Membrane proteins involved in potassium shifts during muscle activity and fatigue

The Impact of Acetazolamide and Methazolamide on Exercise Performance in Normoxia and Hypoxia

Doherty, Connor J., Jou-Chung Chang, Benjamin P. Thompson, Erik R. Swenson, Glen E. Foster, and Paolo B. Dominelli. The impact of acetazolamide and methazolamide on exercise performance in normoxia and hypoxia. High Alt Med Biol. 24:7-18, 2023.-Carbonic anhydrase (CA) inhibitors are commonly prescribed for acute mountain sickness (AMS). In this review, we sought to examine how two CA inhibitors, acetazolamide (AZ) and methazolamide (MZ), affect exercise performance in normoxia and hypoxia. First, we briefly describe the role of CA inhibition in facilitating the increase in ventilation and arterial oxygenation in preventing and treating AMS. Next, we detail how AZ affects exercise performance in normoxia and hypoxia and this is followed by a discussion on MZ. We emphasize that the overarching focus of the review is how the two drugs potentially affect exercise performance, rather than their ability to prevent/treat AMS per se, their interrelationship will be discussed. Overall, we suggest that AZ hinders exercise performance in normoxia, but may be beneficial in hypoxia. Based upon head-to-head studies of AZ and MZ in humans on diaphragmatic and locomotor strength in normoxia, MZ may be a better CA inhibitor when exercise performance is crucial at high altitude.

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.

Paxilline Prevents the Onset of Myotonic Stiffness in Pharmacologically Induced Myotonia: A Preclinical Investigation

Reduced Cl⁻ conductance causes inhibited muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. This represents the pathomechanism of myotonia congenita. Due to the prevailing data suggesting that an increased potassium level is a main contributor, we studied the effect of a modulator of a big conductance Ca²⁺- and voltage-activated K⁺ channels (BK) modulator on contraction and relaxation of slow- and high-twitch muscle specimen before and after the pharmacological induction of myotonia. Human and murine muscle specimens (wild-type and BK^−/−) were exposed to anthracene-9-carboxylic acid (9-AC) to inhibit CLC-1 chloride channels and to induce myotonia in-vitro. Functional effects of BK-channel activation and blockade were investigated by exposing slow-twitch (soleus) and fast-twitch (extensor digitorum longus) murine muscle specimens or human musculus vastus lateralis to an activator (NS1608) and a blocker (Paxilline), respectively. Muscle-twitch force and relaxation times (T_90/10) were monitored. Compared to wild type, fast-twitch muscle specimen of BK^−/− mice resulted in a significantly decreased T_90/10 in presence of 9-AC. Paxilline significantly shortened T_90/10 of murine slow- and fast-twitch muscles as well as human vastus lateralis muscle. Moreover, twitch force was significantly reduced after application of Paxilline in myotonic muscle. NS1608 had opposite effects to Paxilline and aggravated the onset of myotonic activity by prolongation of T_90/10. The currently used standard therapy for myotonia is, in some individuals, not very effective. This in vitro study demonstrated that a BK channel blocker lowers myotonic stiffness and thus highlights its potential therapeutic option in myotonia congenital (MC).

Coordinate adaptations of skeletal muscle and kidney to maintain extracellular [K+] during K+-deficient diet

Extracellular fluid (ECF) potassium concentration ([K+]) is maintained by adaptations of kidney and skeletal muscle, responses heretofore studied separately. We aimed to determine how these organ systems work in concert to preserve ECF [K+] in male C57BL/6J mice fed a K+-deficient diet (0K) versus 1% K+ diet (1K) for 10 days (n = 5-6/group). During 0K feeding, plasma [K+] fell from 4.5 to 2 mM; hindlimb muscle (gastrocnemius and soleus) lost 28 mM K+ (from 115 ± 2 to 87 ± 2 mM) and gained 27 mM Na+ (from 27 ± 0.4 to 54 ± 2 mM). Doubling of muscle tissue [Na+] was not associated with inflammation, cytokine production or hypertension as reported by others. Muscle transporter adaptations in 0K- versus 1K-fed mice, assessed by immunoblot, included decreased sodium pump α2-β2 subunits, decreased K+-Cl- cotransporter isoform 3, and increased phosphorylated (p) Na+,K+,2Cl- cotransporter isoform 1 (NKCC1p), Ste20/SPS-1-related proline-alanine rich kinase (SPAKp), and oxidative stress-responsive kinase 1 (OSR1p) consistent with intracellular fluid (ICF) K+ loss and Na+ gain. Renal transporters' adaptations, effecting a 98% reduction in K+ excretion, included two- to threefold increased phosphorylated Na+-Cl- cotransporter (NCCp), SPAKp, and OSR1p abundance, limiting Na+ delivery to epithelial Na+ channels where Na+ reabsorption drives K+ secretion; and renal K sensor Kir 4.1 abundance fell 25%. Mass balance estimations indicate that over 10 days of 0K feeding, mice lose ~48 μmol K+ into the urine and muscle shifts ~47 μmol K+ from ICF to ECF, illustrating the importance of the concerted responses during K+ deficiency.

Elevation of extracellular osmolarity improves signs of myotonia congenita in vitro: a preclinical animal study

KEY POINTS

During myotonia congenita, reduced chloride (Cl- ) conductance results in impaired muscle relaxation and increased muscle stiffness after forceful voluntary contraction. Repetitive contraction of myotonic muscle decreases or even abolishes myotonic muscle stiffness, a phenomenon called 'warm up'. Pharmacological inhibition of low Cl- channels by anthracene-9-carboxylic acid in muscle from mice and ADR ('arrested development of righting response') muscle from mice showed a relaxation deficit under physiological conditions compared to wild-type muscle. At increased osmolarity up to 400 mosmol L-1 , the relaxation deficit of myotonic muscle almost reached that of control muscle. These effects were mediated by the cation and anion cotransporter, NKCC1, and anti-myotonic effects of hypertonicity were at least partly antagonized by the application of bumetanide.

ABSTRACT

Low chloride-conductance myotonia is caused by mutations in the skeletal muscle chloride (Cl- ) channel gene type 1 (CLCN1). Reduced Cl- conductance of the mutated channels results in impaired muscle relaxation and increased muscle stiffness after forceful voluntary contraction. Exercise decreases muscle stiffness, a phenomena called 'warm up'. To gain further insight into the patho-mechanism of impaired muscle stiffness and the warm-up phenomenon, we characterized the effects of increased osmolarity on myotonic function. Functional force and membrane potential measurements were performed on muscle specimens of ADR ('arrested development of righting response') mice (an animal model for low gCl- conductance myotonia) and pharmacologically-induced myotonia. Specimens were exposed to solutions of increasing osmolarity at the same time as force and membrane potentials were monitored. In the second set of experiments, ADR muscle and pharmacologically-induced myotonic muscle were exposed to an antagonist of NKCC1. Upon osmotic stress, ADR muscle was depolarized to a lesser extent than control wild-type muscle. High osmolarity diminished myotonia and facilitated the warm-up phenomenon as depicted by a faster muscle relaxation time (T90/10 ). Osmotic stress primarily resulted in the activation of the NKCC1. The inhibition of NKCC1 with bumetanide prevented the depolarization and reversed the anti-myotonic effect of high osmolarity. Increased osmolarity decreased signs of myotonia and facilitated the warm-up phenomenon in different in vitro models of myotonia. Activation of NKCC1 activity promotes warm-up and reduces the number of contractions required to achieve normal relaxation kinetics.

Effect of acetazolamide and methazolamide on diaphragm and dorsiflexor fatigue: a randomized controlled trial

Acetazolamide, a carbonic anhydrase (CA) inhibitor used clinically and to prevent acute mountain sickness, worsens skeletal muscle fatigue in animals and humans. In animals, methazolamide, a methylated analog of acetazolamide and an equally potent CA inhibitor, reportedly exacerbates fatigue less than acetazolamide. Accordingly, we sought to determine, in humans, if methazolamide would attenuate diaphragm and dorsiflexor fatigue compared with acetazolamide. Healthy men (dorsiflexor: n = 12; diaphragm: n = 7) performed fatiguing exercise on three occasions, after ingesting acetazolamide (250 mg three times a day) and then in random order, methazolamide (100 mg twice a day) or placebo for 48 h. For both muscles, subjects exercised at a fixed intensity until exhaustion on acetazolamide, with subsequent iso-time and -workload trials. Diaphragm exercise was performed using a threshold-loading device, while dorsiflexor exercise was isometric. Neuromuscular function was determined pre- and postexercise by potentiated transdiaphragmatic twitch pressure and dorsiflexor torque in response to stimulation of the phrenic and fibular nerve, respectively. Diaphragm contractility 3-10 min postexercise was impaired more for acetazolamide than methazolamide or placebo (82 ± 10, 87 ± 9, and 91 ± 8% of pre-exercise value; P < 0.05). Similarly, dorsiflexor fatigue was greater for acetazolamide than methazolamide (mean twitch torque of 61 ± 11 vs. 57 ± 13% of baseline, P < 0.05). In normoxia, methazolamide leads to less neuromuscular fatigue than acetazolamide, indicating a possible benefit for clinical use or in the prophylaxis of acute mountain sickness. NEW & NOTEWORTHY Acetazolamide, a carbonic anhydrase inhibitor, may worsen diaphragm and locomotor muscle fatigue after exercise; whereas, in animals, methazolamide does not impair diaphragm function. Compared with both methazolamide and the placebo, acetazolamide significantly compromised dorsiflexor function at rest and after exhaustive exercise. Similarly, diaphragm function was most compromised on acetazolamide followed by methazolamide and placebo. Methazolamide may be preferable over acetazolamide for clinical use and altitude illness prophylaxis to avoid skeletal muscle dysfunction.

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

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

Limitations in intense exercise performance of athletes - effect of speed endurance training on ion handling and fatigue development

Mechanisms underlying fatigue development and limitations for performance during intense exercise have been intensively studied during the past couple of decades. Fatigue development may involve several interacting factors and depends on type of exercise undertaken and training level of the individual. Intense exercise (½-6 min) causes major ionic perturbations (Ca²⁺ , Cl^- , H⁺ , K⁺ , lactate^- and Na⁺ ) that may reduce sarcolemmal excitability, Ca²⁺ release and force production of skeletal muscle. Maintenance of ion homeostasis is thus essential to sustain force production and power output during intense exercise. Regular speed endurance training (SET), i.e. exercise performed at intensities above that corresponding to maximum oxygen consumption (V̇O2, max ), enhances intense exercise performance. However, most of the studies that have provided mechanistic insight into the beneficial effects of SET have been conducted in untrained and recreationally active individuals, making extrapolation towards athletes' performance difficult. Nevertheless, recent studies indicate that only a few weeks of SET enhances intense exercise performance in highly trained individuals. In these studies, the enhanced performance was not associated with changes in V̇O2, max and muscle oxidative capacity, but rather with adaptations in muscle ion handling, including lowered interstitial concentrations of K⁺ during and in recovery from intense exercise, improved lactate^- -H⁺ transport and H⁺ regulation, and enhanced Ca²⁺ release function. The purpose of this Topical Review is to provide an overview of the effect of SET and to discuss potential mechanisms underlying enhancements in performance induced by SET in already well-trained individuals with special emphasis on ion handling in skeletal muscle.

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

Association Between KCNJ11 Gene E23K Polymorphism and Body Composition Together with its Response to Endurance Training

Objective:

To explore the Association between KCNJ11 gene E23K polymorphism of Chinese and body composition together with its response to endurance training. Method: 102 biologically unrelated Han nationality male new recruits from northern China volunteered to execute a 5000-m running programme, and the intensity is 95–105% individual lactate threshold. The protocol was lasted for 18 weeks, three times per week. The body composition index, including body weight (WT)、lean body weight (LBW), body mass index (BMI) and body fat percentage (Fat%), was measured before and after training. PCR-RFLP was used to detect the KCNJ11 gene E23K polymorphism.

Results:

Hardy-Weinberg equilibrium was observed for the frequency of genotypes in these subjects. Before training, WT, BMI and Fat% in KK group were significantly higher than those in EE and KK group (p<0.05 or p<0.01). There was no significant difference in LBW among groups (P>0.05). After training, the changes of all body composition index in KK group were bigger significantly greater than those in EE and EK groups (p<0.01).

Conclusion:

KCNJ11 gene E23K polymorphism might contribute to individual body composition together with its response to endurance training. The body fat content at baseline in KK was more than those in EE and EK groups, and it may hinder that individual to eliminate their body fat during endurance training.

Inward rectifier potassium currents in mammalian skeletal muscle fibres

Inward rectifying potassium (Kir) channels play a central role in maintaining the resting membrane potential of skeletal muscle fibres. Nevertheless their role has been poorly studied in mammalian muscles. Immunohistochemical and transgenic expression were used to assess the molecular identity and subcellular localization of Kir channel isoforms. We found that Kir2.1 and Kir2.2 channels were targeted to both the surface and the transverse tubular system membrane (TTS) compartments and that both isoforms can be overexpressed up to 3-fold 2 weeks after transfection. Inward rectifying currents (IKir) had the canonical features of quasi-instantaneous activation, strong inward rectification, depended on the external [K(+)], and could be blocked by Ba(2+) or Rb(+). In addition, IKir records show notable decays during large 100 ms hyperpolarizing pulses. Most of these properties were recapitulated by model simulations of the electrical properties of the muscle fibre as long as Kir channels were assumed to be present in the TTS. The model also simultaneously predicted the characteristics of membrane potential changes of the TTS, as reported optically by a fluorescent potentiometric dye. The activation of IKir by large hyperpolarizations resulted in significant attenuation of the optical signals with respect to the expectation for equal magnitude depolarizations; blocking IKir with Ba(2+) (or Rb(+)) eliminated this attenuation. The experimental data, including the kinetic properties of IKir and TTS voltage records, and the voltage dependence of peak IKir, while measured at widely dissimilar bulk [K(+)] (96 and 24 mm), were closely predicted by assuming Kir permeability (PKir) values of ∼5.5 × 10(-6 ) cm s(-1) and equal distribution of Kir channels at the surface and TTS membranes. The decay of IKir records and the simultaneous increase in TTS voltage changes were mostly explained by K(+) depletion from the TTS lumen. Most importantly, aside from allowing an accurate estimation of most of the properties of IKir in skeletal muscle fibres, the model demonstrates that a substantial proportion of IKir (>70%) arises from the TTS. Overall, our work emphasizes that measured intrinsic properties (inward rectification and external [K] dependence) and localization of Kir channels in the TTS membranes are ideally suited for re-capturing potassium ions from the TTS lumen during, and immediately after, repetitive stimulation under physiological conditions.

Extracellular Ca2+-induced force restoration in K+-depressed skeletal muscle of the mouse involves an elevation of [K+]i: implications for fatigue

We examined whether a Ca(2+)-K(+) interaction was a potential mechanism operating during fatigue with repeated tetani in isolated mouse muscles. Raising the extracellular Ca(2+) concentration ([Ca(2+)]o) from 1.3 to 10 mM in K(+)-depressed slow-twitch soleus and/or fast-twitch extensor digitorum longus muscles caused the following: 1) increase of intracellular K(+) activity by 20-60 mM (raised intracellular K(+) content, unchanged intracellular fluid volume), so that the K(+)-equilibrium potential increased by ∼10 mV and resting membrane potential repolarized by 5-10 mV; 2) large restoration of action potential amplitude (16-54 mV); 3) considerable recovery of excitable fibers (∼50% total); and 4) restoration of peak force with the peak tetanic force-extracellular K(+) concentration ([K(+)]o) relationship shifting rightward toward higher [K(+)]o. Double-sigmoid curve-fitting to fatigue profiles (125 Hz for 500 ms, every second for 100 s) showed that prior exposure to raised [K(+)]o (7 mM) increased, whereas lowered [K(+)]o (2 mM) decreased, the rate and extent of force loss during the late phase of fatigue (second sigmoid) in soleus, hence implying a K(+) dependence for late fatigue. Prior exposure to 10 mM [Ca(2+)]o slowed late fatigue in both muscle types, but was without effect on the extent of fatigue. These combined findings support our notion that a Ca(2+)-K(+) interaction is plausible during severe fatigue in both muscle types. We speculate that a diminished transsarcolemmal K(+) gradient and lowered [Ca(2+)]o contribute to late fatigue through reduced action potential amplitude and excitability. The raised [Ca(2+)]o-induced slowing of fatigue is likely to be mediated by a higher intracellular K(+) activity, which prolongs the time before stimulation-induced K(+) efflux depolarizes the sarcolemma sufficiently to interfere with action potentials.

Molecular cloning of ion channels in Felis catus that are related to periodic paralyses in man: a contribution to the understanding of the genetic susceptibility to feline neck ventroflexion and paralysis

Neck ventroflexion in cats has different causes; however, the most common is the hypokalemia associated with flaccid paralysis secondary to chronic renal failure. In humans, the most common causes of acute flaccid paralysis are hypokalemia precipitated by thyrotoxicosis and familial forms linked to mutations in sodium, potassium, and calcium channel genes. Here, we describe the sequencing and analysis of skeletal muscle ion channels in Felis catus that could be related to periodic paralyses in humans, contributing to the understanding of the genetic susceptibility to feline neck ventroflexion and paralysis. We studied genomic DNA from eleven cats, including five animals that were hyperthyroid with hypokalemia, although only one presented with muscle weakness, and six healthy control domestic cats. We identified the ion channel ortholog genes KCNJ2, KCNJ12, KCNJ14, CACNA1S and SCN4A in the Felis catus genome, together with several polymorphic variants. Upon comparative alignment with other genomes, we found that Felis catus provides evidence for a high genomic conservation of ion channel sequences. Although we hypothesized that neck ventroflexion in cats could be associated with a thyrotoxic or familial periodic paralysis channel mutation, we did not identify any previously detected human channel mutation in the hyperthyroid cat presenting hypokalemia. However, based on the small number of affected cats in this study, we cannot yet rule out this molecular mechanism. Notwithstanding, hyperthyroidism should still be considered as a differential diagnosis in hypokalemic feline paralysis.

Inward flux of lactate⁻ through monocarboxylate transporters contributes to regulatory volume increase in mouse muscle fibres

Mouse and rat skeletal muscles are capable of a regulatory volume increase (RVI) after they shrink (volume loss resultant from exposure to solutions of increased osmolarity) and that this RVI occurs mainly by a Na-K-Cl-Cotransporter (NKCC) - dependent mechanism. With high-intensity exercise, increased extracellular osmolarity is accompanied by large increases in extracellular [lactate^-]. We hypothesized that large increases in [lactate^-] and osmolarity augment the NKCC-dependent RVI response observed with a NaCl (or sucrose) - induced increase in osmolarity alone; a response that is dependent on lactate^- influx through monocarboxylate transporters (MCTs). Single mouse muscle fibres were isolated and visualized under light microscopy under varying osmolar conditions. When solution osmolarity was increased by adding NaLac by 30 or 60 mM, fibres lost significantly less volume and regained volume sooner compared to when NaCl was used. Phloretin (MCT1 inhibitor) accentuated the volume loss compared to both NaLac controls, supporting a role for MCT1 in the RVI response in the presence of elevated [lactate^-]. Inhibition of MCT4 (with pCMBS) resulted in a volume loss, intermediate to that seen with phloretin and NaLac controls. Bumetanide (NKCC inhibitor), in combination with pCMBS, reduced the magnitude of volume loss, but volume recovery was complete. While combined phloretin-bumetanide also reduced the magnitude of the volume loss, it also largely abolished the cell volume recovery. In conclusion, RVI in skeletal muscle exposed to raised tonicity and [lactate^-] is facilitated by inward flux of solute by NKCC- and MCT1-dependent mechanisms. This work demonstrates evidence of a RVI response in skeletal muscle that is facilitated by inward flux of solute by MCT-dependent mechanisms. These findings further expand our understanding of the capacities for skeletal muscle to volume regulate, particularly in instances of raised tonicity and lactate^- concentrations, as occurs with high intensity exercise.

Transduction of voltage and Ca2+ signals by Slo1 BK channels

Large-conductance Ca2+ -and voltage-gated K+ channels are activated by an increase in intracellular Ca2+ concentration and/or depolarization. The channel activation mechanism is well described by an allosteric model encompassing the gate, voltage sensors, and Ca2+ sensors, and the model is an excellent framework to understand the influences of auxiliary β and γ subunits and regulatory factors such as Mg2+. Recent advances permit elucidation of structural correlates of the biophysical mechanism.

Effect of intensified training on muscle ion kinetics, fatigue development, and repeated short-term performance in endurance-trained cyclists

The effects of intensified training in combination with a reduced training volume on muscle ion kinetics, transporters, and work capacity were examined. Eight well-trained cyclists replaced their regular training with speed-endurance training (12 × 30 s sprints) 2–3 times per week and aerobic high-intensity training (4–5 × 3–4 min at 90–100% of maximal heart rate) 1–2 times per week for 7 wk and reduced training volume by 70% (intervention period; IP). The duration of an intense exhaustive cycling bout (EX2; 368 ± 6 W), performed 2.5 min after a 2-min intense cycle bout (EX1), was longer ( P < 0.05) after than before IP (4:16 ± 0:34 vs. 3:37 ± 0:28 min:s), and mean and peak power during a repeated sprint test improved ( P < 0.05) by 4% and 3%, respectively. Femoral venous K+ concentration in recovery from EX1 and EX2 was lowered ( P < 0.05) after compared with before IP, whereas muscle interstitial K+ concentration and net muscle K+ release during exercise was unaltered. No changes in muscle lactate and H+ release during and after EX1 and EX2 were observed, but the in vivo buffer capacity was higher ( P < 0.05) after IP. Expression of the ATP-sensitive K+ (KATP) channel (Kir6.2) decreased by IP, with no change in the strong inward rectifying K+ channel (Kir2.1), muscle Na+-K+ pump subunits, monocarboxylate transporters 1 and 4 (MCT1 and MCT4), and Na+/H+ exchanger 1 (NHE1). In conclusion, 7 wk of intensified training with a reduced training volume improved performance during repeated intense exercise, which was associated with a greater muscle reuptake of K+ and muscle buffer capacity but not with the amount of muscle ion transporters.

Relationship between membrane Cl- conductance and contractile endurance in isolated rat muscles

Resting skeletal muscle fibres have a large membrane Cl(-) conductance (G(Cl)) that dampens their excitability. Recently, however, muscle activity was shown to induce PKC-mediated reduction in G(Cl) in rat muscles of 40-90%. To examine the physiological significance of this PKC-mediated G(Cl) reduction for the function of muscles, this study explored effects of G(Cl) reductions on contractile endurance in isolated rat muscles. Contractile endurance was assessed from the ability of muscle to maintain force during prolonged stimulation under conditions when G(Cl) was manipulated by: (i) inhibition of PKC, (ii) reduction of solution Cl(-) or (iii) inhibition of ClC-1 Cl(-) channels using 9-anthracene-carboxylic acid (9-AC). Experiments showed that contractile endurance was optimally preserved by reductions in G(Cl) similar to what occurs in active muscle. Contrastingly, further G(Cl) reductions compromised the endurance. The experiments thus show a biphasic relationship between G(Cl) and contractile endurance in which partial G(Cl) reduction improves endurance while further G(Cl) reduction compromises endurance. Intracellular recordings of trains of action potentials suggest that this biphasic dependency of contractile endurance on G(Cl) reflects that lowering G(Cl) enhances muscle excitability but low G(Cl) also increases the depolarisation of muscle fibres during excitation and reduces their ability to re-accumulate K(+) lost during excitation. If G(Cl) becomes very low, the latter actions dominate causing reduced endurance. It is concluded that the PKC-mediated ClC-1 channel inhibition in active muscle reduces G(Cl) to a level that optimises contractile endurance during intense exercise.

Two types of extracellular action potentials recorded with narrow-tipped pipettes in skeletal muscle of frog, Rana temporaria

Two types of muscle fibre action potentials (APs) were recorded using narrow-tipped extracellular pipettes in isolated sartorius muscles of frog, Rana temporaria. The waveform of type 1 responses (T1 AP, 75% of recordings) was biphasic, 'positive–negative.' The type 2 signals were tri-phasic, 'positive–negative–positive' (T2 AP, 18%). The type of AP was preserved for up to 1 h of collecting data from the given site on the muscle fibre. However, a re-positioning of the recording pipette by a few micrometres along the axis of the studied fibre resulted in a change of the type of AP in 73% of such attempts. In experiments with detubulated muscles, only T1 APs were observed. In experiments using 10 μmol l(−1) Ba2+ filled pipettes, the characteristics of the T1 waveform did not change, but the amplitude of the third phase of T2 APs increased progressively during continuous recording. In experiments with 100 μmol l(−1) Ba2+ filled pipettes, a decline and eventual inversion of the third phase of the T2 signal was observed. Amplitude and duration of the inverted third phase of T2 APs increased progressively with frequency of muscle stimulation. These results can be explained by suggesting that currents generated within the t-tubular system of muscle fibre form the third phase of extracellularly recorded APs. Measurement and analysis of T2 APs present a unique approach in determining the mechanisms controlling accumulation of t-tubular K+ and the role of these mechanisms in regulation of muscle contractility.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

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.

Kir2.6 regulates the surface expression of Kir2.x inward rectifier potassium channels

Precise trafficking, localization, and activity of inward rectifier potassium Kir2 channels are important for shaping the electrical response of skeletal muscle. However, how coordinated trafficking occurs to target sites remains unclear. Kir2 channels are tetrameric assemblies of Kir2.x subunits. By immunocytochemistry we show that endogenous Kir2.1 and Kir2.2 are localized at the plasma membrane and T-tubules in rodent skeletal muscle. Recently, a new subunit, Kir2.6, present in human skeletal muscle, was identified as a gene in which mutations confer susceptibility to thyrotoxic hypokalemic periodic paralysis. Here we characterize the trafficking and interaction of wild type Kir2.6 with other Kir2.x in COS-1 cells and skeletal muscle in vivo. Immunocytochemical and electrophysiological data demonstrate that Kir2.6 is largely retained in the endoplasmic reticulum, despite high sequence identity with Kir2.2 and conserved endoplasmic reticulum and Golgi trafficking motifs shared with Kir2.1 and Kir2.2. We identify amino acids responsible for the trafficking differences of Kir2.6. Significantly, we show that Kir2.6 subunits can coassemble with Kir2.1 and Kir2.2 in vitro and in vivo. Notably, this interaction limits the surface expression of both Kir2.1 and Kir2.2. We provide evidence that Kir2.6 functions as a dominant negative, in which incorporation of Kir2.6 as a subunit in a Kir2 channel heterotetramer reduces the abundance of Kir2 channels on the plasma membrane.

First order phase transition and hysteresis in a cell's maintenance of the membrane potential--An essential role for the inward potassium rectifiers

Hysteretic behavior is found experimentally in the transmembrane potential at low extracellular potassium in mouse lumbrical muscle cells. Adding isoprenaline to the external medium eliminates the bistable, hysteretic region. The system can be modeled mathematically and understood analytically with and without isoprenaline. Inward rectifying potassium channels appear to be essential for the bistability. Relations are derived to express the dimensions of the bistable area in terms of system parameters. The selective advantage and evolutionary origin of inward rectifying channels and hysteretic behavior is discussed.

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.

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.

Effect of extracellular potassium accumulation on muscle fiber conduction velocity: a simulation study

A progressive reduction in muscle fiber conduction velocity is typically observed during fatiguing muscle contraction. Although the exact causes of the conduction velocity decrease have not yet been fully established, increasing evidence suggests that changes in extracellular potassium concentration may be largely responsible. In this study, a mathematical model was developed to examine the effect of extracellular potassium concentration on the muscle fiber action potential and conduction velocity. The model was used to simulate changes in extracellular potassium concentration at a range of temperatures and extracellular potassium accumulation during repetitive stimulation of the muscle fiber at 37 degrees C. The action potential broadened, and its amplitude and conduction velocity decreased as extracellular potassium concentration increased. The potassium-induced changes in action potential shape and conduction velocity were eliminated when the inward rectifier channels were removed from the model. The results support the hypothesis that accumulation of extracellular potassium ions may be a major contributor to the reduction in muscle fiber conduction velocity and loss of membrane excitability during fatiguing contractions. They additionally suggest that inward rectifier currents play a critical role in potassium-induced membrane depolarization, leading to increased sodium inactivation and resulting in the observed reduction in conduction velocity and membrane excitability.

Exercise-induced regulation of phospholemman (FXYD1) in rat skeletal muscle: implications for Na+/K+-ATPase activity

BACKGROUND

Na(+)/K(+)-ATPase activity is upregulated during muscle exercise to maintain ionic homeostasis. One mechanism may involve movement of alpha-subunits to the outer membrane (translocation).

AIM

We investigated the existence of exercise-induced translocation and phosphorylation of phospholemman (PLM, FXYD1) protein in rat skeletal muscle and exercise-induced changes in V(max) and K(m) for Na(+) of the Na(+)/K(+)-ATPase.

METHODS

Two membrane fractionation methods and immunoprecipitation were used.

RESULTS

Both fractionation methods revealed a 200-350% increase in PLM in the sarcolemma after 30 min of treadmill running, while the phosphorylation of Ser-68 of PLM appeared to be unchanged. Exercise did not change V(max) or K(m) for Na(+) of the Na(+)/K(+)-ATPase in muscle homogenate, but induced a 67% increase in V(max) in the sarcolemmal giant vesicle preparation; K(m) for Na(+) remained constant. The main part of the increase in V(max) is related to a 36-53% increase in the level of alpha-subunits; the remainder may be related to increased PLM content. Similar results were obtained with another membrane purification method. In resting muscle, 29% and 32% of alpha(1)- and alpha(2)-subunits, respectively, were co-immunoprecipitated by PLM antibodies. In muscle homogenate prepared after exercise, immunoprecipitation of alpha(1)-subunits was increased to 227%, whereas the fraction of precipitated alpha(2) remained constant.

CONCLUSION

Exercise translocates PLM to the muscle outer membrane and increases its association with mainly the alpha(1)-subunit, which may contribute to the increased V(max) of the Na(+)/K(+)-ATPase.

Expression of aquaporin1, 3, and 4, NKCC1, and NKCC2 in the human endolymphatic sac

OBJECTIVE

To locate aquaporin (AQP) 1, 3, and 4, Na-K-2Cl cotransporter (NKCC) 1 and 2 in the human endolymphatic sac (ES).

METHODS

A sample of human ES was harvested during the removal of vestibular schwannoma via the translabyrinthine approach. The sample was immediately fixed in 4% paraformaldehyde and embedded in OCT compound. Immunohistochemistry was performed with AQP1, 3, and 4, NKCC1, and NKCC2 polyclonal antibodies.

RESULTS

AQP1, AQP3, and NKCC2 were strongly expressed in the epithelial layer of the ES. AQP4 and NKCC1 were weakly expressed in the epithelial layer of the ES.

CONCLUSIONS

As it is impossible to perform quantitative analysis based on the fluorescence intensity of each immunoreactivity, we have presented the existence of AQP1, 3, and 4, NKCC1, and NKCC2 in the ES. The expression of NKCC1 and 2 indicated that the ES may have both secretory and adsorptive functions to maintain the homeostasis of endolymph.

Do multiple ionic interactions contribute to skeletal muscle fatigue?

During intense exercise or electrical stimulation of skeletal muscle the concentrations of several ions change simultaneously in interstitial, transverse tubular and intracellular compartments. Consequently the functional effects of multiple ionic changes need to be considered together. A diminished transsarcolemmal K(+) gradient per se can reduce maximal force in non-fatigued muscle suggesting that K(+) causes fatigue. However, this effect requires extremely large, although physiological, K(+) shifts. In contrast, moderate elevations of extracellular [K(+)] ([K(+)](o)) potentiate submaximal contractions, enhance local blood flow and influence afferent feedback to assist exercise performance. Changed transsarcolemmal Na(+), Ca(2+), Cl(-) and H(+) gradients are insufficient by themselves to cause much fatigue but each ion can interact with K(+) effects. Lowered Na(+), Ca(2+) and Cl(-) gradients further impair force by modulating the peak tetanic force-[K(+)](o) and peak tetanic force-resting membrane potential relationships. In contrast, raised [Ca(2+)](o), acidosis and reduced Cl(-) conductance during late fatigue provide resistance against K(+)-induced force depression. The detrimental effects of K(+) are exacerbated by metabolic changes such as lowered [ATP](i), depleted carbohydrate, and possibly reactive oxygen species. We hypothesize that during high-intensity exercise a rundown of the transsarcolemmal K(+) gradient is the dominant cellular process around which interactions with other ions and metabolites occur, thereby contributing to fatigue.

Na(+)-K (+) pump location and translocation during muscle contraction in rat skeletal muscle

Muscle contraction may up-regulate the number of Na(+)-K(+) pumps in the plasma membrane by translocation of subunits. Since there is still controversy about where this translocation takes place from and if it takes place at all, the present study used different techniques to characterize the translocation. Electrical stimulation and biotin labeling of rat muscle revealed a 40% and 18% increase in the amounts of the Na(+)-K(+) pump alpha(2) subunit and caveolin-3 (Cav-3), respectively, in the sarcolemma. Exercise induced a 36% and 19% increase in the relative amounts of the alpha(2) subunit and Cav-3, respectively, in an outer-membrane-enriched fraction and a 41% and 17% increase, respectively, in sarcolemma giant vesicles. The Na(+)-K(+) pump activity measured with the 3-O-MFPase assay was increased by 37% in giant vesicles from exercised rats. Immunoprecipitation with Cav-3 antibody showed that 17%, 11% and 14% of the alpha(1) subunits were associated with Cav-3 in soleus, extensor digitorum longus, and mixed muscles, respectively. For the alpha(2), the corresponding values were 17%, 5% and 16%. In conclusion; muscle contraction induces translocation of the alpha subunits, which is suggested to be caused partly by structural changes in caveolae and partly by translocation from an intracellular pool.

Impaired calcium release during fatigue

Impaired calcium release from the sarcoplasmic reticulum (SR) has been identified as a contributor to fatigue in isolated skeletal muscle fibers. The functional importance of this phenomenon can be quantified by the use of agents, such as caffeine, which can increase SR Ca2+release during fatigue. A number of possible mechanisms for impaired calcium release have been proposed. These include reduction in the amplitude of the action potential, potentially caused by extracellular K+accumulation, which may reduce voltage sensor activation but is counteracted by a number of mechanisms in intact animals. Reduced effectiveness of SR Ca2+channel opening is caused by the fall in intracellular ATP and the rise in Mg2+concentrations that occur during fatigue. Reduced Ca2+available for release within the SR can occur if inorganic phosphate enters the SR and precipitates with Ca2+. Further progress requires the development of methods that can identify impaired SR Ca2+release in intact, blood-perfused muscles and that can distinguish between the various mechanisms proposed.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Muscle K+, Na+, and Cl− disturbances and Na+-K+ pump inactivation: implications for fatigue

Membrane excitability is a critical regulatory step in skeletal muscle contraction and is modulated by local ionic concentrations, conductances, ion transporter activities, temperature, and humoral factors. Intense fatiguing contractions induce cellular K+ efflux and Na+ and Cl− influx, causing pronounced perturbations in extracellular (interstitial) and intracellular K+ and Na+ concentrations. Muscle interstitial K+ concentration may increase 1- to 2-fold to 11–13 mM and intracellular K+ concentration fall by 1.3- to 1.7-fold; interstitial Na+ concentration may decline by 10 mM and intracellular Na+ concentration rise by 1.5- to 2.0-fold. Muscle Cl− concentration changes reported with muscle contractions are less consistent, with reports of both unchanged and increased intracellular Cl− concentrations, depending on contraction type and the muscles studied. When considered together, these ionic changes depolarize sarcolemmal and t-tubular membranes to depress tetanic force and are thus likely to contribute to fatigue. Interestingly, less severe local ionic changes can also augment subtetanic force, suggesting that they may potentiate muscle contractility early in exercise. Increased Na+-K+-ATPase activity during exercise stabilizes Na+ and K+ concentration gradients and membrane excitability and thus protects against fatigue. However, during intense contraction some Na+-K+ pumps are inactivated and together with further ionic disturbances, likely precipitate muscle fatigue.

A mathematical model of fatigue in skeletal muscle force contraction

The ability for muscle to repeatedly generate force is limited by fatigue. The cellular mechanisms behind muscle fatigue are complex and potentially include breakdown at many points along the excitation-contraction pathway. In this paper we construct a mathematical model of the skeletal muscle excitation-contraction pathway based on the cellular biochemical events that link excitation to contraction. The model includes descriptions of membrane voltage, calcium cycling and crossbridge dynamics and was parameterised and validated using the response characteristics of mouse skeletal muscle to a range of electrical stimuli. This model was used to uncover the complexities of skeletal muscle fatigue. We also parameterised our model to describe force kinetics in fast and slow twitch fibre types, which have a number of biochemical and biophysical differences. How these differences interact to generate different force/fatigue responses in fast- and slow- twitch fibres is not well understood and we used our modelling approach to bring new insights to this relationship.

Chloride conductance in the transverse tubular system of rat skeletal muscle fibres: importance in excitation-contraction coupling and fatigue

Contraction in skeletal muscle fibres is governed by excitation of the transverse-tubular (t-) system, but the properties of the t-system and their importance in normal excitability are not well defined. Here we investigate the properties of the t-system chloride conductance using rat skinned muscle fibres in which the sarcolemma has been mechanically removed but the normal excitation-contraction coupling mechanism kept functional. When the t-system chloride conductance was eliminated, either by removal of all Cl(-) or by block of the chloride channels with 9-anthracene carboxylic acid (9-AC) or by treating muscles with phorbol 12,13-dibutyrate, there was a marked reduction in the threshold electric field intensity required to elicit a t-system action potential (AP) and twitch response. Calculations of the t-system chloride conductance indicated that it constitutes a large proportion of the total chloride conductance observed in intact fibres. Blocking the chloride conductance increased the size of the twitch response and was indicative that Cl(-) normally carries part of the repolarizing current across the t-system membrane on each AP. Block of the t-system chloride conductance also reduced tetanic force responses at higher frequency stimulation (100 Hz) and greatly reduced twitch responses in the period shortly after a brief tetanus, owing to rapid loss of t-system excitability during the AP train. Blocking activity of the Na(+)-K(+) pump in the t-system membrane caused loss of excitability owing to K(+) build-up in the sealed t-system, and this occurred approximately 3-4 times faster when the chloride conductance was blocked. These findings show that the t-system chloride conductance plays a vital role during normal activity by countering the effects of K(+) accumulation in the t-system and maintaining muscle excitability.

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

AMP-activated protein kinase (AMPK), activated by an increase in intracellular AMP-to-ATP ratio, stimulates pathways that can restore ATP levels. We tested the hypothesis that AMPK activation influences extracellular fluid (ECF) K(+) homeostasis. In conscious rats, AMPK was activated with 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) infusion: 38.4 mg x kg bolus then 4 mg x kg(-1) x min(-1) infusion. Plasma [K(+)] and [glucose] both dropped at 1 h of AICAR infusion and [K(+)] dropped to 3.3 +/- 0.04 mM by 3 h, linearly related to the increase in muscle AMPK phosphorylation. AICAR treatment did not increase urinary K(+) excretion. AICAR lowered [K(+)] whether plasma [K(+)] was chronically elevated or lowered. The K(+) infusion rate needed to maintain baseline plasma [K(+)] reached 15.7 +/- 1.3 micromol K(+) x kg(-1) x min(-1) between 120 and 180 min AICAR infusion. In mice expressing a dominant inhibitory form of AMPK in the muscle (Tg-KD1), baseline [K(+)] was not different from controls (4.2 +/- 0.1 mM), but the fall in plasma [K(+)] in response to AICAR (0.25 g/kg) was blunted: [K(+)] fell to 3.6 +/- 0.1 in controls and to 3.9 +/- 0.1 mM in Tg-KD1, suggesting that ECF K(+) redistributes, at least in part, to muscle ICF. In summary, these findings illustrate that activation of AMPK activity with AICAR provokes a significant fall in plasma [K(+)] and suggest a novel mechanism for redistributing K(+) from ECF to ICF.

Changes in interstitial K+ and pH during exercise: implications for blood flow regulation

The analysis of blood samples has clearly demonstrated that exercise is associated with the release of K+ and H+ from muscle. However, blood samples give only incomplete information about the ion changes in the muscle interstitium. Interstitial changes in ion composition may affect the transport properties of the sarcolemmal membrane, may affect fibre excitability and induce fatigue, and may affect sensory nerve endings. Therefore, to better understand muscle function, it is important to quantify the exercise-induced interstitial ion changes. Both interstitial K+ and H+ changes have been quantified with the microdialysis technique. Interstitial K+ accumulation is dependent on the intensity and duration of muscle activity and may reach 10 mmol/L during intense exercise, and the concentration in T-tubules may be even higher. Thus, interstitial K+ can reach a level that affects fibre excitability and the development of fatigue. It has also been demonstrated with microdialysis that the interstitial decrease in pH during muscle activity is larger than the reduction in blood pH. Ion changes in the interstitium may affect blood flow directly or indirectly. Infusion of K+ into the femoral artery in humans has demonstrated that blood flow is affected by changes in K+ as low as 0.1 mmol/L. The vasodilatory effect of K+ can be inhibited with simultaneous barium infusion, indicating that inward rectifier potassium (Kir)channels are involved. Acidosis has a direct effect on blood flow and an indirect effect, mediated by changes in other vasoactive compounds.

Inotropic effects of the K+ channel blocker 3,4-diaminopyridine on fatigued diaphragm muscle

K(+) channels play important roles in skeletal muscle contraction by regulating action potential duration. Blocking these channels, for example with 3,4-diaminopyridine (DAP), augments muscle force considerably, and these force increases are maintained well during fatigue-inducing contractions. The present study tested the hypothesis that K(+) channel blockade also improves force of previously fatigued muscle. Rat diaphragm underwent fatigue-inducing stimulation in vitro with four different stimulation protocols consisting of 20 Hz vs. 50 Hz trains and 1 min vs. 4 min stimulation durations. DAP administered at the onset of the recovery period produced significant force increases irrespective of the amount of antecedent force loss. These force gains considerably exceeded those resulting from normal force recovery in untreated muscle. Furthermore contraction time was prolonged by DAP in all cases, and half-relaxation time was prolonged by DAP in most cases. Several differences were found compared with previous studies of DAP in fresh muscle, including smaller magnitude and slower time course of force increases. Intracellular electrophysiological recordings found smaller effects of DAP on action potential overshoot and time-depolarization integral in previously stimulated compared with fresh muscle. These data indicate that K(+) channel blockade does indeed increase force of fatigued diaphragm, but to an attenuated extent relative to its effects on non-fatigued muscle, which can be explained on the basis of electrophysiological findings. Nonetheless DAP-induced force increases were usually sufficient to restore force to values present prior to the onset of fatigue-inducing stimulation.

Ion channels and ion transporters of the transverse tubular system of skeletal muscle

This review focuses on the electrical properties of the transverse (T) tubular membrane of skeletal muscle, with reference to the contribution of the T-tubular system (TTS) to the surface action potential, the radial spread of excitation and its role in excitation-contraction coupling. Particularly, the most important ion channels and ion transporters that enable proper depolarization and repolarization of the T-tubular membrane are described. Since propagation of excitation along the TTS into the depth of the fibers is a delicate balance between excitatory and inhibitory currents, the composition of channels and transporters is specific to the TTS and different from the surface membrane. The TTS normally enables the radial spread of excitation and the signal transfer to the sarcoplasmic reticulum to release calcium that activates the contractile apparatus. However, due to its structure, even slight shifts of ions may alter its volume, Nernstian potentials, ion permeabilities, and consequently T-tubular membrane potential and excitability.

Training-induced changes in membrane transport proteins of human skeletal muscle

Training improves human physical performance by inducing structural and cardiovascular changes, metabolic changes, and changes in the density of membrane transport proteins. This review focuses on the training-induced changes in proteins involved in sarcolemmal membrane transport. It is concluded that the same type of training affects many transport proteins, suggesting that all transport proteins increase with training, and that both sprint and endurance training in humans increase the density of most membrane transport proteins. There seems to be an upper limit for these changes: intense training for 6-8 weeks substantially increases the density of membrane proteins, whereas years of training (as performed by athletes) have no further effect. Studies suggest that training-induced changes at the protein level are important functionally. The underlying factors responsible for these changes in transport proteins might include changes in substrate concentration, but the existence of "exercise factors" mediating these responses is more likely. Exercise factors might include Ca(2+), mitogen-activated protein kinases, adenosine monophosphate kinases, other kinases, or interleukin-6. Although the magnitudes of training-induced changes have been investigated at the protein level, the underlying signal mechanisms have not been fully described.