AMPK activation with AICAR provokes an acute fall in plasma [K+]

AMPK and glucose deprivation exert an isoform-specific effect on the expression of Na+,K+-ATPase subunits in cultured myotubes

In skeletal muscle, Na+,K+-ATPase (NKA), a heterodimeric (α/β) P-type ATPase, has an essential role in maintenance of Na+ and K+ homeostasis, excitability, and contractility. AMP-activated protein kinase (AMPK), an energy sensor, increases the membrane abundance and activity of NKA in L6 myotubes, but its potential role in regulation of NKA content in skeletal muscle, which determines maximum capacity for Na+ and K+ transport, has not been clearly delineated. We examined whether energy stress and/or AMPK affect expression of NKA subunits in rat L6 and primary human myotubes. Energy stress, induced by glucose deprivation, increased protein content of NKAα1 and NKAα2 in L6 myotubes, while decreasing the content of NKAα1 in human myotubes. Pharmacological AMPK activators (AICAR, A-769662, and diflunisal) modulated expression of NKA subunits, but their effects only partially mimicked those that occurred in response to glucose deprivation, indicating that AMPK does not mediate all effects of energy stress on NKA expression. Gene silencing of AMPKα1/α2 increased protein levels of NKAα1 in L6 myotubes and NKAα1 mRNA levels in human myotubes, while decreasing NKAα2 protein levels in L6 myotubes. Collectively, our results suggest a role for energy stress and AMPK in modulation of NKA expression in skeletal muscle. However, their modulatory effects were not conserved between L6 myotubes and primary human myotubes, which suggests that coupling between energy stress, AMPK, and regulation of NKA expression in vitro depends on skeletal muscle cell model.

Review of the Pathophysiologic and Clinical Aspects of Hypokalemia in Children and Young Adults: an Update

This article examines the regulatory function of the skeletal muscle, renal, and adrenergic systems in potassium homeostasis. The pathophysiologic bases of hypokalemia, systematic approach for an early diagnosis, and therapeutic strategy to avert life-threatening complications are highlighted. By promoting skeletal muscle uptake, intense physical exercise (post), severe trauma, and several toxins produce profound hypokalemia. Hypovolemia due to renal and extra-renal fluid losses and ineffective circulation activate secondary aldosteronism causing urinary potassium wasting. In addition to hypokalemic alkalosis, primary aldosteronism causes low-renin hypertension. Non-aldosterone mineralocorticoid activation leading to low-renin and low-aldosterone hypertension occurs in Liddle's syndrome and apparent mineralocorticoid excess. Although there is enzymatic inhibition of cortisol synthesis in congenital adrenal hyperplasia, precursors of aldosterone produce low-renin hypokalemic hypertension. In addition to the glucocorticoid effect, hypercortisolism activates mineralocorticoid receptors in Cushing's syndrome. Genetic mutations involving furosemide-sensitive Na+-K+-2Cl- co-transporters and thiazide-sensitive Na+-Cl- transporters result in (non-hypertensive) salt-wasting nephropathy. Proximal and distal renal tubular acidosis is associated with hypokalemia. Eating disorders causing hypokalemia include bulimia, laxative abuse, and diuretic misuse. Low urinary potassium (<15 mmol/day) and/or low urinary chloride (<20 mol/L) suggest a gastrointestinal pathology. Co-morbidity of hypokalemia with chronic pulmonary and cardiovascular diseases may increase the fatality rate.

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.

The role of AMPK in regulation of Na+,K+-ATPase in skeletal muscle: does the gauge always plug the sink?

AMP-activated protein kinase (AMPK) is a cellular energy gauge and a major regulator of cellular energy homeostasis. Once activated, AMPK stimulates nutrient uptake and the ATP-producing catabolic pathways, while it suppresses the ATP-consuming anabolic pathways, thus helping to maintain the cellular energy balance under energy-deprived conditions. As much as ~ 20-25% of the whole-body ATP consumption occurs due to a reaction catalysed by Na⁺,K⁺-ATPase (NKA). Being the single most important sink of energy, NKA might seem to be an essential target of the AMPK-mediated energy saving measures, yet NKA is vital for maintenance of transmembrane Na⁺ and K⁺ gradients, water homeostasis, cellular excitability, and the Na⁺-coupled transport of nutrients and ions. Consistent with the model that AMPK regulates ATP consumption by NKA, activation of AMPK in the lung alveolar cells stimulates endocytosis of NKA, thus suppressing the transepithelial ion transport and the absorption of the alveolar fluid. In skeletal muscles, contractions activate NKA, which opposes a rundown of transmembrane ion gradients, as well as AMPK, which plays an important role in adaptations to exercise. Inhibition of NKA in contracting skeletal muscle accentuates perturbations in ion concentrations and accelerates development of fatigue. However, different models suggest that AMPK does not inhibit or even stimulates NKA in skeletal muscle, which appears to contradict the idea that AMPK maintains the cellular energy balance by always suppressing ATP-consuming processes. In this short review, we examine the role of AMPK in regulation of NKA in skeletal muscle and discuss the apparent paradox of AMPK-stimulated ATP consumption.

Energy Provisioning and Inflammasome Activation: The Pivotal Role of AMPK in Sterile Inflammation and Associated Metabolic Disorders

BACKGROUND

Body defenses and metabolic processes probably co-evolved in such a way that rapid, energy-intensive acute inflammatory repair is functionally integrated with energy allocation in a starvation/ infection / injury-prone primitive environment. Disruptive metabolic surplus, aggravated by sedentary lifestyle induces chronic under-activation of AMPK, the master regulator of intracellular energy homeostasis. Sudden increase in chronic, dysregulated 'sterile' inflammatory disorders probably results from a shift towards calorie rich, sanitized, cushioned, injury/ infection free environment, repositioning inflammatory repair pathways towards chronic, non-microbial, 'sterile', 'low grade', and 'parainflammation'. AMPK, (at the helm of energy provisioning) supervises the metabolic regulation of inflammasome activation, a common denominator in lifestyle disorders.

DISCUSSION

In this review, we discuss various pathways linking AMPK under-activation and inflammasome activation. AMPK under-activation, the possible norm in energy-rich sedentary lifestyle, could be the central agency that stimulates inflammasome activation by multiple pathways such as 1: decreasing autophagy, and accumulation of intracellular DAMPs, (particulate crystalline molecules, advanced glycation end-products, oxidized lipids, etc.) 2: stimulating a glycolytic shift (pro-inflammatory) in metabolism, 3: promoting NF-kB activation and decreasing Nrf2 activation, 4: increasing reactive oxygen species (ROS) formation, Unfolded Protein Response (UPR) and Endoplasmic Reticulum (ER) stress.

CONCLUSION

The 'inverse energy crisis' associated with calorie-rich, sedentary lifestyle, advocates dietary and pharmacological interventions for treating chronic metabolic disorders by overcoming / reversing AMPK under-activation.

Negative Regulation of TRPA1 by AMPK in Primary Sensory Neurons as a Potential Mechanism of Painful Diabetic Neuropathy

AMPK is a widely expressed intracellular energy sensor that monitors and modulates energy expenditure. Transient receptor potential ankyrin 1 (TRPA1) channel is a widely recognized chemical and thermal sensor that plays vital roles in pain transduction. In this study, we discovered a functional link between AMPK and TRPA1 in dorsal root ganglion (DRG) neurons, in which AMPK activation rapidly resulted in downregulation of membrane-associated TRPA1 and its channel activity within minutes. Treatment with two AMPK activators, metformin or AICAR, inhibited TRPA1 activity in DRG neurons by decreasing the amount of membrane-associated TRPA1. Metformin induced a dose-dependent inhibition of TRPA1-mediated calcium influx. Conversely, in diabetic db/db mice, AMPK activity was impaired in DRG neurons, and this was associated with a concomitant increase in membrane-associated TRPA1 and mechanical allodynia. Notably, these molecular and behavioral changes were normalized following treatment with AMPK activators. Moreover, high-glucose exposure decreased activated AMPK levels and increased agonist-evoked TRPA1 currents in cultured DRG neurons, and these effects were prevented by treatment with AMPK activators. Our results identify AMPK as a previously unknown regulator of TRPA1 channels. AMPK modulation of TRPA1 could thus serve as an underlying mechanism and potential therapeutic molecular target in painful diabetic neuropathy.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Quantification of Na+,K+ pumps and their transport rate in skeletal muscle: functional significance

During excitation, muscle cells gain Na⁺ and lose K⁺, leading to a rise in extracellular K⁺ ([K⁺]_o), depolarization, and loss of excitability. Recent studies support the idea that these events are important causes of muscle fatigue and that full use of the Na⁺,K⁺-ATPase (also known as the Na⁺,K⁺ pump) is often essential for adequate clearance of extracellular K⁺. As a result of their electrogenic action, Na⁺,K⁺ pumps also help reverse depolarization arising during excitation, hyperkalemia, and anoxia, or from cell damage resulting from exercise, rhabdomyolysis, or muscle diseases. The ability to evaluate Na⁺,K⁺-pump function and the capacity of the Na⁺,K⁺ pumps to fill these needs require quantification of the total content of Na⁺,K⁺ pumps in skeletal muscle. Inhibition of Na⁺,K⁺-pump activity, or a decrease in their content, reduces muscle contractility. Conversely, stimulation of the Na⁺,K⁺-pump transport rate or increasing the content of Na⁺,K⁺ pumps enhances muscle excitability and contractility. Measurements of [³H]ouabain binding to skeletal muscle in vivo or in vitro have enabled the reproducible quantification of the total content of Na⁺,K⁺ pumps in molar units in various animal species, and in both healthy people and individuals with various diseases. In contrast, measurements of 3-O-methylfluorescein phosphatase activity associated with the Na⁺,K⁺-ATPase may show inconsistent results. Measurements of Na⁺ and K⁺ fluxes in intact isolated muscles show that, after Na⁺ loading or intense excitation, all the Na⁺,K⁺ pumps are functional, allowing calculation of the maximum Na⁺,K⁺-pumping capacity, expressed in molar units/g muscle/min. The activity and content of Na⁺,K⁺ pumps are regulated by exercise, inactivity, K⁺ deficiency, fasting, age, and several hormones and pharmaceuticals. Studies on the α-subunit isoforms of the Na⁺,K⁺-ATPase have detected a relative increase in their number in response to exercise and the glucocorticoid dexamethasone but have not involved their quantification in molar units. Determination of ATPase activity in homogenates and plasma membranes obtained from muscle has shown ouabain-suppressible stimulatory effects of Na⁺ and K⁺.

Unaccustomed eccentric contractions impair plasma K+ regulation in the absence of changes in muscle Na+,K+-ATPase content

The Na⁺,K⁺-ATPase (NKA) plays a fundamental role in the regulation of skeletal muscle membrane Na⁺ and K⁺ gradients, excitability and fatigue during repeated intense contractions. Many studies have investigated the effects of acute concentric exercise on K⁺ regulation and skeletal muscle NKA, but almost nothing is known about the effects of repeated eccentric contractions. We therefore investigated the effects of unaccustomed maximal eccentric knee extensor contractions on K⁺ regulation during exercise, peak knee extensor muscle torque, and vastus lateralis muscle NKA content and 3-O-MFPase activity. Torque measurements, muscle biopsies, and venous blood samples were taken before, during and up to 7 days following the contractions in six healthy adults. Eccentric contractions reduced peak isometric muscle torque immediately post-exercise by 26±11% and serum creatine kinase concentration peaked 24 h post-exercise at 339±90 IU/L. During eccentric contractions, plasma [K⁺] rose during Set 1 and remained elevated at ∼4.9 mM during sets 4–10; this was despite a decline in work output by Set 4, which fell by 18.9% at set 10. The rise in plasma [K⁺].work⁻¹ ratio was elevated over Set 2 from Set 4– Set 10. Eccentric contractions had no effect on muscle NKA content or maximal in-vitro 3-O-MFPase activity immediately post- or up to 7 d post-exercise. The sustained elevation in plasma [K⁺] despite a decrease in work performed by the knee extensor muscles suggests an impairment in K⁺ regulation during maximal eccentric contractions, possibly due to increased plasma membrane permeability or to excitation-contraction uncoupling.

AICAR inhibits oxygen consumption by intact skeletal muscle cells in culture

Activation of 5' adenosine monophosphate-activated protein kinase (AMPK) with aminoimidazole carboxamide ribonucleotide (AICAR) increases skeletal muscle glucose uptake and fatty acid oxidation. The purpose of these experiments was to utilize AICAR to enhance palmitate consumption by mitochondria in cultured skeletal muscle cells. In these experiments, we treated C2C12 myotubes or adult single skeletal muscle fibers with varying concentrations of AICAR for different lengths of time. Surprisingly, acute AICAR exposure at most concentrations (0.25-1.5 mM), but not all (0.1 mM), modestly inhibited oxygen consumption even though AICAR increased AMPK phosphorylation. The data suggest that AICAR inhibited oxygen consumption by the cultured muscle in a non-specific manner. The results of these experiments are expected to provide valuable information to investigators interested in using AICAR in cell culture studies.

Activation of AMP-activated protein kinase stimulates Na+,K+-ATPase activity in skeletal muscle cells

Contraction stimulates Na(+),K(+)-ATPase and AMP-activated protein kinase (AMPK) activity in skeletal muscle. Whether AMPK activation affects Na(+),K(+)-ATPase activity in skeletal muscle remains to be determined. Short term stimulation of rat L6 myotubes with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), activates AMPK and promotes translocation of the Na(+),K(+)-ATPase α(1)-subunit to the plasma membrane and increases Na(+),K(+)-ATPase activity as assessed by ouabain-sensitive (86)Rb(+) uptake. Cyanide-induced artificial anoxia, as well as a direct AMPK activator (A-769662) also increase AMPK phosphorylation and Na(+),K(+)-ATPase activity. Thus, different stimuli that target AMPK concomitantly increase Na(+),K(+)-ATPase activity. The effect of AICAR on Na(+),K(+)-ATPase in L6 myotubes was attenuated by Compound C, an AMPK inhibitor, as well as siRNA-mediated AMPK silencing. The effects of AICAR on Na(+),K(+)-ATPase were completely abolished in cultured primary mouse muscle cells lacking AMPK α-subunits. AMPK stimulation leads to Na(+),K(+)-ATPase α(1)-subunit dephosphorylation at Ser(18), which may prevent endocytosis of the sodium pump. AICAR stimulation leads to methylation and dephosphorylation of the catalytic subunit of the protein phosphatase (PP) 2A in L6 myotubes. Moreover, AICAR-triggered dephosphorylation of the Na(+),K(+)-ATPase was prevented in L6 myotubes deficient in PP2A-specific protein phosphatase methylesterase-1 (PME-1), indicating a role for the PP2A·PME-1 complex in AMPK-mediated regulation of Na(+),K(+)-ATPase. Thus contrary to the common paradigm, we report AMPK-dependent activation of an energy-consuming ion pumping process. This activation may be a potential mechanism by which exercise and metabolic stress activate the sodium pump in skeletal muscle.

Na,K-ATPase activity in mouse muscle is regulated by AMPK and PGC-1α

Na,K-ATPase activity, which is crucial for skeletal muscle function, undergoes acute and long-term regulation in response to muscle activity. The aim of the present study was to test the hypothesis that AMP kinase (AMPK) and the transcriptional coactivator PGC-1α are underlying factors in long-term regulation of Na,K-ATPase isoform (α,β and PLM) abundance and Na(+) affinity. Repeated treatment of mice with the AMPK activator AICAR decreased total PLM protein content but increased PLM phosphorylation, whereas the number of α- and β-subunits remained unchanged. The K(m) for Na(+) stimulation of Na,K-ATPase was reduced (higher affinity) after AICAR treatment. PLM abundance was increased in AMPK kinase-dead mice compared with control mice, but PLM phosphorylation and Na,K-ATPase Na(+) affinity remained unchanged. Na,K-ATPase activity and subunit distribution were also measured in mice with different degrees of PGC-1α expression. Protein abundances of α1 and α2 were reduced in PGC-1α +/- and -/- mice, and the β(1)/β(2) ratio was increased with PGC-1α overexpression (TG mice). PLM protein abundance was decreased in TG mice, but phosphorylation status was unchanged. Na,K-ATPase V (max) was decreased in PCG-1α TG and KO mice. Experimentally in vitro induced phosphorylation of PLM increased Na,K-ATPase Na(+) affinity, confirming that PLM phosphorylation is important for Na,K-ATPase function. In conclusion, both AMPK and PGC-1α regulate PLM abundance, AMPK regulates PLM phosphorylation and PGC-1α expression influences Na,K-ATPase α(1) and α(2) content and β(1)/β(2) isoform ratio. Phosphorylation of the Na,K-ATPase subunit PLM is an important regulatory mechanism.

Involvement of a phosphorylation-mediated pathway to regulate the function of NSPL1 in exercise

Skeletal-type neuroendocrine-specific protein like 1 (sk-NSPL1) has been demonstrated to be physiologically important in regulating the membrane translocation of glucose transporter 4 (GLUT4) in skeletal muscles. We investigated the levels of phosphorylation in proteins that are thought to be involved in exercise in wild-type and sk-NSPL1-deficient muscles with specific antibodies and phosphate-metal affinity chromatography resin (p-resin). In both normal skeletal muscle and sk-NSPL1-deficient muscle, adenosine monophosphate (AMP)-dependent kinase (AMPK) and acetyl-CoA carboxylase (ACC) were phosphorylated and adsorbed onto p-resin at high levels after exercise. On the other hand, the effect of 5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR), which is an activator of AMPK, in blood glucose was greatly diminished in mutant mice. P-resin adsorbed sk-NSPL1 in the membrane fraction from wild-type muscle after exercise and AICAR administration. Isolated sk-NSPL1 from wild-type also had increased adsorption onto p-resin after treatment with Ca(2+) and adenosine triphosphate (ATP). After long-term incubation of sk-NSPL1-containing membrane without ATP, sk-NSPL1 adsorption onto anion-exchange resin was drastically reduced. These results suggest that the function of sk-NSPL1 is regulated by a [Ca(2+)](i)- and AMPK-mediated pathway under exercise, and support the hypothesis that sk-NSPL1 is an important factor in the downstream of the exercise-dependent pathway in GLUT4 translocation.

Abnormalities of serum potassium concentration in dialysis-associated hyperglycemia and their correction with insulin: a unique clinical/physiologic exercise in internal potassium balance

The absence of significant losses of potassium in the urine makes dialysis-associated hyperglycemia (DH) a model for the study of the internal potassium balance. Studies of DH have revealed that hyperkalemia is frequent at presentation, insulin infusion is usually the only treatment required, and the magnitude of the decrease in serum potassium concentration (K(+)) during treatment of DH with insulin depends on the starting serum K(+) level, the decreases in serum glucose concentration and tonicity, and the increase in serum total carbon dioxide level. We present an analysis of these findings based on previously studied actions of insulin. Calculations of transcellular potassium shifts based on the combined effects of insulin-the increase in the electrical potential differences (hyperpolarization) of the cell membranes and the correction of the hyperglycemic intracellular dehydration through decrease in serum glucose concentration-produced quantitative predictions of the decrease in serum K(+) similar to the reported changes in serum K(+) during treatment of DH with insulin. The lessons from analyzing serum K(+) changes during treatment of DH with insulin are applicable to other conditions where internal potassium balance is called upon to protect serum K(+), such as the postprandial state. The main questions related to internal potassium balance in DH that await clarification include the structure and function of cell membrane potassium channels, the effect of insulin on these channels, and the mechanisms of feedforward potassium regulation.

Na+,K+-ATPase Na+ affinity in rat skeletal muscle fiber types

Previous studies in expression systems have found different ion activation of the Na(+)/K(+)-ATPase isozymes, which suggest that different muscles have different ion affinities. The rate of ATP hydrolysis was used to quantify Na(+),K(+)-ATPase activity, and the Na(+) affinity of Na(+),K(+)-ATPase was studied in total membranes from rat muscle and purified membranes from muscle with different fiber types. The Na(+) affinity was higher (K(m) lower) in oxidative muscle compared with glycolytic muscle and in purified membranes from oxidative muscle compared with glycolytic muscle. Na(+),K(+)-ATPase isoform analysis implied that heterodimers containing the beta(1) isoform have a higher Na(+) affinity than heterodimers containing the beta(2) isoform. Immunoprecipitation experiments demonstrated that dimers with alpha(1) are responsible for approximately 36% of the total Na,K-ATPase activity. Selective inhibition of the alpha(2) isoform with ouabain suggested that heterodimers containing the alpha(1) isoform have a higher Na(+) affinity than heterodimers containing the alpha(2) isoform. The estimated K(m) values for Na(+) are 4.0, 5.5, 7.5 and 13 mM for alpha(1)beta(1), alpha(2)beta(1), alpha(1)beta(2) and alpha(2)beta(2), respectively. The affinity differences and isoform distributions imply that the degree of activation of Na(+),K(+)-ATPase at physiological Na(+) concentrations differs between muscles (oxidative and glycolytic) and between subcellular membrane domains with different isoform compositions. These differences may have consequences for ion balance across the muscle membrane.

AMP-activated protein kinase activator A-769662 is an inhibitor of the Na(+)-K(+)-ATPase

Muscle contraction and metabolic stress are potent activators of AMP-activated protein kinase (AMPK). AMPK restores energy balance by activating processes that produce energy while inhibiting those that consume energy. The role of AMPK in the regulation of active ion transport is unclear. Our aim was to determine the effect of the AMPK activator A-769662 on Na(+)-K(+)-ATPase function in skeletal muscle cells. Short-term incubation of differentiated rat L6 myotubes with 100 microM A-769662 increased AMPK and acetyl-CoA carboxylase (ACC) phosphorylation in parallel with decreased Na(+)-K(+)-ATPase alpha(1)-subunit abundance at the plasma membrane and ouabain-sensitive (86)Rb(+) uptake. Notably, the effect of A-769662 on Na(+)-K(+)-ATPase was similar in muscle cells that do not express AMPK alpha(1)- and alpha(2)-catalytic subunits. A-769662 directly inhibits the alpha(1)-isoform of the Na(+)-K(+)-ATPase, purified from rat and human kidney cells in vitro with IC(50) 57 microM and 220 microM, respectively. Inhibition of the Na(+)-K(+)-ATPase by 100 microM ouabain decreases sodium pump activity and cell surface abundance, similar to the effect of A-769662, without affecting AMPK and ACC phosphorylation. In conclusion, the AMPK activator A-769662 inhibits Na(+)-K(+)-ATPase activity and decreases the sodium pump cell surface abundance in L6 skeletal muscle cells. The effect of A-769662 on sodium pump is due to direct inhibition of the Na(+)-K(+)-ATPase activity, rather than AMPK activation. This AMPK-independent effect on Na(+)-K(+)-ATPase calls into question the use of A-769662 as a specific AMPK activator for metabolic studies.

Recent advances in understanding integrative control of potassium homeostasis

The potassium homeostatic system is very tightly regulated. Recent studies have shed light on the sensing and molecular mechanisms responsible for this tight control. In addition to classic feedback regulation mediated by a rise in extracellular fluid (ECF) [K(+)], there is evidence for a feedforward mechanism: Dietary K(+) intake is sensed in the gut, and an unidentified gut factor is activated to stimulate renal K(+) excretion. This pathway may explain renal and extrarenal responses to altered K(+) intake that occur independently of changes in ECF [K(+)]. Mechanisms for conserving ECF K(+) during fasting or K(+) deprivation have been described: Kidney NADPH oxidase activation initiates a cascade that provokes the retraction of K(+) channels from the cell membrane, and muscle becomes resistant to insulin stimulation of cellular K(+) uptake. How these mechanisms are triggered by K(+) deprivation remains unclear. Cellular AMP kinase-dependent protein kinase activity provokes the acute transfer of K(+) from the ECF to the ICF, which may be important in exercise or ischemia. These recent advances may shed light on the beneficial effects of a high-K(+) diet for the cardiovascular system.

Narrative review: evolving concepts in potassium homeostasis and hypokalemia

Humans are intermittently exposed to large variations in potassium intake, which range from periods of fasting to ingestion of potassium-rich meals. These fluctuations would abruptly alter plasma potassium concentration if not for rapid mechanisms, primarily in skeletal muscle and the liver, that buffer the changes in plasma potassium concentration by means of transcellular potassium redistribution and feedback control of renal potassium excretion. However, buffers have capacity limits, and even robust feedback control mechanisms require that the perturbation occur before feedback can initiate corrective action. In contrast, feedforward control mechanisms sense the effect of disturbances on the system's homeostasis. This review highlights recent experimental insights into the participation of feedback and feedforward control mechanisms in potassium homeostasis. New data make clear that feedforward homeostatic responses activate when decreased potassium intake is sensed, even when plasma potassium concentration is still within the normal range and before frank hypokalemia ensues, in addition to the classic feedback activation of renal potassium conservation when plasma potassium concentration decreases. Given the clinical importance of dyskalemias in patients, these novel experimental paradigms invite renewed clinical inquiry into this important area.

IAP-targeted therapies for cancer

DNA damage, chromosomal abnormalities, oncogene activation, viral infection, substrate detachment and hypoxia can all trigger apoptosis in normal cells. However, cancer cells acquire mutations that allow them to survive these threats that are part and parcel of the transformation process or that may affect the growth and dissemination of the tumor. Eventually, cancer cells accumulate further mutations that make them resistant to apoptosis mediated by standard cytotoxic chemotherapy or radiotherapy. The inhibitor of apoptosis (IAP) family members, defined by the presence of a baculovirus IAP repeat (BIR) protein domain, are key regulators of cytokinesis, apoptosis and signal transduction. Specific IAPs regulate either cell division, caspase activity or survival pathways mediated through binding to their BIR domains, and/or through their ubiquitin-ligase RING domain activity. These protein-protein interactions and post-translational modifications are the subject of intense investigations that shed light on how these proteins contribute to oncogenesis and resistance to therapy. In the past several years, we have seen multiple approaches of IAP antagonism enter the clinic, and the rewards of such strategies are about to reap benefit. Significantly, small molecule pan-IAP antagonists that mimic an endogenous inhibitor of the IAPs, called Smac, have demonstrated an unexpected ability to sensitize cancer cells to tumor necrosis factor-alpha and to promote autocrine or paracrine production of this cytokine by the tumor cell and possibly, other cells too. This review will focus on these and other developmental therapeutics that target the IAPs in cancer.

Frontiers: skeletal muscle sodium pump regulation: a translocation paradigm

The skeletal muscle sodium pump plays a major role in the removal of K(+) ions from the circulation postprandial, or after a physical activity bout, thereby preventing the development of hyperkalemia and fatigue. Insulin and muscle contractions stimulate Na(+)-K(+)-ATPase activity in skeletal muscle, at least partially via translocation of sodium pump units to the plasma membrane from intracellular stores. The molecular mechanism of this phenomenon is poorly understood. Due to the contradictory reports in the literature, the very existence of the translocation of Na(+)-K(+)-ATPase to the skeletal muscle cell surface is questionable. This review summarizes more than 30 years work on the skeletal muscle sodium pump translocation paradigm. Furthermore, the methodological caveats of major approaches to study the sodium pump translocation in skeletal muscle are discussed. An understanding of the molecular regulation of Na(+)-K(+)-ATPase in skeletal muscle will have important clinical implications for the understanding of the development of complications associated with the metabolic syndrome, such as cardiovascular diseases or increased muscle fatigue in diabetic patients.