Intracellular chloride and the mechanism for its accumulation in rat lumbrical muscle

Periodic paralysis

Periodic paralysis is a rare, dominantly inherited disorder of skeletal muscle in which episodic attacks of weakness are caused by a transient impairment of fiber excitability. Attacks of weakness are often elicited by characteristic environmental triggers, which were the basis for clinically delineating subtypes of periodic paralysis and are an important distinction for optimal disease management. All forms of familial periodic paralysis are caused by mutations of ion channels, often selectively expressed in skeletal muscle, that destabilize the resting potential. The missense mutations usually alter channel function through gain-of-function changes rather than producing a complete loss-of-function null. The knowledge of which channel gene harbors a variant, whether that variant is expected to (or known to) alter function, and how altered function impairs fiber excitability aides in the interpretation of patient signs and symptoms, the interpretation of gene test results, and how to optimize therapeutic intervention for symptom management and improve quality of life.

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.

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.

A highly-selective chloride microelectrode based on a mercuracarborand anion carrier

The chloride gradient plays an important role in regulating cell volume, membrane potential, pH, secretion, and the reversal potential of inhibitory glycine and GABA_A receptors. Measurement of intracellular chloride activity, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{a}}}_{{\boldsymbol{Cl}}}^{{\boldsymbol{i}}}$$\end{document}aCli, using liquid membrane ion-selective microelectrodes (ISM), however, has been limited by the physiochemical properties of Cl⁻ ionophores which have caused poor stability, drift, sluggish response times, and interference from other biologically relevant anions. Most importantly, intracellular \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bf{HC}}{{\bf{O}}}_{{\bf{3}}}^{-}$$\end{document}HCO3− may be up to 4 times more abundant than Cl⁻ (e.g. skeletal muscle) which places severe constraints on the required selectivity of a Cl⁻ – sensing ISM. Previously, a sensitive and highly-selective Cl⁻ sensor was developed in a polymeric membrane electrode using a trinuclear Hg(II) complex containing carborane-based ligands, [9]-mercuracarborand-3, or MC3 for short. Here, we have adapted the use of the MC3 anion carrier in a liquid membrane ion-selective microelectrode and show the MC3-ISM has a linear Nernstian response over a wide range of a_Cl (0.1 mM to 100 mM), is highly selective for Cl⁻ over other biological anions or inhibitors of Cl⁻ transport, and has a 10% to 90% settling  time of 3  sec. Importantly, over the physiological range of a_Cl (1 mM to 100 mM) the potentiometric response of the MC3-ISM is insensitive to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bf{HC}}{{\bf{O}}}_{{\bf{3}}}^{-}$$\end{document}HCO3− or changes in pH. Finally, we demonstrate the biological application of an MC3-ISM by measuring intracellular a_Cl, and the response to an external Cl-free challenge, for an isolated skeletal muscle fiber.

Central Role of Subthreshold Currents in Myotonia

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

Measurement of intracellular ion activity in skeletal muscle fibers: Four microelectrodes or no deal

Allard reviews a new powerful method allowing measurement of intracellular ion activity in isolated skeletal muscle fibers.

A four-electrode method to study dynamics of ion activity and transport in skeletal muscle fibers

Ion movements across biological membranes, driven by electrochemical gradients or active transport mechanisms, control essential cell functions. Membrane ion movements can manifest as electrogenic currents or electroneutral fluxes, and either process can alter the extracellular and/or intracellular concentration of the transported ions. Classic electrophysiological methods allow accurate measurement of membrane ion movements when the transport mechanism produces a net ionic current; however, they cannot directly measure electroneutral fluxes and do not detect any accompanying change in intracellular ion concentrations. Here, we developed a method for simultaneously measuring ion movements and the accompanying dynamic changes in intracellular ion concentrations in intact skeletal muscle fibers under voltage or current clamp in real time. The method combines a two-microelectrode voltage clamp with ion-selective and reference microelectrodes (four-electrode system). We validate the electrical stability of the system and the viability of the preparation for periods of ∼1 h. We demonstrate the power of this method with measurements of intracellular Cl-, H+, and Na+ to show (a) voltage-dependent redistribution of Cl- ions; (b) intracellular pH changes induced by changes in extracellular pCO2; and (c) electroneutral and electrogenic Na+ movements controlled by the Na,K-ATPase. The method is useful for studying a range of transport mechanisms in many cell types, particularly when the transmembrane ion movements are electrically silent and/or when the transport activity measurably changes the intracellular activity of a transported ion.

Recovery from acidosis is a robust trigger for loss of force in murine hypokalemic periodic paralysis

Hypokalemic periodic paralysis causes episodes of muscle weakness. Mi et al. investigate the rest-induced weakness that occurs after vigorous exercise and find that acidosis, as occurs with exercise, leads to accumulation of myoplasmic Cl⁻, which favors a depolarized resting potential when pH returns to normal.

Periodic paralysis is an ion channelopathy of skeletal muscle in which recurrent episodes of weakness or paralysis are caused by sustained depolarization of the resting potential and thus reduction of fiber excitability. Episodes are often triggered by environmental stresses, such as changes in extracellular K⁺, cooling, or exercise. Rest after vigorous exercise is the most common trigger for weakness in periodic paralysis, but the mechanism is unknown. Here, we use knock-in mutant mouse models of hypokalemic periodic paralysis (HypoKPP; Na_V1.4-R669H or Ca_V1.1-R528H) and hyperkalemic periodic paralysis (HyperKPP; Na_V1.4-M1592V) to investigate whether the coupling between pH and susceptibility to loss of muscle force is a possible contributor to exercise-induced weakness. In both mouse models, acidosis (pH 6.7 in 25% CO₂) is mildly protective, but a return to pH 7.4 (5% CO₂) unexpectedly elicits a robust loss of force in HypoKPP but not HyperKPP muscle. Prolonged exposure to low pH (tens of minutes) is required to cause susceptibility to post-acidosis loss of force, and the force decrement can be prevented by maneuvers that impede Cl⁻ entry. Based on these data, we propose a mechanism for post-acidosis loss of force wherein the reduced Cl⁻ conductance in acidosis leads to a slow accumulation of myoplasmic Cl⁻. A rapid recovery of both pH and Cl⁻ conductance, in the context of increased [Cl]_in/[Cl]_out, favors the anomalously depolarized state of the bistable resting potential in HypoKPP muscle, which reduces fiber excitability. This mechanism is consistent with the delayed onset of exercise-induced weakness that occurs with rest after vigorous activity.

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.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

A personal historic perspective on the role of chloride in skeletal and cardiac muscle

During the early decades of the last century, skeletal muscle was held to be impermeable to chloride ions. This theory, based on shaky grounds, was famously falsified by Boyle and Conway in 1941. Two decades later and onwards, the larger part of the resting conductance of skeletal muscle was found to be due to chloride ions, sensitive to the chemical environment, and to be time‐and‐voltage dependent. So, much of the groundwork for the physiological role of chloride ions in skeletal muscle was laid before the game‐changing discovery of chloride channels. The early history of the role of chloride in cardiac muscle, and work on the relative permeability to foreign anions of different muscles are also here covered from a personal perspective.

Sodium Channelopathies of Skeletal Muscle

The Na_V1.4 sodium channel is highly expressed in skeletal muscle, where it carries almost all of the inward Na⁺ current that generates the action potential, but is not present at significant levels in other tissues. Consequently, mutations of SCN4A encoding Na_V1.4 produce pure skeletal muscle phenotypes that now include six allelic disorders: sodium channel myotonia, paramyotonia congenita, hyperkalemic periodic paralysis, hypokalemic periodic paralysis, congenital myasthenia, and congenital myopathy with hypotonia. Mutation-specific alternations of Na_V1.4 function explain the mechanistic basis for the diverse phenotypes and identify opportunities for strategic intervention to modify the burden of disease.

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.

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.

Chloride channels in myotonia congenita assessed by velocity recovery cycles

INTRODUCTION

Myotonia congenita (MC) is caused by congenital defects in the muscle chloride channel CLC-1. This study used muscle velocity recovery cycles (MVRCs) to investigate how membrane function is affected.

METHODS

MVRCs and responses to repetitive stimulation were compared between 18 patients with genetically confirmed MC (13 recessive, 7 dominant) and 30 age-matched, normal controls.

RESULTS

MC patients exhibited increased early supernormality, but this was prevented by treatment with sodium channel blockers. After multiple conditioning stimuli, late supernormality was enhanced in all MC patients, indicating delayed repolarization. These abnormalities were similar between the MC subtypes, but recessive patients showed a greater drop in amplitude during repetitive stimulation.

CONCLUSIONS

MVRCs indicate that chloride conductance only becomes important when muscle fibers are depolarized. The differential responses to repetitive stimulation suggest that, in dominant MC, the affected chloride channels are activated by strong depolarization, consistent with a positive shift of the CLC-1 activation curve.

Physiology and pathophysiology of CLC-1: mechanisms of a chloride channel disease, myotonia

The CLC-1 chloride channel, a member of the CLC-channel/transporter family, plays important roles for the physiological functions of skeletal muscles. The opening of this chloride channel is voltage dependent and is also regulated by protons and chloride ions. Mutations of the gene encoding CLC-1 result in a genetic disease, myotonia congenita, which can be inherited as an autosmal dominant (Thomsen type) or an autosomal recessive (Becker type) pattern. These mutations are scattered throughout the entire protein sequence, and no clear relationship exists between the inheritance pattern of the mutation and the location of the mutation in the channel protein. The inheritance pattern of some but not all myotonia mutants can be explained by a working hypothesis that these mutations may exert a “dominant negative” effect on the gating function of the channel. However, other mutations may be due to different pathophysiological mechanisms, such as the defect of protein trafficking to membranes. Thus, the underlying mechanisms of myotonia are likely to be quite diverse, and elucidating the pathophysiology of myotonia mutations will require the understanding of multiple molecular/cellular mechanisms of CLC-1 channels in skeletal muscles, including molecular operation, protein synthesis, and membrane trafficking mechanisms.

Components of neuronal chloride transport in rat and human neocortex

Considerable evidence indicates disturbances in the ionic gradient of GABAA receptor-mediated inhibition of neurones in human epileptogenic tissues. Two contending mechanisms have been proposed, reduced outward and increased inward Cl⁻ transporters. We investigated the properties of Cl⁻ transport in human and rat neocortical neurones (layer II/III) using intracellular recordings in slices of cortical tissue. We measured the alterations in reversal potential of the pharmacologically isolated inhibitory postsynaptic potential mediated by GABAA receptors (IPSPA) to estimate the ionic gradient and kinetics of Cl⁻ efflux after Cl⁻ injections before and during application of selected blockers of Cl⁻ routes (furosemide, bumetanide, 9-anthracene carboxylic acid and Cs+). Neurones from human epileptogenic cortex exhibited a fairly depolarized reversal potential of GABAA receptor-mediated inhibition (EIPSP-A) of -61.9 ± 8.3 mV. In about half of the neurones, the EIPSP-A averaged -55.2 ± 5.7 mV, in the other half, 68.6 ± 2.3 mV, similar to rat neurones (-68.9 ± 2.6 mV). After injections of Cl⁻, IPSPA recovered in human neurones with an average time constant (τ) of 19.0 ± 9.6 s (rat neurones: 7.2 ± 2.4 s). We calculated Cl⁻ extrusion rates (1/τ) via individual routes from the τ values obtained in different experimental conditions, revealing that, for example, the K+-coupled Cl⁻ transporter KCC2 comprises 45.3% of the total rate in rat neurones. In human neurones, the total rate of Cl⁻ extrusion was 63.9% smaller, and rates via KCC2, the Na+-K+-2Cl⁻ transporter NKCC1 and the voltage-gatedCl− channelClCwere smaller than in rat neurones by 80.0%, 61.7% and 79.9%, respectively. The rate via anion exchangers conversely was 14.4% larger in human than in rat neurones. We propose that (i) KCC2 is the major route of Cl⁻ extrusion in cortical neurones, (ii) reduced KCC2 is the initial step of disturbed Cl⁻ regulation and (iii) reductions in KCC2 contribute to depolarizing EIPSP-A of neurones in human epileptogenic neocortex.

Electrophysiological characterization of neuromuscular synaptic dysfunction in mice

Emerging evidence suggests that synaptic dysfunction occurs prior to neuronal loss in neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS). Therefore, monitoring synaptic activity during early stages of neurodegeneration may provide valuable information for the development of diagnostic and/or therapeutic strategies. Here, we describe an electrophysiological method routinely applied in our laboratory for investigating synaptic activity of the neuromuscular junction (NMJ), the synaptic connection between motoneurons and skeletal muscles. Using conventional intracellular sharp electrodes, both spontaneous synaptic activity (miniature end-plate potentials) and evoked synaptic activity (end-plate potentials) can be readily recorded in acutely isolated nerve-muscle preparations. This method can also be adapted to various simulation protocols for studying short-term plasticity of neuromuscular synapses.

Reducing chloride conductance prevents hyperkalaemia-induced loss of twitch force in rat slow-twitch muscle

Exercise-induced loss of skeletal muscle K(+) can seriously impede muscle performance through membrane depolarization. Thus far, it has been assumed that the negative equilibrium potential and large membrane conductance of Cl(-) attenuate the loss of force during hyperkalaemia. We questioned this idea because there is some evidence that Cl(-) itself can exert a depolarizing influence on membrane potential (V(m)). With this study we tried to identify the possible roles played by Cl(-) during hyperkalaemia. Isolated rat soleus muscles were kept at 25 degrees C and twitch contractions were evoked by current pulses. Reducing [Cl(-)](o) to 5 mM, prior to introducing 12.5 mM K(o), prevented the otherwise occurring loss of force. Reversing the order of introducing these two solutions revealed an additional effect, i.e. the ongoing hyperkalaemia-related loss of force was sped up tenfold after reducing [Cl(-)](o). However, hereafter twitch force recovered completely. The recovery of force was absent at [K(+)](o) exceeding 14 mM. In addition, reducing [Cl(-)](o) increased membrane excitability by 24%, as shown by a shift in the relationship between force and current level. Measurements of V(m) indicated that the antagonistic effect of reducing [Cl(-)](o) on hyperkalaemia-induced loss of force was due to low-Cl(-)-induced membrane hyperpolarization. The involvement of specific Cl(-) conductance was established with 9-anthracene carboxylic acid (9-AC). At 100 microm, 9-AC reduced the loss of force due to hyperkalaemia, while at 200 microm, 9-AC completely prevented loss of force. To study the role of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) in this matter, we added 400 microm of the NKCC inhibitor bumetanide to the incubation medium. This did not affect the hyperkalaemia-induced loss of force. We conclude that Cl(-) exerts a permanent depolarizing influence on V(m). This influence of Cl(-) on V(m), in combination with a large membrane conductance, can apparently have two different effects on hyperkalaemia-induced loss of force. It might exert a stabilizing influence on force production during short periods of hyperkalaemia, but it can add to the loss of force during prolonged periods of hyperkalaemia.

A quantitative analysis of cell volume and resting potential determination and regulation in excitable cells

This paper quantifies recent experimental results through a general physical description of the mechanisms that might control two fundamental cellular parameters, resting potential (Em) and cell volume (Vc), thereby clarifying the complex relationships between them. Em was determined directly from a charge difference (CD) equation involving total intracellular ionic charge and membrane capacitance (Cm). This avoided the equilibrium condition dEm/dt = 0 required in determinations of Em by previous work based on the Goldman-Hodgkin-Katz equation and its derivatives and thus permitted precise calculation of Em even under non-equilibrium conditions. It could accurately model the influence upon Em of changes in Cm or Vc and of membrane transport processes such as the Na+-K+-ATPase and ion cotransport. Given a stable and adequate membrane Na+-K+-ATPase density (N), Vc and Em both converged to unique steady-state values even from sharply divergent initial intracellular ionic concentrations. For any constant set of transmembrane ion permeabilities, this set point of Vc was then determined by the intracellular membrane-impermeant solute content (X-i) and its mean charge valency (zX), while in contrast, the set point of Em was determined solely by zX. Independent changes in membrane Na+ (PNa) or K+ permeabilities (PK) or activation of cation-chloride cotransporters could perturb Vc and Em but subsequent reversal of such changes permitted full recovery of both Vc and Em to the original set points. Proportionate changes in PNa, PK and N, or changes in Cl- permeability (PCl) instead conserved steady-state Vc and Em but altered their rates of relaxation following any discrete perturbation. PCl additionally determined the relative effect of cotransporter activity on Vc and Em, in agreement with recent experimental results. In contrast, changes in Xi- produced by introduction of a finite permeability term to X- (PX) that did not alter zX caused sustained changes in Vc that were independent of Em and that persisted when PX returned to zero. Where such fluxes also altered the effective zX they additionally altered the steady state Em. This offers a basis for the suggested roles of amino acid fluxes in long-term volume regulatory processes in a variety of excitable tissues.

Protective role of extracellular chloride in fatigue of isolated mammalian skeletal muscle

A possible role of extracellular Cl(-) concentration ([Cl(-)](o)) in fatigue was investigated in isolated skeletal muscles of the mouse. When [Cl(-)](o) was lowered from 128 to 10 mM, peak tetanic force was unchanged, fade was exacerbated (wire stimulation electrodes), and a hump appeared during tetanic relaxation in both nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles. Low [Cl(-)](o) increased the rate of fatigue 1) with prolonged, continuous tetanic stimulation in soleus, 2) with repeated intermittent tetanic stimulation in soleus or EDL, and 3) to a greater extent with repeated tetanic stimulation when wire stimulation electrodes were used rather than plate stimulation electrodes in soleus. In nonfatigued soleus muscles, application of 9 mM K(+) with low [Cl(-)](o) caused more rapid and greater tetanic force depression, along with greater depolarization, than was evident at normal [Cl(-)](o). These effects of raised [K(+)](o) and low [Cl(-)](o) were synergistic. From these data, we suggest that normal [Cl(-)](o) provides protection against fatigue involving high-intensity contractions in both fast- and slow-twitch mammalian muscle. This phenomenon possibly involves attenuation of the depolarization caused by stimulation- or exercise-induced run-down of the transsarcolemmal K(+) gradient.

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.

Chloride in smooth muscle

Interest in the functions of intracellular chloride expanded about twenty years ago but mostly this referred to tissues other than smooth muscle. On the other hand, accumulation of chloride above equilibrium seems to have been recognised more readily in smooth muscle. Experimental data is used to show by calculation that the Donnan equilibrium cannot account for the chloride distribution in smooth muscle but it can in skeletal muscle. The evidence that chloride is normally above equilibrium in smooth muscle is discussed and comparisons are made with skeletal and cardiac muscle. The accent is on vascular smooth muscle and the mechanisms of accumulation and dissipation. The three mechanisms by which chloride can be accumulated are described with some emphasis on calculating the driving forces, where this is possible. The mechanisms are chloride/bicarbonate exchange, (Na+K+Cl) cotransport and a novel entity, "pump III", known only from own work. Their contributions to chloride accumulation vary and appear to be characteristic of individual smooth muscles. Thus, (Na+K+Cl) always drives chloride inwards, chloride/bicarbonate exchange is always present but does not always do it and "pump III" is not universal. Three quite different biophysical approaches to assessing chloride permeability are considered and the calculations underlying them are worked out fully. Comparisons with other tissues are made to illustrate that low chloride permeability is a feature of smooth muscle. Some of the functions of the high intracellular chloride concentrations are considered. This includes calculations to illustrate its depolarising influence on the membrane potential, a concept which, experience tells us, some people find confusing. The major topic is the role of chloride in the regulation of smooth muscle contractility. Whilst there is strong evidence that the opening of the calcium-dependent chloride channel leads to depolarisation, calcium entry and contraction in some smooth muscles, it appears that chloride serves a different function in others. Thus, although activation and inhibition of (Na+K+Cl) cotransport is associated with contraction and relaxation respectively, the converse association of inhibition and contraction has been seen. Nevertheless, inhibition of chloride/bicarbonate exchange and "pump III" and stimulation of (K+Cl) cotransport can all cause relaxation and this suggests that chloride is always involved in the contraction of smooth muscle. The evidence that (Na+K+Cl) cotransport more active in experimental hypertension is discussed. This is a common but not universal observation. The information comes almost exclusively from work on cultured cells, usually from rat aorta. Nevertheless, work on smooth muscle freshly isolated from hypertensive rats confirms that (Na+K+Cl) cotransport is activated in hypertension but there are several other differences, of which the depolarisation of the membrane potential may be the most important.Finally, a simple calculation is made which indicates as much as 40% of the energy put into the smooth muscle cell membrane by the sodium pump is necessary to drive (Na+K+Cl) cotransport. Notwithstanding the approximations in this calculation, this suggests that chloride accumulation is energetically expensive. Presumably, this is related to the apparently universal role of chloride in contraction.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Effects of DIDS, a disulfonic stilbene derivative, on chloride movements in toad skeletal muscles

In order to investigate the characteristics of the movement of Cl- ions in toad skeletal muscles we decided to study the relative membrane permeabilities of chloride and nitrate and the effects of DIDS (4,4'-diisothyocyanatostilbene-2,2'-disulphonate) upon the hyperpolarizations produced in muscle fibers when chloride or nitrate ions rapidly replace impermeant sulphate ions in the external solution. For experiments where membrane potential changes were recorded in response to sudden changes in extracellular solutions, small bundles from the semitendinosus muscles were used. We showed that DIDS reduced in a reversible manner the Cl- permeability (pCl) in toad skeletal muscle fibers. The results supporting this conclusion were the following. First, a diminished hyperpolarization in response to a sudden exposure of the fibers to a solution containing Cl-. In these experiments DIDS reduced the pCl/pK ratio to 5.5 from a control value of 12. Second, a smaller transient of the resting potential when [Cl]o was changed from 120 to 30 mM and vice versa.

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.

Use of ion channel blockers in the exploration of possible mechanisms involved in the myopathy of diabetic mice

Changes in the muscle contractions of the phrenic nerve-diaphragm preparation from the diabetic mouse were investigated by means of K(+)- and Cl(-)-channel blockers and the Ca(2+)-mobilizing agent, selenite. The K(+)-channel blockers (UO2(2+) and 4-aminopyridine) cooperated synergistically with the Cl(-)-channel blockers (Cd2+ and 9-anthracenecarboxylic acid) in increasing normal muscle contraction as described previously, but failed to induce this effect in the diaphragm of the diabetic mouse. Treatment with a Cl(-)-channel blocker alone in 0.25 mmol/l Ca2+ Krebs solution induced a myotonic activity accompanied by stimulus-bound repetitive action potential firings. This effect was also diminished in the diaphragm from diabetic mice. The membrane potential of the muscle cells in the diaphragm of the diabetic mouse was slightly but significantly decreased. The membrane input resistance was also increased and was refractory to being further increased by either a Cl(-)-channel blocker or a low Cl(-)-medium. Furthermore, the membrane chloride conductance was found to be decreased, but the membrane K+ conductance remained unchanged in the muscle from diabetic mice. These changes of membrane properties in the muscles from diabetic mice were shown to be similar to those induced by either Cl(-)-channel blockers or a low Cl(-)-medium. In addition, the combined treatment of the diaphragm from diabetic mice with Cd2+ plus UO2(2+) in 0.25 mmol/l Ca2+ Krebs solution and then stepwise replenishment of Ca2+ led to a greater restoration of muscle contractions at a lower cumulative Ca2+ concentration than that was found with the normal diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-dependent heat production by rat skeletal muscle in hypertonic media depends on Na(+)-Cl- co-transport stimulation

1. The rate of energy dissipation (E) in isolated, superfused soleus muscles from young rats was continuously measured under normosmotic and 100-mosM hyperosmotic conditions. The substantial increase of E with respect to basal level in hyperosmolarity (excess E), which is entirely dependent on the presence of extracellular sodium, was largely prevented or inhibited by bumetanide, a potent inhibitor of Na(+)-Cl- co-transport system, or by the removal of chloride from the superfusate (isethionate substitution). Bumetanide or the removal of chloride also acutely decreased basal E, by about 7%. 2. Bumetanide almost entirely suppressed the major, Ca(2+)-dependent part of excess E in hyperosmolarity, as well as the concomitant increase of 45Ca2+ efflux and small increase in resting muscle tension; in contrast, the part of excess E associated with stimulation of Na(+)-H+ exchange in hyperosmolarity was left unmodified. 3. Reduction of 22Na+ influx by bumetanide was more marked in hyperosmolarity than under control conditions, although stimulation of total 22Na+ influx by a 100-mosM stress was not statistically significant. Inhibition of Ca2+ release into the sarcoplasm using dantrolene sodium did not prevent the stimulation of bumetanide-sensitive 22Na+ influx, but rather increased it about fourfold. 4. It is concluded that the largest part of excess E in hyperosmolarity, which is Ca(2+)-dependent energy expenditure, is suppressed when steady-state stimulation of a Na(+)-Cl- co-transport system is inhibited either directly by bumetanide or the removal of extracellular chloride, or indirectly by the blocking of active Na(+)-K+ transport. How the stimulation of Na(+)-Cl- co-transport, by as little as 1 nmol s-1 (g wet muscle weight)-1 during a 100-mosM stress, enhances Ca(2+)-dependent heat by as much as 2.5 mW (g wet muscle weight)-1 remains to be clarified.

Blockers of potassium current and resting membrane potential in rat muscle fibers

Rat diaphragm fibers were equilibrated for several hours in 150 mM KCl; when they were returned to 5 mM KCl the resting potential went back to its original level with a half time of 17 min. This repolarization was blocked by 5 mM BaCl2, a blocker of the inward rectifier K channel. On the other hand, 0.1 mM apamin and 0.02 mM glibenclamide which block the Ca-dependent and ATP sensitive K channels, respectively, and 0.1 mM 9-AC a blocker of the Cl- channel did not affect the repolarization. 5 mM barium decreased the K conductance measured under current-clamp conditions in diaphragm muscle fibers. The possible role of the inward rectifier system in the repolarization following return to normal [K]o is discussed.

Use of ion channel blockers in studying the regulation of skeletal muscle contractions

Effects of K(+)- and Cl(-)-channel blockers on the muscle contraction of mouse diaphragm in response to direct electrical muscle stimulation were studied. K(+)-channel blockers (0.1-1 mmol/l 4-aminopyridine, 0.4-1.2 mmol/l uranyl nitrate and 2-30 mmol/l tetraethylammonium chloride) and a Cl(-)-channel blocker (0.01-0.03 mmol/l 9-anthracene carboxylic acid) increased the contractile amplitudes in a limited extent not to exceed over 50% of control. However, the sequential applications of two different channel blockers at a rather low concentration markedly increased the contractile responses mostly over 300% of control except the combination of 4-aminopyridine and uranyl nitrate. It appears that two K(+)-channel blockers synergistically exerted their effects rather than additionally in the regulation of muscle contractions. Investigation on the possible mechanism of the synergistic action of K(+)-channel blockers suggested that prolongation of action potential durations was in a linear correlation with the increased contractions. On the other hand, the contractile potentiation induced by combination of K(+)- and Cl(-)-channel blockers was attributed to the production of repetitive action potential firings (150 +/- 12 Hz) upon a single electrical stimulation. Similar to Cl(-)-channel blocker, low Cl- as well as low Ca2+ enhanced K(+)-channel blockers in producing contractile potentiation accompanied with stimulus-bound repetitive discharges. Tetrodotoxin at a concentration of 0.03 mumol/l which did not affect the twitches evoked by electrical stimulations completely inhibited the contractile potentiation induced by the combined application of K(+)- and Cl(-)-channel blockers.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of ion channels on the surface membrane of adult rat skeletal muscle

The channels present on the surface membrane of isolated rat flexor digitorum brevis muscle fibers were surveyed using the patch clamp technique. 85 out of 139 fibers had a novel channel which excluded the anions chloride, sulfate, and isethionate with a permeability ratio of chloride to sodium of less than 0.05. The selectivity sequence for cations was Na+ = K+ = Cs+ greater than Ca++ = Mg++ greater than N-Methyl-D-Glucamine. The channel remained closed for long periods, and had a large conductance of approximately 320 pS with several subconductance states at approximately 34 pS levels. Channel activity was not voltage dependent and the reversal potential for cations in muscle fibers of approximately 0 mV results in the channel's behaving as a physiological leakage conductance. Voltage activated potassium channels were present in 65 of the cell attached patches and had conductances of mostly 6, 12, and 25 pS. The voltage sensitivity of the potassium channels was consistent with that of the delayed rectifier current. Only three patches contained chloride channels. The scarcity of chloride channels despite the known high chloride conductance of skeletal muscle suggests that most of the chloride channels must be located in the transverse tubular system.

Rapid co-transport of sodium and chloride ions in giant salivary gland cells of the leech Haementeria ghilianii

1. Double-barrelled Cl(-)-selective microelectrodes were used to measure the apparent intracellular Cl- activity (aiCl) and membrane potential (Em) of leech salivary gland cells. In standard physiological solution buffered with HEPES (10 mM), intracellular Cl- activity (corrected for interference) was 38 +/- 8 mM (n = 11) compared to a value of 12.8 mM expected for passive Cl- distribution. The mean Em was -49.4 +/- 8.2 mV (n = 21) which was about 27 mV negative to the Cl- equilibrium potential. 2. Removal of external Cl- led to a slow fall in aiCl until a steady-state level of 4-11 mM was reached in 30-60 min. Recovery of aiCl on readdition of external Cl- took only 2-3 min. The uptake followed an exponential time course having a single rate constant of 1.73 +/- 0.1 min-1 (n = 5) whereas the loss appeared to occur in two phases. Changes in external Cl- produced immediate changes in Em which were the opposite of those expected for a high Cl- permeability, i.e. Cl- removal produced an immediate hyperpolarization (3-18 mV) and readdition of Cl- produced a transient depolarization (5-22 mV). 3. The intracellular accumulation of Cl- was dependent on the external Cl- activity. Even when the external Cl- concentration was reduced to 3%, the cells accumulated Cl- against an electrochemical gradient. 4. Cl- accumulation was temperature sensitive (Q10 approximately 2). 5. On removal of external Na+, aiCl fell to a level which was close to that expected for passive distribution. The active reaccumulation of Cl-, after intracellular Cl- depletion, was abolished in the absence of external Na+; aiCl slowly increased to its passive level. Steady-state aiCl or its recovery by Cl(-)-depleted cells was not affected by the absence of K+ in the bathing solution. 6. The reaccumulation of Cl- was not affected by furosemide (1-5 mM), bumetanide (10(-4) M), amiloride (10(-3) M) or 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS, 10(-4) M). 7. Removal of external Cl- caused a fall in intracellular Na+ activity (aiNa, measured with Na(+)-selective microelectrodes) from 15.9 +/- 6.8 mM (n = 9) to 2.5 +/- 1.3 mM (n = 3). When external Cl- was readded, aiNa rose to 46.5 +/- 6.6 mM (n = 3) before slowly recovering towards its original value. The maximal change in aiNa was 41.7 +/- 4.5 mM (n = 3) and the rate constant for Na+ uptake was 1.8 +/- 0.4 min-1 (n = 3).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of Na+ and K+ on Cl- distribution in guinea-pig vas deferens smooth muscle: evidence for Na+, K+, Cl- co-transport

1. Smooth muscle cells of the guinea-pig vas deferens after Cl- depletion actively reaccumulate ions to a level many times higher than that predicted by a passive distribution, even when anion exchange (largely responsible for Cl- movements in this preparation) is inhibited by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid). The cells therefore must possess a second mechanism for Cl- accumulation. We have now investigated the ionic requirement of this mechanism using a combination of ion analysis, 36Cl fluxes and direct measurement of the intracellular Cl- activity (aiCl). 2. In the steady state, the Cl- content of tissues was 12-16% less in Na(+)-free solution than in normal Krebs solution. 3. Loss of 36Cl into Cl(-)-free solution was slowed by the absence of Na+ and accelerated on its readdition. Uptake of 36Cl by Cl(-)-depleted tissues was also reduced in the absence of extracellular Na+, particularly at longer time intervals as uptake reached completion. These effects occurred in the presence and absence of CO2-HCO3- and in the presence of DIDS. 4. The initial rate of rise of aiCl on readdition of Cl- to Cl(-)-depleted cells was not significantly affected by the absence of Na+ in the presence of a functional anion exchange, but aiCl stabilized at a lower value than in normal solution. Readdition of Na+ stimulated a rise in aiCl to the control level. Removal and readdition of K+ under these conditions had negligible effects. 5. When anion exchange was inhibited by the presence of DIDS, removal and readdition of Na+ caused, respectively, a marked inhibition and stimulation of the rise in aiCl during Cl- reaccumulation. Under these conditions removal and readdition of K+ had similar effects. 6. The results suggest that Na+, K+, Cl- co-transport is involved in transmembrane movements of Cl- at least when the anion exchange mechanism is blocked. 7. The possibility that the marked effects of changes in external Na+ on transmembrane Cl- movements in the presence of a functional anion exchange mechanism are caused by secondary effects due to changes in intracellular pH as well as to suppression of Na+, K+, Cl- co-transport is discussed.

Intracellular chloride activity in glial cells of the leech central nervous system

1. Chloride-sensitive double-barrelled microelectrodes were used to measure the intracellular Cl- activity (aicl) and membrane potential (Em) in neuropile glial cells of the leech, Hirudo medicinalis. 2. A close relation between the equilibrium potential for Cl- (ECl = -66.1 +/- 4.9 mV; mean +/- S.D.) and the resting potential (Em = -67.8 +/- 5.2 mV; n = 19) was observed in nominally CO2-HCO3(-)-free, HEPES-buffered solutions. A saline buffered with 2% CO2, 11 mM-HCO3- elicited a membrane hyperpolarization and a concomitant decrease of aCl. 3. Changes in ECl followed these of Em with a lag of less than 30 s in response to various extracellular K+ concentrations [( K+]o) or due to bath-application of carbachol or serotonin. 4. Introduction of a Cl(-)-free solution resulted in a transient depolarization indicating a substantial Cl- conductance and a rapid decrease of aiCl to an apparent value of 0.5-0.9 mM. 5. The loop diuretics furosemide (1 mM) and bumetanide (0.2 mM) did not affect the K(+)-induced changes of aiCl. 6. The results indicate a passive Cl- distribution across the membrane of leech neuropile glial cells as a result of a high Cl- conductance.