Membrane potential and contractures in segments cut from rat fast and slow twitch muscles

Positive inotropic effects of Tityus cambridgei and T. serrulatus scorpion venoms on skeletal muscle

Toxins that block voltage-dependent K+ channels and those that modify Na+ channel gating exhibit positive inotropic effect on skeletal muscle. We compared the effect of the venom of Tityus cambridgei (Tc) and Tityus serrulatus (Ts) scorpions on mouse diaphragm force, in vitro. In indirect and direct (using D-tubocurarine 7.3 microM) stimulation, Tc, 10microg/mL, increased the contractile force, an effect prevented by tetrodotoxin (TTX) while Ts, 0.5 microg/mL, potentiated only indirectly stimulated diaphragm, thus indicating its activity is mainly mediated through acetylcholine release from nerve terminal. This effect is prevented by TTX and attenuated by the K+ channel opener cromakalim. In conclusion, our data show that while the positive inotropic effect of both venoms appears associated to the activity of Na+ and K+ channels, only Tc venom acts also directly on skeletal muscle. This finding call for further studies on Tc venom to identify the toxin responsible for its direct inotropic activity as it may have clinical applications.

Wheel-running exercise alters rat diaphragm action potentials and their regulation by K+ channels

Endurance exercise modifies regulatory systems that control skeletal muscle Na+ and K+ fluxes, in particular Na+-K+-ATPase-mediated transport of these ions. Na+ and K+ ion channels also play important roles in the regulation of ionic movements, specifically mediating Na+ influx and K+ efflux that occur during contractions resulting from action potential depolarization and repolarization. Whether exercise alters skeletal muscle electrophysiological properties controlled by these ion channels is unclear. The present study tested the hypothesis that endurance exercise modifies diaphragm action potential properties. Exercised rats spent 8 wk with free access to running wheels, and they were compared with sedentary rats living in conventional rodent housing. Diaphragm muscle was subsequently removed under anesthesia and studied in vitro. Resting membrane potential was not affected by endurance exercise. Muscle from exercised rats had a slower rate of action potential repolarization than that of sedentary animals (P = 0.0098), whereas rate of depolarization was similar in the two groups. The K+ channel blocker 3,4-diaminopyridine slowed action potential repolarization and increased action potential area of both exercised and sedentary muscle. However, these effects were significantly smaller in diaphragm from exercised than sedentary rats. These data indicate that voluntary running slows diaphragm action potential repolarization, most likely by modulating K+ channel number or function.

Methyl jasmonate-induced stimulation of sarcoplasmic reticulum Ca(2+)-ATPase affects contractile responses in rat slow-twitch skeletal muscle

The purpose of this study was to determine whether methyl jasmonate, a stimulator of Ca(2+)-adenosine triphosphatase (ATPase) activity of the purified ATPase from fast-twitch skeletal muscle, could affect contractile responses in small bundles of rat isolated slow-twitch (soleus) fibers. In saponin-skinned fibers, sarcoplasmic reticulum (SR) Ca(2+) loading was performed in pCa 7.0 solution. The amount of Ca(2+) taken up was monitored by use of the amplitude of contraction following application of 10 mM caffeine. Results indicate that the increased loading rate in the presence of methyl jasmonate is likely due to stimulation of the SR Ca(2+)-ATPase. In Triton-skinned fibers, the myofibrillar Ca(2+) sensitivity was not changed by methyl jasmonate (50-200 microM). In intact fibers, the amplitude and the time constant of relaxation of twitch and potassium contracture were reversibly reduced after 2 min of application of methyl jasmonate at a concentration of up to 125 microM. At higher concentrations (>150 microM), effects were not reversible. In the presence of methyl jasmonate (100 microM), the relationship between the amplitude of potassium contractures and the membrane potential shifted to more positive potentials, whereas the steady-state inactivation curve was unchanged. These observations suggest that methyl jasmonate has no effect on voltage sensors. Taken together, our results show that methyl jasmonate is a potent, reversible, and specific stimulator of the SR Ca(2+) pump in slow-twitch skeletal muscle and is an extremely valuable pharmacological tool for improving relaxation and studying calcium-signaling questions.

Differential effects of nandrolone decanoate in fast and slow rat skeletal muscles

PURPOSE

We studied the effects of high doses of an anabolic-androgenic steroid, exercise training, and a combination of steroid and training on mammalian fast- and slow-twitch skeletal muscles at the cellular level.

METHODS

Thirty-two male rats were divided into sedentary and treadmill-trained groups (increased speed and time: 18 m.min-1, 0.5 h.d-1, 5 d.wk-1). Eight animals of each group were treated with nandrolone decanoate (ND) (15 mg.kg-1.wk-1), and others received the same doses of solvent. The animals were killed after 5 wk, and the contractile parameters for isolated small bundles of soleus and extensor digitorum longus (edl) fibers were estimated.

RESULTS

Muscle mass, twitches, and K+ contractures were increased in soleus and edl muscles after the drug treatment and after the exercise training. Caffeine contractures were increased only after the exercise training. The combination of exercise with ND treatment produced greater effects, particularly a significant increase in sensitivity to caffeine and the amplitude of K+ contractures as well as a shortening of the time required to restore contracture. These modifications were more marked in slow than fast muscle.

CONCLUSION

These results show that 5 wk of exercise training produced changes in the contractile responses developed by isolated skeletal muscle cells. The combination of exercise training with ND treatment potentiated these effects, suggesting that there was some modification in the excitation-contraction coupling mechanism. ND treatment also produced a more potent effect in soleus than edl sedentary muscle.

Contractile and electrical properties of sternohyoid muscle in streptozotocin diabetic rats

1. The effects of diabetes on the electrical and contractile function of skeletal muscle are variable, depending on muscle fibre type distribution. The muscles of the upper airway have a characteristic fibre distribution that differs from previously studied muscles, but the effects of diabetes on upper airway muscle function are unknown. Normally, contraction of upper airway muscles, such as the sternohyoids, dilates and/or stabilizes the upper airway, thereby preventing its collapse. Diabetes is associated with obstructive sleep apnoea in which there is collapse of the upper airway due to failure of the upper airway musculature to maintain airway patency. Therefore, the purpose of the present study was to determine the effects of diabetes on the electrical and contractile characteristics of upper airway muscle. 2. Rats were treated with vehicle (sodium citrate buffer; pH 4.5) or with streptozotocin to induce diabetes, confirmed by the presence of hyperglycaemia, and the contractile and electrical properties of the sternohyoid were compared in these two groups. Isometric contractile properties and membrane potentials were determined in isolated sternohyoid muscles in physiological saline solution at 25 degrees C. 3. Streptozotocin had no effect on sternohyoid muscle fatigue, the tension-frequency relationship or membrane potentials, but did increase contraction time, half-relaxation time, twitch tension and tetanic tension. 4. Streptozotocin-induced diabetes has no effect on sternohyoid muscle fatigue or the tension-frequency relationship, but does reduce contractile kinetics and increases force generation. These effects are not due to changes in resting membrane potential. These data are evidence that the association of sleep apnoea and diabetes is not due to effects on upper airway muscle contractile properties.

A gap isolation method to investigate electrical and mechanical properties of fully contracting skeletal muscle fibers

We describe here a single-gap isolation method that allows the simultaneous measurement of electrical activity and tension output from fully contracting segments of frog skeletal muscle fibers. By using single pulses and pulse trains of varying frequency (5-100 Hz), records obtained for both electrical and mechanical fiber response demonstrate that the physiological properties of the fiber segments have been preserved. Action potentials could be recorded free of movement artifacts, even while segments were in fused tetani and developing maximum tensions of more than 600 kN/m2. Single current pulses evoked action potentials that averaged 144 +/- 16 mV (mean +/- SD, n = 8) in amplitude and twitches that averaged 285 +/- 66 kN/m2 and 55 +/- 5 ms (mean +/- SD, n = 20) in magnitude and time to peak, respectively. Trains of action potentials elicited patterns of tension development that exhibited summation, unfused tetani, and fused tetani in a frequency-dependent manner. The AC and DC electrical properties of the single grease gap were modeled with a simple Thévenin equivalent circuit, which satisfactorily predicted the experimental results. Our methodology is easily implemented and potentially applicable to any muscle preparation in which fiber segments with an intact end attached to a piece of tendon can be dissected.

Changes in pharyngeal respiratory muscle force produced by K+ channel blockade

The purpose of the present study was to determine whether the contractility of pharyngeal respiratory muscles can be augmented by altering membranous K+ channel conductance. The effects on twitch force of two K+ channel blockers, tetraethylammonium (TEA, 10 mM) and 4-aminopyridine (4-AP, 0.3 mM), were examined in vitro for sternohyoid and diaphragm muscle strips. Both agents augmented isometric twitch force of both muscles. In response to TEA twitch force of the sternohyoid muscle increased significantly more than that of the diaphragm (by 33 +/- 7 vs. 9 +/- 1%, P = 0.004), whereas with 4-AP the increase in twitch force of the sternohyoid muscle was comparable to that of the diaphragm (55 +/- 15 vs. 64 +/- 6%, P = 0.50). 4-AP shifted the force-frequency relationship of both muscles leftward but did not alter peak tetanic force, so that force with 4-AP exceeded that without drug at stimulation frequencies below 60 Hz. In contrast TEA reduced force at stimulation frequencies > 20 Hz. The isometric contraction times of both muscles was variably prolonged, more so with 4-AP (by 30 +/- 15% for the sternohyoid and 32 +/- 3% for the diaphragm) than with TEA (by 9 +/- 2% for the sternohyoid and 5 +/- 2% for the diaphragm). For the group of muscles and K+ channel blockers, the degree of augmentation of twitch force correlated with the degree of prolongation of contraction time (r = 0.82, P < 0.001), consistent with blocking delayed rectifier K+ channels as the mechanism of increasing muscle force.

Activation of two types of fibres in ferret, Mustela putorius furo, cremaster muscle

Some contractile, histochemical, morphological and electrophysiological properties of ferret, Mustela putorius furo, cremaster muscle have been estimated. Histochemical fibre typing revealed the presence of two types of fibres (type I 66.2%, type II 33.8%). Morphometry performed on ATPase-stained transverse sections showed that type I was composed of a large amount (40%) of small (less than 1400 microns2) cells. In mammalian Ringer two groups of fibres could be recognized on the basis of the values of resting potential (-69.7 mV and -59.1 mV) intracellular sodium activity (8.3 mmol.l-1 and 14.1 mmol.l-1, respectively). In experiments on fibre bundles, the elevation of extracellular potassium concentration to 15-200 mmol.l-1 produced contractures that consisted of a well-defined transient or phasic tension followed by a sustained or tonic tension. Properties of activation and inactivation of the tension analysed in small bundles of cut fibres (lengths 0.5-1.0 cm) were of fast- and slow-twitch type for phasic and tonic phase, respectively. In contrast to the phasic component of K contractures, the tonic phase was abolished by Ca2+ withdrawal and inhibited by Ni2+, Cd2+, Co2+, Gd3+ and gallopamil (D600). In Ca(2+)-free medium the sustained tension was restored by adding Sr2+. It is concluded that in ferret cremaster muscle the presence of slow-twitch fibres would give rise to the tonic component of the K contracture in which an extracellular source of activator Ca2+ is involved. The ability of these fibres to contract with a maintained tension for prolonged periods of time might participate in the temperature regulation of the testes.

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)

Inactivation of excitation-contraction coupling in rat extensor digitorum longus and soleus muscles

K contractures and two-microelectrode voltage-clamp techniques were used to measure inactivation of excitation-contraction coupling in small bundles of fibers from rat extensor digitorum longus (e.d.l.) and soleus muscles at 21 degrees C. The rate of spontaneous relaxation was faster in e.d.l. fibers: the time for 120 mM K contractures to decay to 50% of maximum tension was 9.8 +/- 0.5 s (mean +/- SEM) in e.d.l. and 16.8 +/- 1.7 s in soleus. The rate of decay depended on membrane potential: in e.d.l., the 50% decay time was 14.3 +/- 0.7 s for contractures in 80 mM K (Vm = 25 mV) and 4.9 +/- 0.4 s in 160 mM K (Vm = -3 mV). In contrast to activation, which occurred with less depolarization in soleus fibers, steady state inactivation required more depolarization: after 3 min at -40 mV in 40 mM K, the 200 mM K contracture amplitude in e.d.l. fell to 28 +/- 10% (n = 5) of control, but remained at 85 +/- 2% (n = 6) of control in soleus. These different inactivation properties in e.d.l. and soleus fibers were not influenced by the fact that the 200 mM K solution used to test for steady state inactivation produced contractures that were maximal in soleus fibers but submaximal in e.d.l.: a relatively similar depression was recorded in maximal (200 mM K) and submaximal (60 and 80 mM K) contracture tension. A steady state "pedestal" of tension was observed with maintained depolarization after K contracture relaxation and was larger in soleus than in e.d.l. fibers. The pedestal tension was attributed to the overlap between the activation and inactivation curves for tension vs. membrane potential, which was greater in soleus than in e.d.l. fibers. The K contracture results were confirmed with the two-microelectrode voltage clamp: the contraction threshold increased to more positive potentials at holding potentials of -50 mV in e.d.l. or -40 mV in soleus. At holding potentials of -30 mV in e.d.l. or 0 mV in soleus, contraction could not be evoked by 15-ms pulses to +20 mV. Both K contracture and voltage-clamp experiments revealed that activation in soleus fibers occurred with a smaller transient depolarization and was maintained with greater steady state depolarization than in e.d.l. fibers. The K contracture and voltage-clamp results are described by a model in which contraction depends on the formation of a threshold concentration of activator from a voltage-sensitive molecule that can exist in the precursor, activator, or inactive states.

Potassium contractures in the mouse diaphragm: regional variation in the multiphasic response

1. Submaximal potassium-induced contractures were studied in isolated mouse hemidiaphragms (HD) and compared with extensor digitorum longus (EDL) and soleus. 2. In the HD, the contracture was triphasic: an initial small, fast phase followed by a larger slow phase, and a late phase which was seen on prolonged exposure. 3. Costal segments of the HD responded to elevated potassium with a larger fast phase, while dorsal or ventral segments produced a larger slow phase. 4. In calcium-free solution, the fast phase appeared to be unaffected, while the slow phase was potentiated and its time course abbreviated. The late phase was greatly attenuated or abolished. 5. In the soleus, but not the EDL muscle, contracture amplitude in calcium-free solution was potentiated and the time course was shortened. 6. The characteristics of potassium-induced contractures of both the soleus and EDL muscles are to be seen in the response of the hemidiaphragm, but there is a regional variation in their proportional contribution which suggests a heterogeneity in diaphragm composition.

Purified ryanodine receptor of skeletal muscle sarcoplasmic reticulum forms Ca2+-activated oligomeric Ca2+ channels in planar bilayers

The ryanodine receptor of sarcoplasmic reticulum (SR) from fast-twitch skeletal muscle has been purified and found by electron microscopy to be equivalent to the feet structures that are involved in situ in the junctional association of transverse tubules with terminal cisternae of SR. We now find that when the purified receptor is incorporated into vesicle-derived planar bilayers, it forms Ca2+-specific channels, which are dependent on submicromolar Ca2+ for activity. In the presence of 1 mM ATP, the channel shows essentially no activity at 10 nM Ca2+ but becomes highly activated at 50 nM Ca2+. At suboptimal Ca2+ levels (100 nM), the channel is strongly activated by 1 mM ATP and can be blocked by ruthenium red, both effects being prevented by higher Ca2+ levels (1 microM). Mg2+, added from the cis side at millimolar concentrations, blocks Ca2+ flux through the channel from trans to cis (equivalent to flux from luminal to myoplasmic compartment). Ryanodine stabilizes the open state of the channel and blocks the action of ruthenium red to close the channel. Thus, the purified ryanodine receptor incorporated into a bilayer has the Ca2+-channel characteristics consistent with the calcium release observed in isolated terminal cisternae vesicles. Furthermore, ryanodine induced the appearance of a sublevel gating mode characterized by long open conductance states, which were integral multiples of the smallest observed conductance, 3.8 pS in 50 mM Ca2+. The purified receptor consists essentially of a single-sized high molecular weight polypeptide (Mr. approximately equal to 360,000), which on reconstitution forms the square rectangles diagnostic of the feet structures. We conclude that the identity of the Ca2+-release channel of SR is the foot structure, which consists of an oligomer of the high molecular weight polypeptide.

Membrane Ca2+ interactions and contraction in denervated rat soleus muscle

Under voltage clamp conditions contractile responses and ionic currents of single fibres isolated from rat soleus, denervated for more than 20 days, were recorded in Na-free TEA containing solutions. The relationship between membrane potential and contraction has been analysed under various conditions. The addition of trivalent cations (La3+, Gd3+) resulted in a dose dependent reduction of the contractile response and similar effects were produced by polymyxin B (0.05-0.5 mM). By contrast in the presence of phospholipase D (1-5 U/ml) contractions were significantly increased for all values of depolarization. The time course of the change of tension amplitude after the application of Ca-free medium, was dependent on the amplitude, the duration and the frequency of the depolarization. Upon depolarization glycerol-treated fibres generated contractile responses which were similar to those recorded in normal muscle and were also dependent on [Ca]o. It is proposed that in denervated soleus muscle the negatively charged phospholipids at the outside of the membrane were involved in the depolarization-contraction coupling by means of their Ca binding properties. The quantity of Ca binding sites would be dependent on [Ca]o and membrane potential and their binding properties modified during and/or following variation in membrane potential.

Effects of external cations and calcium-channel blockers on depolarization-contraction coupling in denervated rat twitch skeletal muscles

In the double mannitol gap arrangement the contraction was estimated in single fibres isolated from rat extensor digitorum longus (e.d.l.) muscles that had been denervated for 2-48 days. Denervation induced large changes in the characteristics of the action potential and of the twitch. Up to 15-20 days after denervation the contraction-depolarization curve was sigmoidal and the maximum amplitude of the contraction was not modified by variation of [Ca]o or [Na]o. After 15-20 days of denervation a bell-shaped curve described the relation between contraction and potential. The maximum amplitude was dependent upon the [Ca]o. In Ca-free solution no contractile response was obtained. In Na-free, Ca-containing solution the relationship between contraction and potential was not modified by the addition of divalent cations Co2+, Cd2+, Mn2+, Mg2+, Ni2+, or Ba2+. The contraction, which appeared in Ca-free solution, was restored by adding Sr2+. D600, verapamil and bepridil failed to change the amplitude of the contraction while a marked reduction was found with dihydropyridines. The reduction was overcome by increasing [Ca]o. The present results suggest that the slow calcium current is not involved in the generation of the contractile responses developed by denervated muscles in Na-free (TEA) solution.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.

Changes in the ionic currents sensitivity to inhibitors in twitch rat skeletal muscles following denervation

Under voltage clamp conditions, using the double mannitol gap technique, ionic currents developed by fast (e.d.l.) and slow (soleus) twitch muscle fibers of the rat were analysed at different times following denervation and the results compared with those obtained in normal cells. In slow fibers, denervation caused the appearance of a new population of TTX-resistant Na+ channels (dissociation constant K2 = 2,800 nM) compared with the normal TTX-sensitive Na+ channels (K1 = 9 nM). This new population of Na channels appeared in 5 days and contributed about 32% of the total Na conductance. Denervated fast fibres developed a slow component in the delayed outward current which was found to be typical of slow innervated muscles. This component appeared 5 to 20 days after nerve section. These changes are associated with modifications of potassium channels' sensitivity for specific inhibitors (TEA and 4-AP). After denervation, the delayed outward current in the two types of muscles becomes resistant to 4-AP whereas TEA, which blocks the total delayed outward current in innervated fibers (dissociation constant of 21.4 mM) becomes more effective in blocking the fast component (dissociation constant of 0.61 mM) and less effective in blocking the slow component in denervated cells. The analysis of the characteristics of the TEA sensitive and TEA insensitive components of the outward current leads to the proposal that these components were related to the fast and to the slow components previously described in fast and slow twitch mammalian skeletal muscles.

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.