Modulation of the isoprenaline-induced membrane hyperpolarization of mouse skeletal muscle cells

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

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

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

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

Exchange protein directly activated by cAMP (Epac) 1 plays an essential role in stress-induced exercise capacity by regulating PGC-1α and fatty acid metabolism in skeletal muscle

Exchange protein directly activated by cAMP (Epac) mediates cAMP-mediated cell signal independent of protein kinase A (PKA). Mice lacking Epac1 displayed metabolic defect suggesting possible functional involvement of skeletal muscle and exercise capacity. Epac1 was highly expressed, but not Epac 2, in the extensor digitorum longus (EDL) and soleus muscles. The exercise significantly increased protein expression of Epac 1 in EDL and soleus muscle of wild-type (WT) mice. A global proteomics and pathway analyses revealed that Epac 1 deficiency mainly affected "the energy production and utilization" process in the skeletal muscle. We have tested their forced treadmill exercise tolerance. Epac1^-/- mice exhibited significantly reduced exercise capacity in the forced treadmill exercise and lower number of type 1 fibers than WT mice. The basal protein level of proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) was reduced in the Epac1^-/- mice. Furthermore, increasing expression of PGC-1α by exercise was also significantly attenuated in the skeletal muscle of Epac1^-/- mice. The expressions of downstream target genes of PGC-1α, which involved in uptake and oxidation of fatty acids, ERRα and PPARδ, and fatty acid content were lower in muscles of Epac1^-/-, suggesting a role of Epac1 in forced treadmill exercise capacity by regulating PGC-1α pathway and lipid metabolism in skeletal muscle. Taken together, Epac1 plays an important role in exercise capacity by regulating PGC-1α and fatty acid metabolism in the skeletal muscle.

Lack of β2-AR improves exercise capacity and skeletal muscle oxidative phenotype in mice

β(2)-adrenergic receptor (β(2)-AR) agonists have been used as ergogenics by athletes involved in training for strength and power in order to increase the muscle mass. Even though anabolic effects of β(2)-AR activation are highly recognized, less is known about the impact of β(2)-AR in endurance capacity. We presently used mice lacking β(2)-AR [β(2)-knockout (β(2) KO)] to investigate the role of β(2)-AR on exercise capacity and skeletal muscle metabolism and phenotype. β(2) KO mice and their wild-type controls (WT) were studied. Exercise tolerance, skeletal muscle fiber typing, capillary-to-fiber ratio, citrate synthase activity and glycogen content were evaluated. When compared with WT, β(2) KO mice displayed increased exercise capacity (61%) associated with higher percentage of oxidative fibers (21% and 129% of increase in soleus and plantaris muscles, respectively) and capillarity (31% and 20% of increase in soleus and plantaris muscles, respectively). In addition, β(2) KO mice presented increased skeletal muscle citrate synthase activity (10%) and succinate dehydrogenase staining. Likewise, glycogen content (53%) and periodic acid-Schiff staining (glycogen staining) were also increased in β(2) KO skeletal muscle. Altogether, these data provide evidence that disruption of β(2)-AR improves oxidative metabolism in skeletal muscle of β(2) KO mice and this is associated with increased exercise capacity.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Isoprenaline-stimulated differential adrenergic response of K+ channels in skeletal muscle under hypokalaemic conditions

The mechanism underlying the hyperpolarization induced by isoprenaline in mouse lumbrical muscle fibres was studied using cell-attached patch and intracellular membrane potential ( V(m)) recordings. Sarcolemmal inwardly rectifying K(+) channels (K(IR): 45 pS) and Ca(2+)-activated K(+) channels (BK: 181 pS) were identified. Exposure to isoprenaline closed K(IR) channels and increased BK channel activity. This increase was observed as a shift from 50 to -40 mV in the voltage dependence of channel activation. Isoprenaline prevented hysteresis of V(m) when the extracellular [K(+)] fell below 3.8 mM. This hysteresis was due to the properties of the K(IR). The effects of chloride transport and isoprenaline on V(m) did not interact purely competitively, but isoprenaline could prevent the depolarization induced by hyperosmotic media equally as well as bumetanide, which inhibits the Na(+)/K(+)/2Cl(-) cotransporter. In lumbrical muscle this leads to hyperpolarization, but this might vary among muscles. The switch from K(IR) to BK as the component of total K(+) conductance was due to isoprenaline.

Effects of chloride transport on bistable behaviour of the membrane potential in mouse skeletal muscle

The lumbrical skeletal muscle fibres of mice exhibited electrically bistable behaviour due to the nonlinear properties of the inwardly rectifying potassium conductance. When the membrane potential (V(m)) was measured continuously using intracellular microelectrodes, either a depolarization or a hyperpolarization was observed following reduction of the extracellular potassium concentration (K+o) from 5.7 mM to values in the range 0.76-3.8 mM, and V(m) showed hysteresis when K+o was slowly decreased and then increased within this range. Hypertonicity caused membrane depolarization by enhancing chloride import through the Na+-K+-2Cl- cotransporter and altered the bistable behaviour of the muscle fibres. Addition of bumetanide, a potent inhibitor of the Na+-K+-2Cl- cotransporter, and of anthracene-9-carboxylic acid, a blocker of chloride channels, caused membrane hyperpolarization particularly under hypertonic conditions, and also altered the bistable behaviour of the cells. Hysteresis loops shifted with hypertonicity to higher K+o values and with bumetanide to lower values. The addition of 80 microM BaCl2 or temperature reduction from 35 to 27 degrees C induced a depolarization of cells that were originally hyperpolarized. In the K+o range of 5.7-22.8 mM, cells in isotonic media (289 mmol x kg(-1)) responded nearly Nernstianly to K+o reduction, i.e. 50 mV per decade; in hypertonic media this dependence was reduced to 36 mV per decade (319 mmol x kg(-1)) or to 31 mV per decade (340 mmol x kg(-1)). Our data can explain apparent discrepancies in DeltaV(m) found in the literature. We conclude that chloride import through the Na+-K+-2Cl- cotransporter and export through Cl- channels influenced the V(m) and the bistable behaviour of mammalian skeletal muscle cells. The possible implication of this bistable behaviour in hypokalaemic periodic paralysis is discussed.

The role of K+ channels in the force recovery elicited by Na+-K+ pump stimulation in Ba2+-paralysed rat skeletal muscle

The present experiments were performed to assess the role of K+ channels in hormonal stimulation of the Na+-K+ pump and to determine the contribution of Na+-K+ pumps to the recovery of excitability and contractility in depolarized skeletal muscle. In soleus muscle, Ba2+ (0.02 and 1 mM) was found to inhibit 42K+ efflux and 42K+ influx. Both in the absence and the presence of Ba2+ (1 mM), salbutamol and calcitonin gene-related peptide (CGRP) induced a marked decrease in intracellular Na+ and stimulation of 42K+ uptake. In soleus muscles Ba2+ (0.1 and 1.0 mM) decreased twitch and tetanic force. Subsequent stimulation of the Na+-K+ pumps by salbutamol, CGRP or repeated electrical stimulation produced a highly significant restoration of force development, which was suppressed by ouabain, but not by glibenclamide. Also, in extensor digitorum longus muscles Ba2+ (0.1 mM) produced a considerable force decline, which was partly restored by salbutamol and CGRP. The area of compound action potentials (M-waves) elicited by indirect stimulation was decreased by Ba2+ (0.1 mM). This was associated with a concomitant decrease in tetanic force and depolarization. Salbutamol, CGRP or repeated electrical stimulation all elicited marked recovery of M-wave area, force and membrane potential. All recordings showed close correlations between these three parameters. The data add further support to the concept that due to its electrogenic nature and large transport capacity, the Na+-K+ pump is a rapid and efficient mechanism for the maintenance of excitability in skeletal muscle, acting independently of Ba2+- or ATP-sensitive K+ channel function.

The influence of bumetanide on the membrane potential of mouse skeletal muscle cells in isotonic and hypertonic media

1. Increasing the medium osmolality, with a non-ionic osmoticant, from control (289 mOsm) to 319 mOsm or 344 mOsm in the lumbrical muscle cell of the mouse, resulted in a depolarization of the membrane potential (Vm) of 5.9 mV and 10.9 mV, respectively. 2. In control medium, the blockers of chloride related cotransport bumetanide and furosemide, induced a hyperpolarization of -3.6 and -3.0 mV and prevented the depolarization due to hypertonicity. When bumetanide was added in hypertonic media Vm fully repolarized to control values. 3. In a medium of 266 mOsm, the hyperpolarization by bumetanide was absent. 4. At 344 mOsm the half-maximal effective concentration (IC50) was 0.5 microM for bumetanide and 21 microM for furosemide. 5. In solutions containing 1.25 mM sodium the depolarization by hypertonicity was reduced to 2.3 mV. 6. Reducing chloride permeability, by anthracene 9 carboxylic acid (9-AC) in 289 mOsm, induced a small but significant hyperpolarization of -2.6 mV. Increasing medium osmolality to 344 mOsm enlarged this hyperpolarization significantly to -7.6 mV. 7. In a solution of 344 mOsm containing 100 microM ouabain, the bumetanide-induced hyperpolarization of Vm was absent. 8. The results indicate that a Na-K-2Cl cotransporter is present in mouse lumbrical muscle fibre and that its contribution to Vm is dependent on medium osmolality.