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.

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 β 2 -adrenoceptor agonists. A missense substitution corresponding to a mutation in the skeletal muscle voltage-gated Na + channel (Na v 1.4, 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 22 Na 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 86 Rb uptake was 83% larger than in WT. Salbutamol stimulated 86 Rb 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 86 Rb 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.

Action potential (AP) excitation requires a transient dominance of depolarizing membrane currents over the repolarizing membrane currents that stabilize the resting membrane potential. Such stabilizing currents, in turn, depend on passive membrane conductance (G m ), which in skeletal muscle fibers covers membrane conductances for K + (G K ) and Cl − (G Cl ). Myotonic disorders and studies with metabolically poisoned muscle have revealed capacities of G K and G Cl to inversely interfere with muscle excitability. However, whether regulation of G K and G Cl occur in AP-firing muscle under normal physiological conditions is unknown. This study establishes a technique that allows the determination of G Cl and G K with a temporal resolution of seconds in AP-firing muscle fibers. With this approach, we have identified and quantified a biphasic regulation of G m in active fast-twitch extensor digitorum longus fibers of the rat. Thus, at the onset of AP firing, a reduction in G Cl of ∼70% caused G m to decline by ∼55% in a manner that is well described by a single exponential function characterized by a time constant of ∼200 APs (phase 1). When stimulation was continued beyond ∼1,800 APs, synchronized elevations in G K (∼14-fold) and G Cl (∼3-fold) caused G m to rise sigmoidally to ∼400% of its level before AP firing (phase 2). Phase 2 was often associated with a failure to excite APs. When AP firing was ceased during phase 2, G m recovered to its level before AP firing in ∼1 min. Experiments with glibenclamide (K ATP channel inhibitor) and 9-anthracene carboxylic acid (ClC-1 Cl − channel inhibitor) revealed that the decreased G m during phase 1 reflected ClC-1 channel inhibition, whereas the massively elevated G m during phase 2 reflected synchronized openings of ClC-1 and K ATP channels. In conclusion, G Cl and G K are acutely regulated in AP-firing fast-twitch muscle fibers. Such regulation may contribute to the physiological control of excitability in active muscle.

In several pathological and experimental conditions, the passive membrane conductance of muscle fibers (G m ) and their excitability are inversely related. Despite this capacity of G m 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 G m 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 G m via ClC-1 channel inhibition but after ∼1,800 APs, G m rose substantially, causing AP excitation failure. This late increase of G m reflected activation of ClC-1 and K ATP channels. The present study has explored regulation of G m in AP-firing slow-twitch fibers of soleus muscle and compared it to G m dynamics in fast-twitch fibers. It further explored aspects of the cellular signaling that conveyed regulation of G m in AP-firing fibers. Thus, in both fiber types, AP firing first triggered protein kinase C (PKC)-dependent ClC-1 channel inhibition that reduced G m by ∼50%. Experiments with dantrolene showed that AP-triggered SR Ca 2+ 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 G m enhanced muscle fiber excitability. In fast-twitch fibers, the late rise in G m was accelerated by glucose-free conditions, whereas it was postponed when intermittent resting periods were introduced during AP firing. Remarkably, elevation of G m was never encountered in AP-firing slow-twitch fibers, even after 15,000 APs. These observations implicate metabolic depression in the elevation of G m in AP-firing fast-twitch fibers. It is concluded that regulation of G m is a general phenomenon in AP-firing muscle, and that differences in G m regulation may contribute to the different phenotypes of fast- and slow-twitch muscle.