in unstimulated force by verapamil reported in a previous studies in glibenclamide-exposed fibers is due to a reduction in Ca(2+) load by reducing Ca(2+) influx through CaV1.1 channels between and during contractions.

Disruption of KATP channel expression in skeletal muscle by targeted oligonucleotide delivery promotes activity-linked thermogenesis

Despite the medical, social, and economic impact of obesity, only a few therapeutic options, focused largely on reducing caloric intake, are currently available and these have limited success rates. A major impediment is that any challenge by caloric restriction is counterbalanced by activation of systems that conserve energy to prevent body weight loss. Therefore, targeting energy-conserving mechanisms to promote energy expenditure is an attractive strategy for obesity treatment. Here, in order to suppress muscle energy efficiency, we target sarcolemmal ATP-sensitive potassium (KATP) channels which have previously been shown to be important in maintaining muscle energy economy. Specifically, we employ intramuscular injections of cell-penetrating vivo-morpholinos to prevent translation of the channel pore-forming subunit. This intervention results in significant reduction of KATP channel expression and function in treated areas, without affecting the channel expression in nontargeted tissues. Furthermore, suppression of KATP channel function in a group of hind limb muscles causes a substantial increase in activity-related energy consumption, with little effect on exercise tolerance. These findings establish a proof-of-principle that selective skeletal muscle targeting of sarcolemmal KATP channel function is possible and that this intervention can alter overall bodily energetics without a disabling impact on muscle mechanical function.

Properties of single FDB fibers following a collagenase digestion for studying contractility, fatigue, and pCa-sarcomere shortening relationship

The objective of this study was to optimize the approach to obtain viable single flexor digitorum brevis (FDB) fibers following a collagenase digestion. A first aim was to determine the culture medium conditions for the collagenase digestion. The MEM yielded better fibers in terms of morphology and contractility than the DMEM. The addition of FBS to culture media was crucial to prevent fiber supercontraction. The addition of FBS to the physiological solution used during an experiment was also beneficial, especially during fatigue. Optimum FBS concentration in MEM was 10% (vol/vol), and for the physiological solution, it ranged between 0.2 and 1.0%. A second aim was to document the stability of single FDB fibers. If tested the day of the preparation, most fibers (∼80%) had stable contractions for up to 3 h, normal stimulus duration strength to elicit contractions, and normal and stable resting membrane potential during prolonged microelectrode penetration. A third aim was to document their fatigue kinetics. Major differences in fatigue resistance were observed between fibers as expected from the FDB fiber-type composition. All sarcoplasmic [Ca(2+)] and sarcomere length parameters returned to their prefatigue levels after a short recovery. The pCa-sarcomere shortening relationship of unfatigued fibers is very similar to the pCa-force curve reported in other studies. The pCa-sarcomere shortening from fatigue data is complicated by large decreases in sarcomere length between contractions. It is concluded that isolation of single fibers by a collagenase digestion is a viable preparation to study contractility and fatigue kinetics.

A syndrome of congenital hyperinsulinism and rhabdomyolysis is caused by KCNJ11 mutation

BACKGROUND

Congenital hyperinsulinism is a genetically heterogeneous disorder, but mutations in the components of the ATP-sensitive potassium channel K(ATP) account for more than a third of all isolated congenital hyperinsulinism cases. The association between congenital hyperinsulinism and rhabdomyolysis has not been reported.

OBJECTIVE

To describe significant skeletal muscle manifestations in a family with a novel mutation in KCNJ11 (encoding the Kir6.2 component of K(ATP)).

METHODS

Cross-sectional analysis of members of a large multiplex consanguineous family with congenital hyperinsulinism and rhabdomyolysis using autozygosity mapping and exome sequencing.

RESULTS

Five affected members of an extended consanguineous Saudi family were recruited along with relevant unaffected relatives. We were able to map an apparently novel syndrome of congenital hyperinsulinism and severe rhabdomyolysis leading to acute renal failure to a single locus that harbours KCNJ11 in which we identified a novel homozygous mutation.

CONCLUSIONS

This study expands the phenotype associated with KCNJ11 loss of function in humans and calls for increased awareness of rhabdomyolysis as a potential late-onset life-threatening complication of KCNJ11-related congenital hyperinsulinism.

Sarcolemmal ATP-sensitive potassium channels modulate skeletal muscle function under low-intensity workloads

ATP-sensitive potassium (K_ATP) channels have the unique ability to adjust membrane excitability and functions in accordance with the metabolic status of the cell. Skeletal muscles are primary sites of activity-related energy consumption and have K_ATP channels expressed in very high density. Previously, we demonstrated that transgenic mice with skeletal muscle–specific disruption of K_ATP channel function consume more energy than wild-type littermates. However, how K_ATP channel activation modulates skeletal muscle resting and action potentials under physiological conditions, particularly low-intensity workloads, and how this can be translated to muscle energy expenditure are yet to be determined. Here, we developed a technique that allows evaluation of skeletal muscle excitability in situ, with minimal disruption of the physiological environment. Isometric twitching of the tibialis anterior muscle at 1 Hz was used as a model of low-intensity physical activity in mice with normal and genetically disrupted K_ATP channel function. This workload was sufficient to induce K_ATP channel opening, resulting in membrane hyperpolarization as well as reduction in action potential overshoot and duration. Loss of K_ATP channel function resulted in increased calcium release and aggravated activity-induced heat production. Thus, this study identifies low-intensity workload as a trigger for opening skeletal muscle K_ATP channels and establishes that this coupling is important for regulation of myocyte function and thermogenesis. These mechanisms may provide a foundation for novel strategies to combat metabolic derangements when energy conservation or dissipation is required.

Anti-inflammatory interventions and skeletal muscle injury: benefit or detriment?

Exercise, eccentric contractions, acute trauma, and disease are all causal mechanisms of skeletal muscle injury. After skeletal muscle is injured, it undergoes sequential phases of degeneration, inflammation, regeneration, and fibrosis. Events that occur in response to inflammation trigger regenerative processes. However, since inflammation causes pain, decreases skeletal muscle function, has a negative effect on performance, and contributes to fibrosis, which is one of the leading causes of delayed regeneration, the general practice has been to reduce inflammation. The problem with this approach is that preventing inflammation may hinder recovery. Current treatment options for inflammation are not necessarily effective and, in some cases, they may be unsafe. This review focuses on the question of whether the most beneficial course of treatment should be to block inflammation or if it is sensible to allow inflammatory processes to progress naturally. If blocking inflammation is perceived as a beneficial approach, it is not yet known at what time point during the inflammatory response it is most sensible to interfere. To address these issues, this review evaluates the effects of various anti-inflammatory agents on recovery processes in response to exercise-induced, traumatic, and disease-associated models of skeletal muscle injury. A collective analysis such as this should lay the foundation for future work that systematically manipulates the inflammatory response to most effectively promote regeneration and functional recovery in injured skeletal muscle, while reducing the negative effects of inflammatory processes such as pain and fibrosis.

Skeletal muscle fatigue--regulation of excitation-contraction coupling to avoid metabolic catastrophe

ATP provides the energy in our muscles to generate force, through its use by myosin ATPases, and helps to terminate contraction by pumping Ca(2+) back into the sarcoplasmic reticulum, achieved by Ca(2+) ATPase. The capacity to use ATP through these mechanisms is sufficiently high enough so that muscles could quickly deplete ATP. However, this potentially catastrophic depletion is avoided. It has been proposed that ATP is preserved not only by the control of metabolic pathways providing ATP but also by the regulation of the processes that use ATP. Considering that contraction (i.e. myosin ATPase activity) is triggered by release of Ca(2+), the use of ATP can be attenuated by decreasing Ca(2+) release within each cell. A lower level of Ca(2+) release can be accomplished by control of membrane potential and by direct regulation of the ryanodine receptor (RyR, the Ca(2+) release channel in the terminal cisternae). These highly redundant control mechanisms provide an effective means by which ATP can be preserved at the cellular level, avoiding metabolic catastrophe. This Commentary will review some of the known mechanisms by which this regulation of Ca(2+) release and contractile response is achieved, demonstrating that skeletal muscle fatigue is a consequence of attenuation of contractile activation; a process that allows avoidance of metabolic catastrophe.

Compartmentation of membrane processes and nucleotide dynamics in diffusion-restricted cardiac cell microenvironment

Orchestrated excitation-contraction coupling in heart muscle requires adequate spatial arrangement of systems responsible for ion movement and metabolite turnover. Co-localization of regulatory and transporting proteins into macromolecular complexes within an environment of microanatomical cell components raises intracellular diffusion barriers that hamper the mobility of metabolites and signaling molecules. Compared to substrate diffusion in the cytosol, diffusional restrictions underneath the sarcolemma are much larger and could impede ion and nucleotide movement by a factor of 10(3)-10(5). Diffusion barriers thus seclude metabolites within the submembrane space enabling rapid and vectorial effector targeting, yet hinder energy supply from the bulk cytosolic space implicating the necessity for a shunting transfer mechanism. Here, we address principles of membrane protein compartmentation, phosphotransfer enzyme-facilitated interdomain energy transfer, and nucleotide signal dynamics at the subsarcolemma-cytosol interface. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

The KATP channel Kir6.2 subunit content is higher in glycolytic than oxidative skeletal muscle fibers

It has long been suggested that in skeletal muscle, the ATP-sensitive K(+) channel (K(ATP)) channel is important in protecting energy levels and that abolishing its activity causes fiber damage and severely impairs function. The responses to a lack of K(ATP) channel activity vary between muscles and fibers, with the severity of the impairment being the highest in the most glycolytic muscle fibers. Furthermore, glycolytic muscle fibers are also expected to face metabolic stress more often than oxidative ones. The objective of this study was to determine whether the t-tubular K(ATP) channel content differs between muscles and fiber types. K(ATP) channel content was estimated using a semiquantitative immunofluorescence approach by staining cross sections from soleus, extensor digitorum longus (EDL), and flexor digitorum brevis (FDB) muscles with anti-Kir6.2 antibody. Fiber types were determined using serial cross sections stained with specific antimyosin I, IIA, IIB, and IIX antibodies. Changes in Kir6.2 content were compared with changes in CaV1.1 content, as this Ca(2+) channel is responsible for triggering Ca(2+) release from sarcoplasmic reticulum. The Kir6.2 content was the lowest in the oxidative soleus and the highest in the glycolytic EDL and FDB. At the individual fiber level, the Kir6.2 content within a muscle was in the order of type IIB > IIX > IIA ≥ I. Interestingly, the Kir6.2 content for a given fiber type was significantly different between soleus, EDL, and FDB, and highest in FDB. Correlations of relative fluorescence intensities from the Kir6.2 and CaV1.1 antibodies were significant for all three muscles. However, the variability in content between the three muscles or individual fibers was much greater for Kir6.2 than for CaV1.1. It is suggested that the t-tubular K(ATP) channel content increases as the glycolytic capacity increases and as the oxidative capacity decreases and that the expression of K(ATP) channels may be linked to how often muscles/fibers face metabolic stress.

Chronic AMPK activation evokes the slow, oxidative myogenic program and triggers beneficial adaptations in mdx mouse skeletal muscle

A therapeutic approach for Duchenne muscular dystrophy (DMD) is to up-regulate utrophin in skeletal muscle in an effort to compensate for the lack of dystrophin. We previously hypothesized that promotion of the slow, oxidative myogenic program, which triggers utrophin up-regulation, can attenuate the dystrophic pathology in mdx animals. Since treatment of healthy mice with the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) enhances oxidative capacity and elicits a fast-to-slow fiber-type transition, we evaluated the effects of chronic AMPK stimulation on skeletal muscle phenotype and utrophin expression in mdx mice. Daily AICAR administration (500 mg/kg/day, 30 days) of 5-7-week-old mdx animals induced an elevation in mitochondrial cytochrome c oxidase enzyme activity, an increase in myosin heavy-chain type IIa-positive fibers and slower twitch contraction kinetics in the fast, glycolytic extensor digitorum longus muscle. Utrophin expression was significantly enhanced in response to AICAR, which occurred coincident with an elevated β-dystroglycan expression along the sarcolemma. These adaptations were associated with an increase in sarcolemmal structural integrity under basal conditions, as well as during damaging eccentric contractions ex vivo. Notably, peroxisome proliferator-activated receptor γ co-activator-1α (PGC-1α) and silent information regulator two ortholog 1 protein contents were significantly higher in muscle from mdx mice compared with wild-type littermates and AICAR further increased PGC-1α expression. Our data show that AICAR-evoked muscle plasticity results in beneficial phenotypic adaptations in mdx mice and suggest that the contextually novel application of this compound for muscular dystrophy warrants further study.

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.

Sarcolemmal ATP-sensitive K(+) channels control energy expenditure determining body weight

Metabolic processes that regulate muscle energy use are major determinants of bodily energy balance. Here, we find that sarcolemmal ATP-sensitive K(+) (K(ATP)) channels, which couple membrane excitability with cellular metabolic pathways, set muscle energy expenditure under physiological stimuli. Disruption of K(ATP) channel function provoked, under conditions of unaltered locomotor activity and blood substrate availability, an extra energy cost of cardiac and skeletal muscle performance. Inefficient fuel metabolism in K(ATP) channel-deficient striated muscles reduced glycogen and fat body depots, promoting a lean phenotype. The propensity to lesser body weight imposed by K(ATP) channel deficit persisted under a high-fat diet, yet obesity restriction was achieved at the cost of compromised physical endurance. Thus, sarcolemmal K(ATP) channels govern muscle energy economy, and their downregulation in a tissue-specific manner could present an antiobesity strategy by rendering muscle increasingly thermogenic at rest and less fuel efficient during exercise.

Impaired exercise tolerance and skeletal muscle myopathy in sulfonylurea receptor-2 mutant mice

By sensing intracellular energy levels, ATP-sensitive potassium (K(ATP)) channels help regulate vascular tone, glucose metabolism, and cardioprotection. SUR2 mutant mice lack full-length K(ATP) channels in striated and smooth muscle and display a complex phenotype of hypertension and coronary vasospasm. SUR2 mutant mice also display baseline cardioprotection and can withstand acute sympathetic stress better than normal mice. We now studied response to a form of chronic stress, namely that induced by 4 wk of daily exercise on SUR2 mutant mice. Control mice increased exercise capacity by 400% over the training period, while SUR2 mutant mice showed little increase in exercise capacity. Unexercised SUR2 mutant showed necrotic and regenerating fibers in multiple muscle skeletal muscles, including quadriceps, tibialis anterior, and diaphragm muscles. Unlike exercised control animals, SUR2 mutant mice did not lose weight, presumably due to less overall exertion. Unexercised SUR2 mutant mice showed a trend of mildly reduced cardiac function, measured by fractional shortening, (46 +/- 4% vs. 57 +/- 7% for SUR2 mutant and control, respectively), and this decrease was not exacerbated by chronic exercise exposure. Despite an improved response to acute sympathetic stress and baseline cardioprotection, exercise intolerance results from lack of SUR2 K(ATP) channels in mice.

How ATP inhibits the open K(ATP) channel

ATP-sensitive potassium (K_ATP) channels are composed of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits. Binding of ATP to Kir6.2 leads to inhibition of channel activity. Because there are four subunits and thus four ATP-binding sites, four binding events are possible. ATP binds to both the open and closed states of the channel and produces a decrease in the mean open time, a reduction in the mean burst duration, and an increase in the frequency and duration of the interburst closed states. Here, we investigate the mechanism of interaction of ATP with the open state of the channel by analyzing the single-channel kinetics of concatenated Kir6.2 tetramers containing from zero to four mutated Kir6.2 subunits that possess an impaired ATP-binding site. We show that the ATP-dependent decrease in the mean burst duration is well described by a Monod-Wyman-Changeux model in which channel closing is produced by all four subunits acting in a single concerted step. The data are inconsistent with a Hodgkin-Huxley model (four independent steps) or a dimer model (two independent dimers). When the channel is open, ATP binds to a single ATP-binding site with a dissociation constant of 300 μM.

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.

Targeted mutation of mouse skeletal muscle sodium channel produces myotonia and potassium-sensitive weakness

Hyperkalemic periodic paralysis (HyperKPP) produces myotonia and attacks of muscle weakness triggered by rest after exercise or by K+ ingestion. We introduced a missense substitution corresponding to a human familial HyperKPP mutation (Met1592Val) into the mouse gene encoding the skeletal muscle voltage-gated Na+ channel NaV1.4. Mice heterozygous for this mutation exhibited prominent myotonia at rest and muscle fiber-type switching to a more oxidative phenotype compared with controls. Isolated mutant extensor digitorum longus muscles were abnormally sensitive to the Na+/K+ pump inhibitor ouabain and exhibited age-dependent changes, including delayed relaxation and altered generation of tetanic force. Moreover, rapid and sustained weakness of isolated mutant muscles was induced when the extracellular K+ concentration was increased from 4 mM to 10 mM, a level observed in the muscle interstitium of humans during exercise. Mutant muscle recovered from stimulation-induced fatigue more slowly than did control muscle, and the extent of recovery was decreased in the presence of high extracellular K+ levels. These findings demonstrate that expression of the Met1592ValNa+ channel in mouse muscle is sufficient to produce important features of HyperKPP, including myotonia, K+-sensitive paralysis, and susceptibility to delayed weakness during recovery from fatigue.

A patient suffering from hypokalemic periodic paralysis is deficient in skeletal muscle ATP-sensitive K channels

Hypokalemic periodic paralysis (HOPP) is a rare disease associated with attacks of muscle weakness and hypokalemia. In the present study, immunoprecipitation/Western blotting has shown that a HOPP patient was deficient in sarcolemmal K(ATP) channels. Real-time RT-PCR has revealed that HOPP has decreased mRNA levels of Kir6.2, a pore-forming K(ATP) channel subunit, without affecting the expression of other K(ATP) channel-forming proteins. Based on these findings, we conclude that HOPP could be associated with impaired expression of Kir6.2 which leads to deficiency in skeletal muscle K(ATP) channels, which may explain the symptoms and clinical signs of this disease.

KATP channel deficiency in mouse flexor digitorum brevis causes fibre damage and impairs Ca2+ release and force development during fatigue in vitro

Activation of the K(ATP) channels results in faster fatigue rates as the channels depress action potential amplitude, whereas abolishing the channel activity has no effect in whole extensor digitorum longus (EDL) and soleus muscles. In this study, we examined the effects of abolished K(ATP) channel activity during fatigue at 37 degrees C on free intracellular Ca(2+) (Ca(2+)(i)) and tetanic force using single muscle fibres and small muscle bundles from the flexor digitorum brevis (FDB). K(ATP) channel deficient muscle fibres were obtained (i) pharmacologically by exposing wild-type fibres to glibenclamide, and (ii) genetically using null mice for the Kir6.2 gene (Kir6.2(-/-) mice). Fatigue was elicited using 200 ms tetanic contractions every second for 3 min. This study demonstrated for the first time that abolishing K(ATP) channel activity at 37 degrees C resulted in faster fatigue rates, where decreases in peak Ca(2+)(i) and tetanic force were faster in K(ATP) channel deficient fibres than in control wild-type fibres. Furthermore, several contractile dysfunctions were also observed in K(ATP) channel deficient muscle fibre. They included partially or completely supercontracted single muscle fibres, greater increases in unstimulated Ca(2+)(i) and unstimulated force, and lower force recovery. We propose that the observed faster rate of fatigue in K(ATP) channel deficient fibres is because the decreases in peak Ca(2+)(i) and force caused by contractile dysfunctions prevail over the expected slower decreases when the channels do not depress action potential amplitude.

The mouse dystrophin muscle promoter/enhancer drives expression of mini-dystrophin in transgenic mdx mice and rescues the dystrophy in these mice

Successful gene therapy for Duchenne muscular dystrophy (DMD) requires the restoration of dystrophin protein in skeletal muscles. To achieve this goal, appropriate regulatory elements that impart tissue-specific transgene expression need to be identified. Currently, most muscle-directed gene therapy studies utilize the muscle creatine kinase promoter. We have previously described a muscle enhancer element (mDME-1) derived from the mouse dystrophin gene that increases transcription from the mouse dystrophin muscle promoter. Here, we explore the use of this native mouse dystrophin muscle promoter/enhancer to drive expression of a human dystrophin minigene in transgenic mice. We show that the dystrophin promoter can provide tissue-specific transgene expression and that the mini-dystrophin protein is expressed at the sarcolemma of skeletal muscles from mdx mice, where it restores the dystrophin-associated glycoprotein complex. The level of transgene expression obtained is sufficient to protect mdx muscles from the morphological and physiological symptoms of muscular dystrophy, as well as from exercise-induced damage. Therefore, the dystrophin muscle promoter/enhancer sequence represents an alternative for use in gene therapy vectors for the treatment of DMD.

Behavioral phenotyping of mice lacking the K ATP channel subunit Kir6.2

ATP-sensitive potassium (K(ATP)) channels are expressed in various tissues and cell-types where they act as so-called metabolic sensors that couple metabolic state to cellular excitability. The pore of most K(ATP) channel types is built by Kir6.2 subunits. Analysis of a general Kir6.2 knockout (KO) mouse has identified a variety of different functional roles for central and peripheral K(ATP) channels in situations of metabolic demand. However, the widespread distribution of these channels suggests that they might influence cellular physiology and animal behavior under metabolic control conditions. As a comprehensive behavioral description of Kir6.2 KO mice under physiological control conditions has not yet been carried out, we subjected Kir6.2 KO and corresponding wild-type (WT) mice to a test battery to assess emotional behavior, motor activity and coordination, species-typical behaviors and cognition. The results indicated that in these test situations Kir6.2 KO mice were less active, had impaired motor coordination, and appeared to differ from controls in their emotional reactivity. Differences between KO and WT mice were generally attenuated in test situations that resembled the home cage environment. Moreover, in their home cages KO mice were more active than WT mice. Thus, our results suggest that loss of Kir6.2-containing K(ATP) channels does affect animal behavior under metabolic control conditions, especially in novel situations. These findings assign novel functional roles to K(ATP) channels beyond those previously described. However, according to the widespread expression of K(ATP) channels, these effects are complex, being dependent on details of test apparatus, procedure and prior experience.

Membrane proteins involved in potassium shifts during muscle activity and fatigue

Muscle activity is associated with potassium displacements, which may cause fatigue. It was reported previously that the density of the large-conductance Ca2+-dependent K+ (BK(Ca)) channel is higher in the T tubule membrane than in the sarcolemmal membrane and that the opposite is the case for the ATP-sensitive K+ (K(ATP)) channel. In the present experiments, we investigated the subcellular localizations of the strong inward rectifier 2.1 K+ (Kir2.1) channel and the Na+-K+-2Cl- (NKCC)1 cotransporter with Western blot analysis of different muscle fractions. Furthermore, muscle function was studied while trying to manipulate the opening probability or transport capacity of these proteins during electrical stimulation of isolated soleus muscles. All experiments were made with excised muscle from male Wistar rats. Kir2.1 channels were almost undetectable in the sarcolemmal membrane but present in the T tubule membrane, whereas NKCC1 cotransporters were present in the sarcolemmal membrane. For muscles incubated in a buffer containing pinacidil, NS1619, Ba2+, or bumetanide, there was a faster reduction in peak force (P < 0.05). Furthermore, bumetanide incubation reduced the peak force at the onset of electrical stimulation (P < 0.05). Thus the effects on muscle force indicate that these drugs can affect K+-transporting proteins and thereby influence K+ accumulation, especially in the T tubules, suggesting that K(ATP) and BK(Ca) channels are responsible for K+ release and decrease in force during repeated muscle contractions, whereas Kir2.1 and NKCC1 may have a role in K+ reuptake.

Cytoplasmic ATP-sensing domains regulate gating of skeletal muscle ClC-1 chloride channels

ClC proteins are a family of chloride channels and transporters that are found in a wide variety of prokaryotic and eukaryotic cell types. The mammalian voltage-gated chloride channel ClC-1 is important for controlling the electrical excitability of skeletal muscle. Reduced excitability of muscle cells during metabolic stress can protect cells from metabolic exhaustion and is thought to be a major factor in fatigue. Here we identify a novel mechanism linking excitability to metabolic state by showing that ClC-1 channels are modulated by ATP. The high concentration of ATP in resting muscle effectively inhibits ClC-1 activity by shifting the voltage gating to more positive potentials. ADP and AMP had similar effects to ATP, but IMP had no effect, indicating that the inhibition of ClC-1 would only be relieved under anaerobic conditions such as intense muscle activity or ischemia, when depleted ATP accumulates as IMP. The resulting increase in ClC-1 activity under these conditions would reduce muscle excitability, thus contributing to fatigue. We show further that the modulation by ATP is mediated by cystathionine beta-synthase-related domains in the cytoplasmic C terminus of ClC-1. This defines a function for these domains as gating-modulatory domains sensitive to intracellular ligands, such as nucleotides, a function that is likely to be conserved in other ClC proteins.

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.

KATP channel: relation with cell metabolism and role in the cardiovascular system

ATP-sensitive potassium channel (K(ATP)) is one kind of inwardly rectifying channel composed of two kinds of subunits: the pore forming subunits and the regulatory subunits. K(ATP) channels exist in the sarcolemmal, mitochondrial and nuclear membranes of various tissues. Cell metabolism regulates K(ATP) gene expression and metabolism products regulate the channel by direct interactions, while K(ATP) controls membrane potentials and regulate cell activities including energy metabolism, apoptosis and gene expression. K(ATP) channels from different cell organelles are linked by some signal molecules and they can respond to common stimulation in a coordinate way. In the cardiovascular system K(ATP) has important functions. The most prominent is that opening of this channel can protect cardiac myocytes against ischemic injuries. The sarcolemmal K(ATP) may provide a basic protection against ischemia by energy sparing, while both the sarcolemmal K(ATP) and mitochondrial K(ATP) channels are necessary for the ischemia preconditioning. K(ATP) channels also have important functions including homeostasis maintenance and vascular tone regulation under physiological conditions. Further elucidation of the role of K(ATP) in the cardiovascular system will help us to regulate cell metabolism or prevent damage caused by abnormal channel functions.

Roles of KATP channels as metabolic sensors in acute metabolic changes

Physiological and pathophysiological roles of K(ATP) channels have been clarified recently in genetically engineered mice. The Kir6.2-containing K(ATP) channels in pancreatic ss-cells and the hypothalamus are essential in the regulation of glucose-induced insulin secretion and hypoglycemia-induced glucagon secretion, respectively, and are involved in glucose uptake in skeletal muscles, thus playing a key role in the maintenance of glucose homeostasis. Disruption of Kir6.1-containing K(ATP) channels in mice leads to spontaneous vascular spasm mimicking vasospastic (Prinzmetal) angina in humans, indicating that the Kir6.1-containing K(ATP) channels in vascular smooth muscles participate in the regulation of vascular tonus, especially in coronary arteries. Together with protective roles of K(ATP) channels against cardiac ischemia and hypoxia-induced seizure propagation, it is now clear that K(ATP) channels, as metabolic sensors, are critical in the maintenance of homeostasis against acute metabolic changes.

Roles of ATP-sensitive K+ channels as metabolic sensors: studies of Kir6.x null mice

ATP-sensitive K+ channels (KATP channels) are present in various tissues, including pancreatic beta-cells, heart, skeletal muscles, vascular smooth muscles, and brain. KATP channels are hetero-octameric proteins composed of inwardly rectifying K+ channel (Kir6.x) and sulfonylurea receptor (SUR) subunits. Different combinations of Kir6.x and SUR subunits comprise KATP channels with distinct electrophysiological and pharmacological properties. Recent studies of genetically engineered mice have provided insight into the physiological and pathophysiological roles of Kir6.x-containing KATP channels. Analysis of Kir6.2 null mice has shown that Kir6.2/SUR1 channels in pancreatic beta-cells and the hypothalamus are essential in glucose-induced insulin secretion and hypoglycemia-induced glucagon secretion, respectively, and that Kir6.2/SUR2 channels are involved in glucose uptake in skeletal muscles. Kir6.2-containing KATP channels in brain also are involved in protection from hypoxia-induced generalized seizure. In cardiovascular tissues, Kir6.1-containing KATP channels are involved in regulation of vascular tonus. In addition, the Kir6.1 null mouse is a model of Prinzmetal angina in humans. Our studies of Kir6.2 null and Kir6.1 null mice reveal that KATP channels are critical metabolic sensors in acute metabolic changes, including hyperglycemia, hypoglycemia, ischemia, and hypoxia.

Statistical analyses of repeated measures in physiological research: a tutorial

Experimental designs involving repeated measurements on experimental units are widely used in physiological research. Often, relatively many consecutive observations on each experimental unit are involved and the data may be quite nonlinear. Yet evidently, one of the most commonly used statistical methods for dealing with such data sets in physiological research is the repeated-measurements ANOVA model. The problem herewith is that it is not well suited for data sets with many consecutive measurements; it does not deal with nonlinear features of the data, and the interpretability of the model may be low. The use of inappropriate statistical models increases the likelihood of drawing wrong conclusions. The aim of this article is to illustrate, for a reasonably typical repeated-measurements data set, how fundamental assumptions of the repeated-measurements ANOVA model are inappropriate and how researchers may benefit from adopting different modeling approaches using a variety of different kinds of models. We emphasize intuitive ideas rather than mathematical rigor. We illustrate how such models represent alternatives that 1) can have much higher interpretability, 2) are more likely to meet underlying assumptions, 3) provide better fitted models, and 4) are readily implemented in widely distributed software products.

ATP-sensitive K+ channel knockout compromises the metabolic benefit of exercise training, resulting in cardiac deficits

Exercise training elicits a metabolic and cardiovascular response that underlies fitness. The molecular mechanisms that orchestrate this adaptive response and secure the wide-ranging gains of a regimented exercise program are poorly understood. Formed through association of the Kir6.2 pore and the sulfonylurea receptor, the stress-responsive ATP-sensitive K(+) channels (K(ATP) channels), with their metabolic-sensing capability and broad tissue expression, are potential candidates for integrating the systemic adaptive response to repetitive exercise. Here, the responses of mice lacking functional Kir6.2-containing K(ATP) channels (Kir6.2-KO) were compared with wild-type controls following a 28-day endurance swimming protocol. While chronic aquatic training resulted in lighter, leaner, and fitter wild-type animals, the Kir6.2-KO manifested less augmentation in exercise capacity and lacked metabolic improvement in body fat composition and glycemic handling with myocellular defects. Moreover, the repetitive stress of swimming unmasked a survival disadvantage in the Kir6.2-KO, associated with pathologic calcium-dependent structural damage in the heart and impaired cardiac performance. Thus, Kir6.2-containing K(ATP) channel activity is required for attainment of the physiologic benefits of exercise training without injury.