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

Resting membrane potential and intracellular [Na+] at rest, during fatigue and during recovery in rat soleus muscle fibres in situ

Large trans-sarcolemmal ionic shifts occur with fatiguing exercise or stimulation of isolated muscles. However, it is unknown how resting membrane potential (EM) and intracellular sodium concentration ([Na+]i) change with repeated contractions in living mammals. We investigated (i) whether [Na+]i (peak, kinetics) can reveal changes of Na+-K+ pump activity during brief or fatiguing stimulation and (ii) how resting EM and [Na+]i change during fatigue and recovery of rat soleus muscle in situ. Muscles of anaesthetised rats were stimulated with brief (10 s) or repeated tetani (60 Hz for 200 ms, every 2 s, for 30 s or 300 s) with isometric force measured. Double-barrelled ion-sensitive microelectrodes were used to quantify resting EM and [Na+]i. Post-stimulation data were fitted using polynomials and back-extrapolated to time zero recovery. Mean pre-stimulation resting EM (layer 2-7 fibres) was -71 mV (surface fibres were more depolarised), and [Na+]i was 14 mM. With deeper fibres, 10 s stimulation (2-150 Hz) increased [Na+]i to 38-46 mM whilst simultaneously causing hyperpolarisations (7.3 mV for 2-90 Hz). Fatiguing stimulation for 30 s or 300 s led to end-stimulation resting EM of -61 to -53 mV, which recovered rapidly (T1/2, 8-22 s). Mean end-stimulation [Na+]i increased to 86-101 mM with both fatigue protocols and the [Na+]i recovery time-course (T1/2, 21-35 s) showed no difference between protocols. These combined findings suggest that brief stimulation hyperpolarises the resting EM, likely via maximum Na+-induced stimulation of the Na+-K+ pump. Repeated tetani caused massive depolarisation and elevations of [Na+]i that together lower force, although they likely interact with other factors to cause fatigue. [Na+]i recovery kinetics provided no evidence of impaired Na+-K+ pump activity with fatigue. KEY POINTS: It is uncertain how resting membrane potential, intracellular sodium concentration ([Na+]i), and sodium-potassium (Na+-K+) pump activity change during repeated muscle contractions in living mammals. For rat soleus muscle fibres in situ, brief tetanic stimulation for 10 s led to raised [Na+]i, anticipated to evoke maximal Na+-induced stimulation of the Na+-K+ pump causing an immediate hyperpolarisation of the sarcolemma. More prolonged stimulation with repeated tetanic contractions causes massive elevations of [Na+]i, which together with large depolarisations (via K+ disturbances) likely reduce force production. These effects occurred without impairment of Na+-K+ pump function. Together these findings suggest that rapid activation of the Na+-K+ pump occurs with brief stimulation to maintain excitability, whereas more prolonged stimulation causes rundown of the trans-sarcolemmal K+ gradient (hence depolarisation) and Na+ gradient, which in combination can impair contraction to contribute to fatigue in living mammals.

Novel loss-of-function variants expand ABCC9-related intellectual disability and myopathy syndrome

Loss-of-function mutation of ABCC9, the gene encoding the SUR2 subunit of ATP sensitive-potassium (KATP) channels, was recently associated with autosomal recessive ABCC9-related intellectual disability and myopathy syndrome (AIMS). Here we identify nine additional subjects, from seven unrelated families, harbouring different homozygous loss-of-function variants in ABCC9 and presenting with a conserved range of clinical features. All variants are predicted to result in severe truncations or in-frame deletions within SUR2, leading to the generation of non-functional SUR2-dependent KATP channels. Affected individuals show psychomotor delay and intellectual disability of variable severity, microcephaly, corpus callosum and white matter abnormalities, seizures, spasticity, short stature, muscle fatigability and weakness. Heterozygous parents do not show any conserved clinical pathology but report multiple incidences of intra-uterine fetal death, which were also observed in an eighth family included in this study. In vivo studies of abcc9 loss-of-function in zebrafish revealed an exacerbated motor response to pentylenetetrazole, a pro-convulsive drug, consistent with impaired neurodevelopment associated with an increased seizure susceptibility. Our findings define an ABCC9 loss-of-function-related phenotype, expanding the genotypic and phenotypic spectrum of AIMS and reveal novel human pathologies arising from KATP channel dysfunction.

Ischemic preconditioning increases spinal excitability and voluntary activation during maximal plantar flexion contractions in men

The enigmatic benefits of acute limb ischemic preconditioning (IP) in enhancing muscle force and exercise performance have intrigued researchers. This study sought to unravel the underlying mechanisms, focusing on increased neural drive and the role of spinal excitability while excluding peripheral factors. Soleus Hoffmann (H)-reflex /M-wave recruitment curves and unpotentiated supramaximal responses were recorded before and after IP or a low-pressure control intervention. Subsequently, the twitch interpolation technique was applied during maximal voluntary contractions to assess conventional parameters of neural output. Following IP, there was an increase in both maximum normalized force and voluntary activation (VA) for the plantar flexor group, with negligible peripheral alterations. Greater benefits were observed in participants with lower VA levels. Despite greater H-reflex gains, soleus volitional (V)-wave and sEMG amplitudes remained unchanged. In conclusion, IP improves muscle force via enhanced neural drive to the muscles. This effect appears associated, at least in part, to reduced presynaptic inhibition and/or increased motoneuron excitability. Furthermore, the magnitude of the benefit is inversely proportional to the skeletal muscle's functional reserve, making it particularly noticeable in under-recruited muscles. These findings have implications for the strategic application of the IP procedure across diverse populations.

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.

Skeletal muscle delimited myopathy and verapamil toxicity in SUR2 mutant mouse models of AIMS

ABCC9-related intellectual disability and myopathy syndrome (AIMS) arises from loss-of-function (LoF) mutations in the ABCC9 gene, which encodes the SUR2 subunit of ATP-sensitive potassium (KATP ) channels. KATP channels are found throughout the cardiovascular system and skeletal muscle and couple cellular metabolism to excitability. AIMS individuals show fatigability, muscle spasms, and cardiac dysfunction. We found reduced exercise performance in mouse models of AIMS harboring premature stop codons in ABCC9. Given the roles of KATP channels in all muscles, we sought to determine how myopathy arises using tissue-selective suppression of KATP and found that LoF in skeletal muscle, specifically, underlies myopathy. In isolated muscle, SUR2 LoF results in abnormal generation of unstimulated forces, potentially explaining painful spasms in AIMS. We sought to determine whether excessive Ca2+ influx through CaV 1.1 channels was responsible for myopathology but found that the Ca2+ channel blocker verapamil unexpectedly resulted in premature death of AIMS mice and that rendering CaV 1.1 channels nonpermeable by mutation failed to reverse pathology; results which caution against the use of calcium channel blockers in AIMS.

Pharmacological FGF21 signals to glutamatergic neurons to enhance leptin action and lower body weight during obesity

OBJECTIVE

Fibroblast growth factor 21 (FGF21) is a peripherally-derived endocrine hormone that acts on the central nervous system (CNS) to regulate whole body energy homeostasis. Pharmacological administration of FGF21 promotes weight loss in obese animal models and human subjects with obesity. However, the central targets mediating these effects are incompletely defined.

METHODS

To explore the mechanism for FGF21's effects to lower body weight, we pharmacologically administer FGF21 to genetic animal models lacking the obligate FGF21 co-receptor, β-klotho (KLB), in either glutamatergic (Vglut2-Cre) or GABAergic (Vgat-Cre) neurons. In addition, we abolish FGF21 signaling to leptin receptor (LepR-Cre) positive cells. Finally, we examine the synergistic effects of FGF21 and leptin to lower body weight and explore the importance of physiological leptin levels in FGF21-mediated regulation of body weight.

RESULTS

Here we show that FGF21 signaling to glutamatergic neurons is required for FGF21 to modulate energy expenditure and promote weight loss. In addition, we demonstrate that FGF21 signals to leptin receptor-expressing cells to regulate body weight, and that central leptin signaling is required for FGF21 to fully stimulate body weight loss during obesity. Interestingly, co-administration of FGF21 and leptin synergistically leads to robust weight loss.

CONCLUSIONS

These data reveal an important endocrine crosstalk between liver- and adipose-derived signals which integrate in the CNS to modulate energy homeostasis and body weight regulation.

Neuroanatomical organization and functional roles of PVN MC4R pathways in physiological and behavioral regulations

OBJECTIVE

The paraventricular nucleus of hypothalamus (PVN), an integrative center in the brain, orchestrates a wide range of physiological and behavioral responses. While the PVN melanocortin 4 receptor (MC4R) signaling (PVNMC4R+) is involved in feeding regulation, the neuroanatomical organization of PVNMC4R+ connectivity and its role in other physiological regulations are incompletely understood. Here we aimed to better characterize the input-output organization of PVNMC4R+ neurons and test their physiological functions beyond feeding.

METHODS

Using a combination of viral tools, we mapped PVNMC4R+ circuits and tested the effects of chemogenetic activation of PVNMC4R+ neurons on thermoregulation, cardiovascular control, and other behavioral responses beyond feeding.

RESULTS

We found that PVNMC4R+ neurons innervate many different brain regions that are known to be important not only for feeding but also for neuroendocrine and autonomic control of thermoregulation and cardiovascular function, including but not limited to the preoptic area, median eminence, parabrachial nucleus, pre-locus coeruleus, nucleus of solitary tract, ventrolateral medulla, and thoracic spinal cord. Contrary to these broad efferent projections, PVNMC4R+ neurons receive monosynaptic inputs mainly from other hypothalamic nuclei (preoptic area, arcuate and dorsomedial hypothalamic nuclei, supraoptic nucleus, and premammillary nucleus), the circumventricular organs (subfornical organ and vascular organ of lamina terminalis), the bed nucleus of stria terminalis, and the parabrachial nucleus. Consistent with their broad efferent projections, chemogenetic activation of PVNMC4R+ neurons not only suppressed feeding but also led to an apparent increase in heart rate, blood pressure, and brown adipose tissue temperature. These physiological changes accompanied acute transient hyperactivity followed by hypoactivity and resting-like behavior.

CONCLUSIONS

Our results elucidate the neuroanatomical organization of PVNMC4R+ circuits and shed new light on the roles of PVNMC4R+ pathways in autonomic control of thermoregulation, cardiovascular function, and biphasic behavioral activation.

ADH5-mediated NO bioactivity maintains metabolic homeostasis in brown adipose tissue

Brown adipose tissue (BAT) thermogenic activity is tightly regulated by cellular redox status, but the underlying molecular mechanisms are incompletely understood. Protein S-nitrosylation, the nitric-oxide-mediated cysteine thiol protein modification, plays important roles in cellular redox regulation. Here we show that diet-induced obesity (DIO) and acute cold exposure elevate BAT protein S-nitrosylation, including UCP1. This thermogenic-induced nitric oxide bioactivity is regulated by S-nitrosoglutathione reductase (GSNOR; alcohol dehydrogenase 5 [ADH5]), a denitrosylase that balances the intracellular nitroso-redox status. Loss of ADH5 in BAT impairs cold-induced UCP1-dependent thermogenesis and worsens obesity-associated metabolic dysfunction. Mechanistically, we demonstrate that Adh5 expression is induced by the transcription factor heat shock factor 1 (HSF1), and administration of an HSF1 activator to BAT of DIO mice increases Adh5 expression and significantly improves UCP1-mediated respiration. Together, these data indicate that ADH5 controls BAT nitroso-redox homeostasis to regulate adipose thermogenesis, which may be therapeutically targeted to improve metabolic health.

Lower Ca2+ enhances the K+-induced force depression in normal and HyperKPP mouse muscles

Hyperkalemic periodic paralysis (HyperKPP) manifests as stiffness or subclinical myotonic discharges before or during periods of episodic muscle weakness or paralysis. Ingestion of Ca²⁺ alleviates HyperKPP symptoms, but the mechanism is unknown because lowering extracellular [Ca²⁺] ([Ca²⁺]_e) has no effect on force development in normal muscles under normal conditions. Lowering [Ca²⁺]_e, however, is known to increase the inactivation of voltage-gated cation channels, especially when the membrane is depolarized. Two hypotheses were tested: (1) lowering [Ca²⁺]_e depresses force in normal muscles under conditions that depolarize the cell membrane; and (2) HyperKPP muscles have a greater sensitivity to low Ca²⁺-induced force depression because many fibers are depolarized, even at a normal [K⁺]_e. In wild type muscles, lowering [Ca²⁺]_e from 2.4 to 0.3 mM had little effect on tetanic force and membrane excitability at a normal K⁺ concentration of 4.7 mM, whereas it significantly enhanced K⁺-induced depression of force and membrane excitability. In HyperKPP muscles, lowering [Ca²⁺]_e enhanced the K⁺-induced loss of force and membrane excitability not only at elevated [K⁺]_e but also at 4.7 mM K⁺. Lowering [Ca²⁺]_e increased the incidence of generating fast and transient contractures and gave rise to a slower increase in unstimulated force, especially in HyperKPP muscles. Lowering [Ca²⁺]_e reduced the efficacy of salbutamol, a β2 adrenergic receptor agonist and a treatment for HyperKPP, to increase force at elevated [K⁺]_e. Replacing Ca²⁺ by an equivalent concentration of Mg²⁺ neither fully nor consistently reverses the effects of lowering [Ca²⁺]_e. These results suggest that the greater Ca²⁺ sensitivity of HyperKPP muscles primarily relates to (1) a greater effect of Ca²⁺ in depolarized fibers and (2) an increased proportion of depolarized HyperKPP muscle fibers compared with control muscle fibers, even at normal [K⁺]_e.

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.

Pathophysiology of skeletal muscle disturbances in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Chronic Fatigue Syndrome or Myalgic Encephaloymelitis (ME/CFS) is a frequent debilitating disease with an enigmatic etiology. The finding of autoantibodies against ß2-adrenergic receptors (ß2AdR) prompted us to hypothesize that ß2AdR dysfunction is of critical importance in the pathophysiology of ME/CFS. Our hypothesis published previously considers ME/CFS as a disease caused by a dysfunctional autonomic nervous system (ANS) system: sympathetic overactivity in the presence of vascular dysregulation by ß2AdR dysfunction causes predominance of vasoconstrictor influences in brain and skeletal muscles, which in the latter is opposed by the metabolically stimulated release of endogenous vasodilators (functional sympatholysis). An enigmatic bioenergetic disturbance in skeletal muscle strongly contributes to this release. Excessive generation of these vasodilators with algesic properties and spillover into the systemic circulation could explain hypovolemia, suppression of renin (paradoxon) and the enigmatic symptoms. In this hypothesis paper the mechanisms underlying the energetic disturbance in muscles will be explained and merged with the first hypothesis. The key information is that ß2AdR also stimulates the Na+/K+-ATPase in skeletal muscles. Appropriate muscular perfusion as well as function of the Na+/K+-ATPase determine muscle fatigability. We presume that dysfunction of the ß2AdR also leads to an insufficient stimulation of the Na+/K+-ATPase causing sodium overload which reverses the transport direction of the sodium-calcium exchanger (NCX) to import calcium instead of exporting it as is also known from the ischemia-reperfusion paradigm. The ensuing calcium overload affects the mitochondria, cytoplasmatic metabolism and the endothelium which further worsens the energetic situation (vicious circle) to explain postexertional malaise, exercise intolerance and chronification. Reduced Na+/K+-ATPase activity is not the only cause for cellular sodium loading. In poor energetic situations increased proton production raises intracellular sodium via sodium-proton-exchanger subtype-1 (NHE1), the most important proton-extruder in skeletal muscle. Finally, sodium overload is due to diminished sodium outward transport and enhanced cellular sodium loading. As soon as this disturbance would have occurred in a severe manner the threshold for re-induction would be strongly lowered, mainly due to an upregulated NHE1, so that it could repeat at low levels of exercise, even by activities of everyday life, re-inducing mitochondrial, metabolic and vascular dysfunction to perpetuate the disease.

Impaired skeletal muscle mitochondrial pyruvate uptake rewires glucose metabolism to drive whole-body leanness

Metabolic cycles are a fundamental element of cellular and organismal function. Among the most critical in higher organisms is the Cori Cycle, the systemic cycling between lactate and glucose. Here, skeletal muscle-specific Mitochondrial Pyruvate Carrier (MPC) deletion in mice diverted pyruvate into circulating lactate. This switch disinhibited muscle fatty acid oxidation and drove Cori Cycling that contributed to increased energy expenditure. Loss of muscle MPC activity led to strikingly decreased adiposity with complete muscle mass and strength retention. Notably, despite decreasing muscle glucose oxidation, muscle MPC disruption increased muscle glucose uptake and whole-body insulin sensitivity. Furthermore, chronic and acute muscle MPC deletion accelerated fat mass loss on a normal diet after high fat diet-induced obesity. Our results illuminate the role of the skeletal muscle MPC as a whole-body carbon flux control point. They highlight the potential utility of modulating muscle pyruvate utilization to ameliorate obesity and type 2 diabetes.

Liver Derived FGF21 Maintains Core Body Temperature During Acute Cold Exposure

Fibroblast Growth Factor 21 (FGF21) elicits an array of metabolic effects. However, the physiological role of FGF21 during thermal challenges is not clear. In this study, we assessed the tissue source of FGF21 and its site of action to regulate core body temperature in response to cold. Using mice lacking FGF21 specifically in the liver (FGF21 LivKO) or adipose tissues (FGF21 AdipoKO), we performed a series of cold exposure studies to examine the tissue specific induction of FGF21 in response to cold. We also examined the physiological site of FGF21 action during cold exposure by impairing FGF21 signaling to adipose tissues or the central nervous system (CNS) using genetic ablation of the FGF21 co-receptor β-klotho in adipose tissues (KLB AdipoKO) or pharmacological blockage of FGF21 signaling. We found that only liver-derived FGF21 enters circulation during acute cold exposure and is critical for thermoregulation. While FGF21 signaling directly to adipose tissues during cold is dispensable for thermoregulation, central FGF21 signaling is necessary for maximal sympathetic drive to brown adipose tissue to maintain thermoregulation during cold. These data demonstrate a previously unrecognized role for FGF21 in the maintenance of body temperature in response to cold.

Understanding the physiology of the asymptomatic diaphragm of the M1592V hyperkalemic periodic paralysis mouse

When muscles become paralyzed in crises of hyperkalemic periodic paralysis, patients do not stop breathing. Here is why.

The diaphragm muscle of hyperkalemic periodic paralysis (HyperKPP) patients and of the M1592V HyperKPP mouse model rarely suffers from the myotonic and paralytic symptoms that occur in limb muscles. Enigmatically, HyperKPP diaphragm expresses the mutant NaV1.4 channel and, more importantly, has an abnormally high Na⁺ influx similar to that in extensor digitorum longus (EDL) and soleus, two hindlimb muscles suffering from the robust HyperKPP abnormalities. The objective was to uncover the physiological mechanisms that render HyperKPP diaphragm asymptomatic. A first mechanism involves efficient maintenance of resting membrane polarization in HyperKPP diaphragm at various extracellular K⁺ concentrations compared with larger membrane depolarizations in HyperKPP EDL and soleus. The improved resting membrane potential (EM) results from significantly increased Na⁺ K⁺ pump electrogenic activity, and not from an increased protein content. Action potential amplitude was greater in HyperKPP diaphragm than in HyperKPP soleus and EDL, providing a second mechanism for the asymptomatic behavior of the HyperKPP diaphragm. One suggested mechanism for the greater action potential amplitude is lower intracellular Na⁺ concentration because of greater Na⁺ K⁺ pump activity, allowing better Na⁺ current during the action potential depolarization phase. Finally, HyperKPP diaphragm had a greater capacity to generate force at depolarized EM compared with wild-type diaphragm. Action potential amplitude was not different between wild-type and HyperKPP diaphragm. There was also no evidence for an increased activity of the Na⁺–Ca²⁺ exchanger working in the reverse mode in the HyperKPP diaphragm compared with the wild-type diaphragm. So, a third mechanism remains to be elucidated to fully understand how HyperKPP diaphragm generates more force compared with wild type. Although the mechanism for the greater force at depolarized resting EM remains to be determined, this study provides support for the modulation of the Na⁺ K⁺ pump as a component of therapy to alleviate weakness in HyperKPP.

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.

Loss of ATP-Sensitive Potassium Channel Surface Expression in Heart Failure Underlies Dysregulation of Action Potential Duration and Myocardial Vulnerability to Injury

The search for new approaches to treatment and prevention of heart failure is a major challenge in medicine. The adenosine triphosphate-sensitive potassium (K_ATP) channel has been long associated with the ability to preserve myocardial function and viability under stress. High surface expression of membrane K_ATP channels ensures a rapid energy-sparing reduction in action potential duration (APD) in response to metabolic challenges, while cellular signaling that reduces surface K_ATP channel expression blunts APD shortening, thus sacrificing energetic efficiency in exchange for greater cellular calcium entry and increased contractile force. In healthy hearts, calcium/calmodulin-dependent protein kinase II (CaMKII) phosphorylates the Kir6.2 K_ATP channel subunit initiating a cascade responsible for K_ATP channel endocytosis. Here, activation of CaMKII in a transaortic banding (TAB) model of heart failure is coupled with a 35–40% reduction in surface expression of K_ATP channels compared to hearts from sham-operated mice. Linkage between K_ATP channel expression and CaMKII is verified in isolated cardiomyocytes in which activation of CaMKII results in downregulation of K_ATP channel current. Accordingly, shortening of monophasic APD is slowed in response to hypoxia or heart rate acceleration in failing compared to non-failing hearts, a phenomenon previously shown to result in significant increases in oxygen consumption. Even in the absence of coronary artery disease, failing myocardium can be further injured by ischemia due to a mismatch between metabolic supply and demand. Ischemia-reperfusion injury, following ischemic preconditioning, is diminished in hearts with CaMKII inhibition compared to wild-type hearts and this advantage is largely eliminated when myocardial K_ATP channel expression is absent, supporting that the myocardial protective benefit of CaMKII inhibition in heart failure may be substantially mediated by K_ATP channels. Recognition of CaMKII-dependent downregulation of K_ATP channel expression as a mechanism for vulnerability to injury in failing hearts points to strategies targeting this interaction for potential preventives or treatments.

Disruption of ATP-sensitive potassium channel function in skeletal muscles promotes production and secretion of musclin

Sarcolemmal ATP-sensitive potassium (KATP) channels control skeletal muscle energy use through their ability to adjust membrane excitability and related cell functions in accordance with cellular metabolic status. Mice with disrupted skeletal muscle KATP channels exhibit reduced adipocyte size and increased fatty acid release into the circulation. As yet, the molecular mechanisms underlying this link between skeletal muscle KATP channel function and adipose mobilization have not been established. Here, we demonstrate that skeletal muscle-specific disruption of KATP channel function in transgenic (TG) mice promotes production and secretion of musclin. Musclin is a myokine with high homology to atrial natriuretic peptide (ANP) that enhances ANP signaling by competing for elimination. Augmented musclin production in TG mice is driven by a molecular cascade resulting in enhanced acetylation and nuclear exclusion of the transcription factor forkhead box O1 (FOXO1) - an inhibitor of transcription of the musclin encoding gene. Musclin production/secretion in TG is paired with increased mobilization of fatty acids and a clear trend toward increased circulating ANP, an activator of lipolysis. These data establish KATP channel-dependent musclin production as a potential mechanistic link coupling "local" skeletal muscle energy consumption with mobilization of bodily resources from fat. Understanding such mechanisms is an important step toward designing interventions to manage metabolic disorders including those related to excess body fat and associated co-morbidities.

Distinct α2 Na,K-ATPase membrane pools are differently involved in early skeletal muscle remodeling during disuse

Location, location, location. The Na-K pump of skeletal muscle is regulated differently at neuromuscular junctions.

The Na,K-ATPase is essential for the contractile function of skeletal muscle, which expresses the α1 and α2 subunit isoforms of Na,K-ATPase. The α2 isozyme is predominant in adult skeletal muscles and makes a greater contribution in working compared with noncontracting muscles. Hindlimb suspension (HS) is a widely used model of muscle disuse that leads to progressive atrophy of postural skeletal muscles. This study examines the consequences of acute (6–12 h) HS on the functioning of the Na,K-ATPase α1 and α2 isozymes in rat soleus (disused) and diaphragm (contracting) muscles. Acute disuse dynamically and isoform-specifically regulates the electrogenic activity, protein, and mRNA content of Na,K-ATPase α2 isozyme in rat soleus muscle. Earlier disuse-induced remodeling events also include phospholemman phosphorylation as well as its increased abundance and association with α2 Na,K-ATPase. The loss of α2 Na,K-ATPase activity results in reduced electrogenic pump transport and depolarized resting membrane potential. The decreased α2 Na,K-ATPase activity is caused by a decrease in enzyme activity rather than by altered protein and mRNA content, localization in the sarcolemma, or functional interaction with the nicotinic acetylcholine receptors. The loss of extrajunctional α2 Na,K-ATPase activity depends strongly on muscle use, and even the increased protein and mRNA content as well as enhanced α2 Na,K-ATPase abundance at this membrane region after 12 h of HS cannot counteract this sustained inhibition. In contrast, additional factors may regulate the subset of junctional α2 Na,K-ATPase pool that is able to recover during HS. Notably, acute, low-intensity muscle workload restores functioning of both α2 Na,K-ATPase pools. These results demonstrate that the α2 Na,K-ATPase in rat skeletal muscle is dynamically and acutely regulated by muscle use and provide the first evidence that the junctional and extrajunctional pools of the α2 Na,K-ATPase are regulated differently.

Musclin is an activity-stimulated myokine that enhances physical endurance

Exercise remains the most effective way to promote physical and metabolic wellbeing, but molecular mechanisms underlying exercise tolerance and its plasticity are only partially understood. In this study we identify musclin-a peptide with high homology to natriuretic peptides (NP)-as an exercise-responsive myokine that acts to enhance exercise capacity in mice. We use human primary myoblast culture and in vivo murine models to establish that the activity-related production of musclin is driven by Ca(2+)-dependent activation of Akt1 and the release of musclin-encoding gene (Ostn) transcription from forkhead box O1 transcription factor inhibition. Disruption of Ostn and elimination of musclin secretion in mice results in reduced exercise tolerance that can be rescued by treatment with recombinant musclin. Reduced exercise capacity in mice with disrupted musclin signaling is associated with a trend toward lower levels of plasma atrial NP (ANP) and significantly smaller levels of cyclic guanosine monophosphate (cGMP) and peroxisome proliferator-activated receptor gamma coactivator 1-α in skeletal muscles after exposure to exercise. Furthermore, in agreement with the established musclin ability to interact with NP clearance receptors, but not with NP guanyl cyclase-coupled signaling receptors, we demonstrate that musclin enhances cGMP production in cultured myoblasts only when applied together with ANP. Elimination of the activity-related musclin-dependent boost of ANP/cGMP signaling results in significantly lower maximum aerobic capacity, mitochondrial protein content, respiratory complex protein expression, and succinate dehydrogenase activity in skeletal muscles. Together, these data indicate that musclin enhances physical endurance by promoting mitochondrial biogenesis.

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.