Ca2+-dependent slow action potentials in normal and dystrophic mouse skeletal muscle

Dysferlin and myoferlin regulate transverse tubule formation and glycerol sensitivity

Dysferlin is a membrane-associated protein implicated in muscular dystrophy and vesicle movement and function in muscles. The precise role of dysferlin has been debated, partly because of the mild phenotype in dysferlin-null mice (Dysf). We bred Dysf mice to mice lacking myoferlin (MKO) to generate mice lacking both myoferlin and dysferlin (FER). FER animals displayed progressive muscle damage with myofiber necrosis, internalized nuclei, and, at older ages, chronic remodeling and increasing creatine kinase levels. These changes were most prominent in proximal limb and trunk muscles and were more severe than in Dysf mice. Consistently, FER animals had reduced ad libitum activity. Ultrastructural studies uncovered progressive dilation of the sarcoplasmic reticulum and ectopic and misaligned transverse tubules in FER skeletal muscle. FER muscle, and Dysf- and MKO-null muscle, exuded lipid, and serum glycerol levels were elevated in FER and Dysf mice. Glycerol injection into muscle is known to induce myopathy, and glycerol exposure promotes detachment of transverse tubules from the sarcoplasmic reticulum. Dysf, MKO, and FER muscles were highly susceptible to glycerol exposure in vitro, demonstrating a dysfunctional sarcotubule system, and in vivo glycerol exposure induced severe muscular dystrophy, especially in FER muscle. Together, these findings demonstrate the importance of dysferlin and myoferlin for transverse tubule function and in the genesis of muscular dystrophy.

Stimulation pulse characteristics and electrode configuration determine site of excitation in isolated mammalian skeletal muscle: implications for fatigue

We examined whether electrical field stimulation with varying characteristics could excite isolated mammalian skeletal muscle through different sites. Supramaximal (20-V, 0.1-ms) pulse stimulation with transverse wire or parallel plate electrodes evoked similar forces in nonfatigued slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles from mice. d-tubocurarine shifted the twitch force-stimulation strength relationship toward higher pulse strengths with both electrode configurations in soleus muscle, suggesting that weaker pulses excite muscle via neuromuscular transmission. With wire stimulation, movement of the recording electrode along the muscle caused a delay between the stimulus artifact and the peak of the action potential, consistent with action potential propagation along the sarcolemma. TTX abolished all contractions evoked with 20-V, 0.1-ms pulses, suggesting that excitation occurred via voltage-dependent Na+ channels and, hence, muscle action potentials. TTX did not prevent force development with ≥0.4-ms pulses in soleus or 1-ms pulses in EDL muscle. Furthermore, myoplasmic Ca2+ (i.e., the fura 2 ratio) and sarcomere shortening were greater during tetanic stimulation with 2.0-ms than with 0.5-ms pulses in flexor digitorum brevis fibers from rats. TTX prevented all shortening and Ca2+ release with 0.5-ms, but not 2.0-ms, pulses, indicating that longer pulses can directly trigger Ca2+ release. Hence, proper interpretation of mechanistic studies requires precise understanding of how muscles are excited; otherwise, incorrect conclusions can be made. Using this new understanding, we showed that disrupted propagation of action potentials along the surface membrane is a major cause of fatigue in soleus muscle that is focally and continuously stimulated at 125 Hz.

Calcium action potentials in innervated and denervated rat muscle fibres

We studied the generation of calcium action potentials (Ca APs) in innervated and denervated fibres of the extensor digitorum longus of the rat in a tetraethylammonium (TEA) sulphate solution plus 3-4 diaminopiridine (3-4 DAP). The main results are the following: (1) more than 90% of the innervated fibres were capable of developing well-sustained Ca APs that were blocked by Cd or nifedipine; (2) the incidence of Ca APs obtained from the denervated fibres was substantially lower than in the control preparations; (3) no relation was found between the appearance of Ca APs in the denervated fibres and the resting membrane potential (Vm), stimulus duration (500-2000 ms) or holding potential (-80, -100 mV); (4) The percentage of denervated fibres that exhibited Ca APs was increased significantly with the following procedures. First, by raising the external Ca concentration to 14 mM; second, by depleting the intracellular K concentration by overnight exposure of the muscles to a free K-Cs solution; (c) and third, by incubating the muscles in 500 nM apamin, a venom that inhibits the K conductance activated by Ca. Several factors may be involved in the lower incidence of Ca APs obtained in denervated fibres: (1) a diminished Ca current due to a reduction in the driving force as a result of an increment in the intracellular Ca concentration; (2) a persistence of a shunting K conductance that is not inhibited by TEA and 3-4 DAP; (3) a shift in the voltage dependence of the activation and inactivation parameters of the Ca current or the appearance of a new type of Ca channel with a different kinetics.

Oxygen consumption stimulated by increases in permeability of Na+ of mouse diaphragm muscle in vitro

1. The effects on oxygen consumption of agents that modify Na(+)-permeability were examined in mouse diaphragm muscle perfused with a bathing solution that contained (+)-tubocurarine in a flow-through mode. Twitch tension and levels of Na+ and K+ in the muscle were also measured. 2. Unstimulated preparations decreased the concentration of oxygen in the bathing solution, indicating a basal level of oxygen consumption. Electrical stimulation of the muscle further decreased the concentration of oxygen. Potassium cyanide eliminated both the basal and the stimulated consumption of oxygen. 3. Veratridine facilitated the effect of stimulation at 0.1 Hz on both the consumption of oxygen and the twitch tension. Ouabain antagonized those effects. 4. Twitch contractions were blocked in the presence of dantrolene sodium or by pretreatment with ethylene glycol. Electrical stimulation of such preparations still caused a residual but considerably decreased consumption of oxygen. Ouabain and tetrodotoxin reduced the residual consumption of oxygen. 5. Ouabain significantly increased the levels of Na+ in the tissue, while veratridine alone did not. The effect of ouabain was further potentiated by the simultaneous presence of veratridine. 6. These results indicate that the enhancement of the sarcolemmal permeability to Na+ increases the rate oxygen consumption. This concept supports the hypothesis that Na+ homeostasis depends on energy consumption.

Ca-dependent slow action potentials in neuromuscular diseases

Slow Ca-dependent action potentials were studied in skeletal muscle fibers from different Neuromuscular Diseases (NMD). Biopsies were obtained from: 3 myopathies [Fascioscapulohumeral Dystrophy (FSH) and Polymyositis (PM)], 6 patients with other diseases (CD) [Amyotrophic Lateral Sclerosis (ALS), Central Core Disease, Mitochondrial Myopathy, Polyneuritis (PN), von Eulenberg's Paramyotonia], and 8 normal control muscles. Experiments were carried out in muscle fibers under current-clamp conditions. Membrane currents other than Ca ones were abolished or greatly diminished. Muscle fibers produced any of 3 types of responses, when stimulated by depolarizing pulses: fully developed Ca-action potentials (CaAP), abortive non-regenerative Ca responses (NrR), or only capacitive passive responses (WR). The 3 types of responses were not dependent on the basal conditions of the fibers. The frequency of observation of CaAPs was significantly higher in myopathic disease. In myopathies, 46% of the muscle fibers had CaAPs, while only 22% of fibers from CD and 15% of the fibers from normal muscles showed CaAPs. No differences were observed in the resting constants as well as in the CaAPs parameters between normal and diseased muscle fibers.

Ca-dependent slow action potentials in human skeletal muscle

Slow Ca-action potentials (CaAP) were studied in normal human skeletal muscle fibers obtained during surgery (fibers with both ends cut). Control studies also were carried out with intact as well as cut rat skeletal muscle fibers. Experiments were performed in hypertonic Cl-free saline with 10 or 84 mM Ca and K-channel blockers; muscles were preincubated in a saline containing Cs and tetraethylammonium. A current-clamp technique with two intracellular microelectrodes was used. In human muscle, 14.5% of the fibers showed fully developed CaAPs, 21% displayed nonregenerative Ca responses, and 64.5% showed only passive responses; CaAPs were never observed in 10 mM Ca. In rat muscle, nearly 90% of the fibers showed CaAPs, which were not affected by the cut-end condition. Human and rat muscle fibers had similar membrane potential and conductance in the resting state. In human muscle (22-32 degrees C, 84 mM Ca), the threshold and peak potential during a CaAP were +26 +/- 6 mV and +70 +/- 3 mV, respectively, and the duration measured at threshold level was 1.7 +/- 0.5 sec. In rat muscle, the duration was four times longer. During a CaAP, membrane conductance was assumed to be a leak conductance in parallel with a Ca and a K conductance. In human muscle (22-32 degrees C, 84 mM Ca, 40 micron fiber diameter), values were 0.4 +/- 0.1 microS, 1.1 +/- 0.7 microS, and 0.9 +/- 0.4 microS, respectively. Rat muscle (22-24 degrees C, 84 mM Ca) showed leak and K conductances similar to those found in human fibers. Ca-conductance in rat muscle was double the values obtained in human muscle fibers.

Fatigue during continuous 20 Hz stimulation of the rat phrenic nerve diaphragm preparation

The site and mechanism of action of fatigue was investigated in the isolated rat phrenic nerve diaphragm preparation, with indirect and direct stimulation at 20 Hz and recording of tension and EMG. An equal decay of the subtetanic tension during indirect and direct stimulation, and a parallel decay of tension and EMG, suggested a mechanism of fatigue localized to structures that were 'seen' by the EMG electrodes. A comparison of the responses to sub- and supra-maximum direct stimulation did not show increased fatigue at sub-maximum stimulation. Therefore, the fatigue was probably not caused by an increased threshold of the excitability of the sarcolemma. However, prolongation of the stimulus pulse during direct stimulation from 0.5 ms to 5 ms in the fatigued preparation caused a two-phasic recovery of tension. The initial phase, but not the slow phase, was inhibited by tetrodotoxin (TTX). Thus a recovery of sarcolemma action potentials could explain the initial phase. The slow phase was probably caused by a mechanism localized at more distal potential-dependent sites, probably in the T tubules.

Comparison of force and stiffness in normal and dystrophic mouse muscles

Isometric force and stiffness of fast- and slow-twitch muscles of affected and normal mice of the 129/ReJ dy/dy strain were studied at rest and during active contraction at a variety of lengths. Dystrophic muscles developed less force in response to stimulation, but the resting stiffness was not reduced as much, particularly at long muscle lengths. This is consistent with the replacement of muscle fibers by connective tissue that is considerably less elastic. When second and third stimuli are superimposed on the rising phase of a twitch in a normal muscle, a less-than-linear summation of force and stiffness generation (early depression) is followed by a more-than-linear summation (later facilitation). Dystrophic muscles showed a smaller early depression and a greater later facilitation of force and active muscle stiffness. Many of these phenomena can be predicted from a simple model of Ca2+ release and binding to troponin.

Skeletal muscle Ca2+ channels

Ca2+ channels are widely distributed among different cell types. We shall describe in this paper kinetic properties of voltage-dependent slow Ca2+ channels in mammalian and frog skeletal muscle fibres. In addition, recent data on a fast-activated Ca2+ channel will be presented. Finally, the possible physiological role of the channel will be considered.

Effect of theophylline on membrane potential and contractile force in hamster diaphragm muscle in vitro

Theophylline enhances the force of diaphragmatic contraction and delays fatigue. The mechanism is not known, but recent evidence suggests it may act at the cell membrane. To test this hypothesis, we studied the effect of theophylline on resting membrane potential and tension in hamster diaphragm cells. Muscle strips were obtained from five adult hamsters and placed in Krebs solution, aerated with 95% O2, 5% CO2. Resting membrane potential was measured using 3-M KCl-filled glass microelectrodes; 15-22 fibers in each strip were sampled. Force frequency curves (twitch to 100 Hz) were obtained. The muscle bath was then changed to one containing 100 mg/liter (0.55) theophylline. Resting membrane potential was -76 +/- 3 mV (mean +/- S.D.) in Krebs solution and increased to -85 +/- 3 mV (P less than 0.01) with added theophylline. Tension increased from 5% (at 100 Hz) to 20% (at 10 Hz) with theophylline. Hyperpolarization indicates an increase in intracellular to extracellular potassium concentration. Net potassium outflow occurs with each contraction, causing the cell membrane to become depolarized with repeated contractions, ultimately leading to fatigue. The hyperpolarization of the skeletal muscle cell membrane observed with theophylline may play an important role in prolonging time to fatigue.

A fast-activated inward calcium current in twitch muscle fibres of the frog (Rana montezume)

Voltage-clamp experiments were performed at 18 degrees C in intact twitch muscle fibres of the frog using the three micro-electrode technique. Membrane currents were recorded in the presence of 120 mM-tetraethylammonium-methanesulphonate and 10 mM-Ca2+. The recording solution was made hypertonic by adding 350 mM-sucrose to avoid contraction. Two components of inward current in the absence of external Na+ were observed. Depolarization induced a fast-activated inward current of small amplitude in addition to the well-known slow, transient Ca2+ current (ICa,s). Both components of inward current persisted in the presence of tetrodotoxin. They practically disappeared on replacing external Ca2+ with Mg2+ and were blocked by millimolar additions of Cd2+ to the bath. Thus, the fast-activated component of inward current was also carried by Ca2+ (ICa,f). Neither ICa,f nor ICa,s were reduced by 5 microM-diltiazem. During 400 ms depolarizations ICa,f was detected at approximately -60 mV, 30 mV more negative than the membrane potentials at which ICa,s appeared. At about 0 mV the time constant for activation was 5 ms for ICa and 150 ms for ICa,s. ICa,f did not significantly decline during depolarizations up to 2s in duration at membrane potentials between -60 and -30 mV. ICa,f tended to disappear as a function of time on exposure to the hypertonic recording solution. Its maximum amplitude decreased from about -25 microA/cm2 during the first 5 min to about -5 microA/cm2 after 25 min while ICa.s remained practically unchanged (maximum peak amplitude of about -60 microA/cm2). These results indicate the existence of two types of voltage-dependent CA2+ channels in intact muscle fibres. The kinetic properties of fast-activated Ca2+ channels suggest that they significantly activate during a single twitch.

Which active forms of physostigmine generate contractions in diaphragm muscles of mice?

Physostigmine-induced contractions of isolated small bundles (100 micron diameter) of muscle fibres isolated from the diaphragm of C57 BL mice were studied under various conditions. At different external pH the changes in the maximal amplitude of the contraction were related to the external activity of the permeant neutral form of the drug. However changes in the internal pH showed that the amplitude of the contractile response depended directly on the internal activity of the protonated form of the drug. The amplitude of the contraction was enhanced by caffeine treatment and depended on the external calcium concentrations. In mammalian skeletal muscle, physostigmine appeared to produce contractions due to its anticholinesterase property. The release of calcium seemed to occur from an intracellular store different from that involved in the action of caffeine.