Role of physiological ClC-1 Cl- ion channel regulation for the excitability and function of working skeletal muscle.

Electrical membrane properties of skeletal muscle fibers have been thoroughly studied over the last five to six decades. This has shown that muscle fibers from a wide range of species, including fish, amphibians, reptiles, birds, and mammals, are all characterized by high resting membrane permeability for Cl(-) ions. Thus, in resting human muscle, ClC-1 Cl(-) ion channels account for ∼80% of the membrane conductance, and because active Cl(-) transport is limited in muscle fibers, the equilibrium potential for Cl(-) lies close to the resting membrane potential. These conditions-high membrane conductance and passive distribution-enable ClC-1 to conduct membrane current that inhibits muscle excitability. This depressing effect of ClC-1 current on muscle excitability has mostly been associated with skeletal muscle hyperexcitability in myotonia congenita, which arises from loss-of-function mutations in the CLCN1 gene. However, given that ClC-1 must be drastically inhibited (∼80%) before myotonia develops, more recent studies have explored whether acute and more subtle ClC-1 regulation contributes to controlling the excitability of working muscle. Methods were developed to measure ClC-1 function with subsecond temporal resolution in action potential firing muscle fibers. These and other techniques have revealed that ClC-1 function is controlled by multiple cellular signals during muscle activity. Thus, onset of muscle activity triggers ClC-1 inhibition via protein kinase C, intracellular acidosis, and lactate ions. This inhibition is important for preserving excitability of working muscle in the face of activity-induced elevation of extracellular K(+) and accumulating inactivation of voltage-gated sodium channels. Furthermore, during prolonged activity, a marked ClC-1 activation can develop that compromises muscle excitability. Data from ClC-1 expression systems suggest that this ClC-1 activation may arise from loss of regulation by adenosine nucleotides and/or oxidation. The present review summarizes the current knowledge of the physiological factors that control ClC-1 function in active muscle

Comparison of regulated passive membrane conductance in action potential–firing fast- and slow-twitch muscle

In several pathological and experimental conditions, the passive membrane conductance of muscle fibers (Gm) and their excitability are inversely related. Despite this capacity of Gm to determine muscle excitability, its regulation in active muscle fibers is largely unexplored. In this issue, our previous study (Pedersen et al. 2009. J. Gen. Physiol. doi:10.1085/jgp.200910291) established a technique with which biphasic regulation of Gm in action potential (AP)-firing fast-twitch fibers of rat extensor digitorum longus muscles was identified and characterized with temporal resolution of seconds. This showed that AP firing initially reduced Gm via ClC-1 channel inhibition but after ∼1,800 APs, Gm rose substantially, causing AP excitation failure. This late increase of Gm reflected activation of ClC-1 and KATP channels. The present study has explored regulation of Gm in AP-firing slow-twitch fibers of soleus muscle and compared it to Gm dynamics in fast-twitch fibers. It further explored aspects of the cellular signaling that conveyed regulation of Gm in AP-firing fibers. Thus, in both fiber types, AP firing first triggered protein kinase C (PKC)-dependent ClC-1 channel inhibition that reduced Gm by ∼50%. Experiments with dantrolene showed that AP-triggered SR Ca2+ release activated this PKC-mediated ClC-1 channel inhibition that was associated with reduced rheobase current and improved function of depolarized muscles, indicating that the reduced Gm enhanced muscle fiber excitability. In fast-twitch fibers, the late rise in Gm was accelerated by glucose-free conditions, whereas it was postponed when intermittent resting periods were introduced during AP firing. Remarkably, elevation of Gm was never encountered in AP-firing slow-twitch fibers, even after 15,000 APs. These observations implicate metabolic depression in the elevation of Gm in AP-firing fast-twitch fibers. It is concluded that regulation of Gm is a general phenomenon in AP-firing muscle, and that differences in Gm regulation may contribute to the different phenotypes of fast- and slow-twitch muscle

Cardiac dysfunction in a porcine model of pediatric malnutrition

Half a million children die annually of severe acute malnutrition and cardiac dysfunction may contribute to the mortality. However, cardiac function remains poorly examined in cases of severe acute malnutrition.To determine malnutrition-induced echocardiographic disturbances and longitudinal changes in plasma pro-atrial natriuretic peptide and cardiac troponin-T in a pediatric porcine model.Five-week old piglets (Duroc-x-Danish Landrace-x-Yorkshire) were fed a nutritionally inadequate maize-flour diet to induce malnutrition (MAIZE, n = 12) or a reference diet (AGE-REF, n = 12) for 7 weeks. Outcomes were compared to a weight-matched reference group (WEIGHT-REF, n = 8). Pro-atrial natriuretic peptide and cardiac troponin-T were measured weekly. Plasma pro-atrial natriuretic peptide decreased in both MAIZE and AGE-REF during the first 3 weeks but increased markedly in MAIZE relative to AGE-REF during week 5-7 (p ≤ 0.001). There was overall no difference in plasma cardiac troponin-T between groups. However, further analysis revealed that release of cardiac troponin-T in plasma was more frequent in AGE-REF compared with MAIZE (OR: 4.8; 95%CI: 1.2-19.7; p = 0.03). However, when release occurred, cardiac troponin-T concentration was 6.9-fold higher (95%CI: 3.0-15.9; p < 0.001) in MAIZE compared to AGE-REF. At week 7, the mean body weight in MAIZE was lower than AGE-REF (8.3 vs 32.4 kg, p < 0.001), whereas heart-weight relative to body-weight was similar across the three groups. The myocardial performance index was 86% higher in MAIZE vs AGE-REF (p < 0.001) and 27% higher in MAIZE vs WEIGHT-REF (p = 0.025).Malnutrition associates with cardiac dysfunction in a pediatric porcine model by increased myocardial performance index and pro-atrial natriuretic peptide and it associates with cardiac injury by elevated cardiac troponin-T. Clinical studies are needed to see if the same applies for children suffering from malnutrition

Na+,K+-pump stimulation improves contractility in isolated muscles of mice with hyperkalemic periodic paralysis

In patients with hyperkalemic periodic paralysis (HyperKPP), attacks of muscle weakness or paralysis are triggered by K+ ingestion or rest after exercise. Force can be restored by muscle work or treatment with β2-adrenoceptor agonists. A missense substitution corresponding to a mutation in the skeletal muscle voltage-gated Na+ channel (Nav1.4, Met1592Val) causing human HyperKPP was targeted into the mouse SCN4A gene (mutants). In soleus muscles prepared from these mutant mice, twitch, tetanic force, and endurance were markedly reduced compared with soleus from wild type (WT), reflecting impaired excitability. In mutant soleus, contractility was considerably more sensitive than WT soleus to inhibition by elevated [K+]o. In resting mutant soleus, tetrodotoxin (TTX)-suppressible 22Na uptake and [Na+]i were increased by 470 and 58%, respectively, and membrane potential was depolarized (by 16 mV, P < 0.0001) and repolarized by TTX. Na+,K+ pump–mediated 86Rb uptake was 83% larger than in WT. Salbutamol stimulated 86Rb uptake and reduced [Na+]i both in mutant and WT soleus. Stimulating Na+,K+ pumps with salbutamol restored force in mutant soleus and extensor digitorum longus (EDL). Increasing [Na+]i with monensin also restored force in soleus. In soleus, EDL, and tibialis anterior muscles of mutant mice, the content of Na+,K+ pumps was 28, 62, and 33% higher than in WT, respectively, possibly reflecting the stimulating effect of elevated [Na+]i on the synthesis of Na+,K+ pumps. The results confirm that the functional disorders of skeletal muscles in HyperKPP are secondary to increased Na+ influx and show that contractility can be restored by acute stimulation of the Na+,K+ pumps. Calcitonin gene-related peptide (CGRP) restored force in mutant soleus but caused no detectable increase in 86Rb uptake. Repeated excitation and capsaicin also restored contractility, possibly because of the release of endogenous CGRP from nerve endings in the isolated muscles. These observations may explain how mild exercise helps locally to prevent severe weakness during an attack of HyperKPP

Loss of force induced by high extracellular [K+] in rat muscle: effect of temperature, lactic acid and β2-agonist

Loss of K+ from active muscles, leading to increased [K+]o, has been proposed to cause muscle fatigue by reducing excitability. Since exercise increases muscle temperature, we investigated the influence of temperature on muscle [K+]o sensitivity. Intact rat soleus or extensor digitorum longus (EDL) muscles were mounted on force transducers and stimulated electrically to evoke short isometric tetani at regular intervals. In each experiment, control force at 4 mM K+ was initially determined at every temperature used. In soleus muscles at 20 °C, 9 mM K+ reduced force to 33 ± 5 % of control force. Increasing the temperature to 30 °C restored force to 89 ± 5 % of control force. Likewise, at 30 °C 11 mM K+ reduced force to 16 ± 4 % and increasing the temperature to 35 °C restored force to 35 ± 5 %. Similar results were obtained using EDL. The force recovery induced by elevating temperature, reflecting reduced [K+]o sensitivity, was associated with improved excitability assessed from compound action potentials. Force recovery induced by a temperature elevation from 20 to 30 °C was associated with hyperpolarization (5 mV), reduced [Na+]i and a 93 % increase in Na+-K+ pump activity. The force recovery was blocked by ouabain. Since intensive exercise leads to lactic acidosis and increased plasma catecholamines, the effect of these two factors was also investigated. At 11 mM K+, force was completely restored by combining temperature elevation (30 to 35 °C), L-lactic acid (10 mM) and the β2-agonist salbutamol (10−5 M). We suggest an exercise scenario where the depressing action of exercise-induced hyperkalaemia is counteracted by elevated muscle temperature, lactic acidosis and catecholamines