Alterations in triad ultrastructure following repetitive stimulation and intracellular changes associated with exercise in amphibian skeletal muscle

Nernst-Planck-Gaussian finite element modelling of Ca2+ electrodiffusion in amphibian striated muscle transverse tubule-sarcoplasmic reticular triadic junctional domains

Introduction

Intracellular Ca2+ signalling regulates membrane permeabilities, enzyme activity, and gene transcription amongst other functions. Large transmembrane Ca2+ electrochemical gradients and low diffusibility between cell compartments potentially generate short-lived, localised, high-[Ca2+] microdomains. The highest concentration domains likely form between closely apposed membranes, as at amphibian skeletal muscle transverse tubule-sarcoplasmic reticular (T-SR, triad) junctions.

Materials and methods

Finite element computational analysis characterised the formation and steady state and kinetic properties of the Ca2+ microdomains using established empirical physiological and anatomical values. It progressively incorporated Fick diffusion and Nernst-Planck electrodiffusion gradients, K+, Cl-, and Donnan protein, and calmodulin (CaM)-mediated Ca2+ buffering. It solved for temporal-spatial patterns of free and buffered Ca2+, Gaussian charge differences, and membrane potential changes, following Ca2+ release into the T-SR junction.

Results

Computational runs using established low and high Ca2+ diffusibility (D Ca2+) limits both showed that voltages arising from intracytosolic total [Ca2+] gradients and the counterions little affected microdomain formation, although elevated D Ca2+ reduced attained [Ca2+] and facilitated its kinetics. Contrastingly, adopting known cytosolic CaM concentrations and CaM-Ca2+ affinities markedly increased steady-state free ([Ca2+]free) and total ([Ca2+]), albeit slowing microdomain formation, all to extents reduced by high D Ca2+. However, both low and high D Ca2+ yielded predictions of similar, physiologically effective, [Ca2+-CaM]. This Ca2+ trapping by the relatively immobile CaM particularly increased [Ca2+] at the junction centre. [Ca2+]free, [Ca2+-CaM], [Ca2+], and microdomain kinetics all depended on both CaM-Ca2+ affinity and D Ca2+. These changes accompanied only small Gaussian (∼6 mV) and surface charge (∼1 mV) effects on tubular transmembrane potential at either D Ca2+.

Conclusion

These physical predictions of T-SR Ca2+ microdomain formation and properties are compatible with the microdomain roles in Ca2+ and Ca2+-CaM-mediated signalling but limited the effects on tubular transmembrane potentials. CaM emerges as a potential major regulator of both the kinetics and the extent of microdomain formation. These possible cellular Ca2+ signalling roles are discussed in relation to possible feedback modulation processes sensitive to the μM domain but not nM bulk cytosolic, [Ca2+]free, and [Ca2+-CaM], including ryanodine receptor-mediated SR Ca2+ release; Na+, K+, and Cl- channel-mediated membrane excitation and stabilisation; and Na+/Ca2+ exchange transport.

Finite element analysis predicts Ca2+ microdomains within tubular-sarcoplasmic reticular junctions of amphibian skeletal muscle

A finite element analysis modelled diffusional generation of steady-state Ca²⁺ microdomains within skeletal muscle transverse (T)-tubular-sarcoplasmic reticular (SR) junctions, sites of ryanodine receptor (RyR)-mediated SR Ca²⁺ release. It used established quantifications of sarcomere and T-SR anatomy (radial diameter [Formula: see text]; axial distance [Formula: see text]). Its boundary SR Ca²⁺ influx densities,[Formula: see text], reflected step impositions of influxes, [Formula: see text] deduced from previously measured Ca²⁺ signals following muscle fibre depolarization. Predicted steady-state T-SR junctional edge [Ca²⁺], [Ca²⁺]_edge, matched reported corresponding experimental cytosolic [Ca²⁺] elevations given diffusional boundary efflux [Formula: see text] established cytosolic Ca²⁺ diffusion coefficients [Formula: see text] and exit length [Formula: see text]. Dependences of predicted [Ca²⁺]_edge upon [Formula: see text] then matched those of experimental [Ca²⁺] upon Ca²⁺ release through their entire test voltage range. The resulting model consistently predicted elevated steady-state T-SR junctional ~ µM-[Ca²⁺] elevations radially declining from maxima at the T-SR junction centre along the entire axial T-SR distance. These [Ca²⁺] heterogeneities persisted through 10⁴- and fivefold, variations in D and w around, and fivefold reductions in d below, control values, and through reported resting muscle cytosolic [Ca²⁺] values, whilst preserving the flux conservation ([Formula: see text] condition, [Formula: see text]. Skeletal muscle thus potentially forms physiologically significant ~ µM-[Ca²⁺] T-SR microdomains that could regulate cytosolic and membrane signalling molecules including calmodulin and RyR, These findings directly fulfil recent experimental predictions invoking such Ca²⁺ microdomains in observed regulatory effects upon Na⁺ channel function, in a mechanism potentially occurring in similar restricted intracellular spaces in other cell types.

Sarcoplasmic reticular Ca2+-ATPase inhibition paradoxically upregulates murine skeletal muscle Nav1.4 function

Skeletal muscle Na⁺ channels possess Ca²⁺- and calmodulin-binding sites implicated in Nav1.4 current (I_Na) downregulation following ryanodine receptor (RyR1) activation produced by exchange protein directly activated by cyclic AMP or caffeine challenge, effects abrogated by the RyR1-antagonist dantrolene which itself increased I_Na. These findings were attributed to actions of consequently altered cytosolic Ca²⁺, [Ca²⁺]_i, on Na_v1.4. We extend the latter hypothesis employing cyclopiazonic acid (CPA) challenge, which similarly increases [Ca²⁺]_i, but through contrastingly inhibiting sarcoplasmic reticular (SR) Ca²⁺-ATPase. Loose patch clamping determined Na⁺ current (I_Na) families in intact native murine gastrocnemius skeletal myocytes, minimising artefactual [Ca²⁺]_i perturbations. A bespoke flow system permitted continuous I_Na comparisons through graded depolarizing steps in identical stable membrane patches before and following solution change. In contrast to the previous studies modifying RyR1 activity, and imposing control solution changes, CPA (0.1 and 1 µM) produced persistent increases in I_Na within 1-4 min of introduction. CPA pre-treatment additionally abrogated previously reported reductions in I_Na produced by 0.5 mM caffeine. Plots of peak current against voltage excursion demonstrated that 1 µM CPA increased maximum I_Na by ~ 30%. It only slightly decreased half-maximal activating voltages (V_0.5) and steepness factors (k), by 2 mV and 0.7, in contrast to the V_0.5 and k shifts reported with direct RyR1 modification. These paradoxical findings complement previously reported downregulatory effects on Nav1.4 of RyR1-agonist mediated increases in bulk cytosolic [Ca²⁺]. They implicate possible local tubule-sarcoplasmic triadic domains containing reduced [Ca²⁺]_TSR in the observed upregulation of Nav1.4 function following CPA-induced SR Ca²⁺ depletion.

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.

The determinants of transverse tubular volume in resting skeletal muscle

The transverse tubular (t)-system of skeletal muscle couples sarcolemmal electrical excitation with contraction deep within the fibre. Exercise, pathology and the composition of the extracellular fluid (ECF) can alter t-system volume (t-volume). T-volume changes are thought to contribute to fatigue, rhabdomyolysis and disruption of excitation–contraction coupling. However, mechanisms that underlie t-volume changes are poorly understood. A multicompartment, history-independent computer model of rat skeletal muscle was developed to define the minimum conditions for t-volume stability. It was found that the t-system tends to swell due to net ionic fluxes from the ECF across the access resistance. However, a stable t-volume is possible when this is offset by a net efflux from the t-system to the cell and thence to the ECF, forming a net ion cycle ECF→t-system→sarcoplasm→ECF that ultimately depends on Na⁺/K⁺-ATPase activity. Membrane properties that maximize this circuit flux decrease t-volume, including P_Na(t) > P_Na(s), P_K(t) < P_K(s) and N_(t) < N_(s) [P, permeability; N, Na⁺/K⁺-ATPase density; (t), t-system membrane; (s), sarcolemma]. Hydrostatic pressures, fixed charges and/or osmoles in the t-system can influence the magnitude of t-volume changes that result from alterations in this circuit flux. Using a parameter set derived from literature values where possible, this novel theory of t-volume was tested against data from previous experiments where t-volume was measured during manipulations of ECF composition. Predicted t-volume changes correlated satisfactorily. The present work provides a robust, unifying theoretical framework for understanding the determinants of t-volume.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

Local calcium signals induced by hyper-osmotic stress in mammalian skeletal muscle cells

Strenuous activitiy of skeletal muscle leads to temporary osmotic dysbalance and isolated skeletal muscle fibers exposed to osmotic stress respond with characteristic micro-domain calcium signals. It has been suggested that osmotic stress targets transverse tubular (TT) dihydropyridine receptors (DHPRs) which normally serve as voltage-dependent activators of Ca release via ryanodine receptor (RyR1s) of the sarcoplasmic reticulum (SR). Here, we pursued this hypothesis by imaging the response to hyperosmotic solutions in both mouse skeletal muscle fibers and myotubes. Ca fluctuations in the cell periphery of fibers exposed to osmotic stress were accompanied by a substantial dilation of the peripheral TT. The Ca signals were completely inhibited by a conditioning depolarization that inactivates the DHPR. Dysgenic myotubes, lacking the DHP-receptor-alpha1-subunit, showed strongly reduced, yet not completely inhibited activity when stimulated with solutions of elevated tonicity. The results point to a modulatory, even though not essential, role of the DHP receptor for osmotic stress-induced Ca signals in skeletal muscle.

Control of cell volume in skeletal muscle

Regulation of cell volume is a fundamental property of all animal cells and is of particular importance in skeletal muscle where exercise is associated with a wide range of cellular changes that would be expected to influence cell volume. These complex electrical, metabolic and osmotic changes, however, make rigorous study of the consequences of individual factors on muscle volume difficult despite their likely importance during exercise. Recent charge-difference modelling of cell volume distinguishes three major aspects to processes underlying cell volume control: (i) determination by intracellular impermeant solute; (ii) maintenance by metabolically dependent processes directly balancing passive solute and water fluxes that would otherwise cause cell swelling under the influence of intracellular membrane-impermeant solutes; and (iii) volume regulation often involving reversible short-term transmembrane solute transport processes correcting cell volumes towards their normal baselines in response to imposed discrete perturbations. This review covers, in turn, the main predictions from such quantitative analysis and the experimental consequences of comparable alterations in extracellular pH, lactate concentration, membrane potential and extracellular tonicity. The effects of such alterations in the extracellular environment in resting amphibian muscles are then used to reproduce the intracellular changes that occur in each case in exercising muscle. The relative contributions of these various factors to the control of cell volume in resting and exercising skeletal muscle are thus described.