Potentiation of the twitch responses by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase in frog atrial fibres

In frog atrial fibres, cyclopiazonic acid as well as thapsigargin, which are inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, induced a significant increase in the twitch amplitude without detectable changes in its kinetics. The measurements performed on chemically skinned fibres show that cyclopiazonic acid has no effect on the properties of contractile proteins. In the presence of a T-type Ca2+ channel blocker or L-type Ca2+ channel blocker, cyclopiazonic acid still induced a potentiation of the twitch while no effect was found in the presence of a Na(+)-Ca2+ exchange blocker. The effect of cyclopiazonic acid was not related to any modification in myofibrillar Ca2+ sensitivity or in Ca2+ influx through Ca2+ channels. It is proposed that the inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase resulted in a potentiation of the effect of the Ca2+ influx and that the major role of the sarcoplasmic reticulum was to limit the intracellular Ca2+ concentration.

Calcium currents and contraction in frog atrial trabeculae

The properties of two types of calcium current (ICa(T) and ICa(L)) and their relationship with contraction were studied in isolated frog atrial trabeculae using the double mannitol gap technique. In Ringer solution containing TTX the calcium currents observed are similar to the ICa(T) and ICa(L) found in single frog atrial cells. The results obtained by using inorganic calcium channel blockers have shown that the phasic contraction appears only when ICa(T) and ICa(L) are simultaneously present. It is proposed that ICa(T) loads the sarcoplasmic reticulum (SR) while ICa(L) triggers Ca2+ release from the SR.

Effect of cyclopiazonic acid on contractile responses in slow and fast bundles of cremaster skeletal muscle from the ferret

The effects of cyclopiazonic acid (CPA) on twitch force, calcium (Ca2+) uptake and release by the sarcoplasmic reticulum (SR), and Ca2+ sensitivity of contractile apparatus were studied using intact and chemically skinned cremaster fibers and compared with those on the extensor digitorum longus and soleus. In cremaster muscles treated with CPA (0.5-5 microM) a potentiation of the twitch was observed, associated with an increase in time to peak and in time of relaxation. In Triton-skinned fibers, CPA, at concentrations less than 10 microM, exerted no significant effect on the contractile apparatus of either slow- or fast-twitch fibers. In slow-twitch fibers, a dose-dependent increase in Ca2+ sensitivity was associated with a decrease in maximal tension, at CPA concentrations > 10 microM. In saponin-skinned fibers, during the uptake phase, CPA at > 10 microM induced a dose-dependent decrease in caffeine contracture. The possibility of an action on the SR Ca2+ release channel was excluded by testing the effect of CPA during the releasing phase. The enhancing effect of CPA (0.5-5 microM) on mechanical activity could be explained by an inhibition of the SR Ca2+ ATPase in skeletal muscle cells without an effect on the contractile proteins. Our results strongly suggest that CPA (< 10 microM) has a highly specific effect on the SR Ca2+ pump in the fast- and slow-twitch fibers and therefore could be a good tool to study the mechanisms of Ca2+ regulation in skeletal muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of contractile responses due to simulated weightlessness in rat soleus muscle

Some contractile and electrophysiological properties of muscle fibers isolated from the slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles of rats were compared with those measured in SOL muscles from suspended rats. In suspended SOL (21 days of tail-suspension) membrane potential (Em), intracellular sodium activity (aiNa) and the slope of the relationship between Em and log [K]o were typical of fast-twitch muscles. The relation between the maximal amplitude of K-contractures vs Em was steeper for control SOL than for EDL and suspended SOL muscles. After suspension, in SOL muscles the contractile threshold and the inactivation curves for K-contractures were shifted to more positive Em. Repriming of K-contractures was unaffected by suspension. The exposure of isolated fibers to perchlorate (ClO4-)-containing (6-40 mM) solutions resulted in a similar concentration-dependent shift to more negative Em of activation curves for EDL and suspended SOL muscles. On exposure to a Na-free TEA solution, SOL from control and suspended rats, in contrast to EDL muscles, generated slow contractile responses. Suspended SOL showed a reduced sensitivity to the contracture-producing effect of caffeine compared to control muscles. These results suggested that the modifications observed due to suspension could be encounted by changes in the characteristics of muscle fibers from slow to fast-twitch type.

Effects of cyclopiazonic acid on Ca(2+)-activated tension production in skinned skeletal muscle fibres of the ferret

The effect of cyclopiazonic acid, an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, was investigated on force generation by the contractile apparatus in Triton-skinned fibres from extensor digitorum longus and soleus. Concentrations of cyclopiazonic acid lower than 10 microM were without effect on Ca(2+)-activated tension in both types of muscles. In contrast, in soleus, cyclopiazonic acid (20, 50, 100 microM) was found to shift reversibly the relation-tension pCa (-log[Ca2+]) towards lower free Ca2+ and to decrease maximal Ca(2+)-activated tension, in a dose-dependent manner. In extensor digitorum longus, the Ca2+ sensitivity was significantly increased only at a high cyclopiazonic acid concentration (100 microM) and for all the concentrations tested between 5 to 100 microM maximal Ca(2+)-activated tension was unchanged. These results suggest that cyclopiazonic acid has a direct effect on contractile proteins, in a dose-dependent manner. Ca2+ sensitivity and Ca(2+)-activated maximal tension of the contractile apparatus were differentially affected in fast- (extensor digitorum longus) and slow-twitch (soleus) fibres from ferret skeletal muscle.

Calcium sensitivity of skinned ferret EDL, soleus, and cremaster fibers

The properties of the contractile system at different times of the year in the ferret extensor digitorum longus (EDL), soleus and cremaster muscles were examined by using chemically skinned (Triton X-100) preparations. The results show clear differences in calcium sensitivity between these skeletal muscles. The apparent calcium threshold for activation was lower in soleus than in EDL, while calcium concentrations ([Ca2+]) required to obtain the half-maximal tension, expressed as pCa50 (-log[Ca2+]), was lower in EDL than in soleus muscle. In fact, pCa50 obtained in fast and slow fibers by fitting the experimental data points by a modified Hill equation was 5.92 +/- 0.02 (n = 9) and 6.09 +/- 0.03 (n = 11) respectively. So EDL appears to be a typical fast-twitch muscle and soleus a typical slow-twitch muscle. Adult ferret cremaster muscle was composed of two types of fibers during the quiescent period similar to EDL and soleus, and only one type that was intermediate between EDL and soleus in the breeding period, as assessed by pCa50 values. These annual modifications in calcium activation of adult ferret cremaster muscle could be related to changes in the function of these muscles and may be correlated with seasonal variations of sexual activity.

Effect of haemodynamic pressure overload of the adult ferret right ventricle on inotropic responsiveness to external calcium and rest periods

The inotropic effects of external calcium concentration ([Ca2+]o] and rest periods have been compared in papillary muscles isolated from control (n = 4) and pressure-overloaded right (n = 5) ventricles of adult ferrets. Hypertrophy was induced by pulmonary artery clipping for 30-45 days. Under control conditions (3 mM [Ca2+]o, 0.1 Hz), the isometric twitch force of hypertrophied muscles was decreased by 75%, time to peak was increased by 30% and time to half-relaxation was increased by 50% compared with non-hypertrophied preparations. The sensitivity of contraction to [Ca2+]o was decreased in hypertrophied muscles compared with control ([Ca2+] required for half-maximal contraction: 4.1 mM vs 1.7 mM) and the maximal contraction reached at high [Ca2+]o was smaller in pressure-overloaded muscles compared with control (8.3 +/- 2.0 mN mm-2 vs 19.0 +/- 2.1 mN mm-2 respectively). In both groups, rest periods longer than the steady-state interval were initially accompanied by a potentiation of the first post-rest contraction compared with steady-state. Peak potentiation occurred after a rest of 120 s in hypertrophied muscles and after a rest of 60 s in control. The maximal relative potentiation, i.e. compared with the steady-state twitch, was higher in hypertrophied muscles (+75%) than in control (+20%). After peak potentiation, the amplitude of the first post-rest contraction progressively decreased with increasing periods of rest, although at a slower rate in hypertrophy compared with control. The time constants of post-rest decay were 1203 +/- 99 s and 528 +/- 24 s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of cyclopiazonic acid, an inhibitor of the Ca++ ATPase of sarcoplasmic reticulum, on Ca++ transport, contraction and relaxation in cardiac muscle

Cyclopiazonic acid is a potent inhibitor of cardiac sarcoplasmic reticulum Ca++ ATPase. It scarely affects inotropism but significantly impairs lusitropism suggesting a greater role for cardiac sarcoplasmic reticulum in the control of cardiac relaxation than in the control of cardiac contraction.

Intracellular Na activity measurements in the control and hypertrophied heart of the ferret: an ion-sensitive micro-electrode study

Because of the role of intracellular Na on cardiac contractility and of the depressed isometric contractile response of the hypertrophied myocardium, the effects of pressure overload on the intracellular Na activity (aiNa) have been investigated in papillary muscles isolated from the ferret right ventricle. In animals subjected to pulmonary artery clipped for 1-2 months, right ventricle-to-body weight ratio was increased by about 39% in comparison with the control group. aiNa was measured in quiescent papillary muscles, by means of Na-sensitive micro-electrodes, at room temperature (19-22 degrees C). aiNa values were, in the control ventricular cells, 7.8 +/- 1.1 mM (mean +/- SD; n = 20) and in the hypertrophied ones, 8.0 +/- 1.2 mM (n = 49). During superfusion by medium with a reduced extracellular Na concentration ([Na]0), aiNa declined in control and pressure-overloaded muscles to similar steady-state levels at a given [Na]0. aiNa fall was mono-exponential and was characterized by a smaller time constant in the hypertrophied group upon total withdrawal of Na0 (control 209 +/- 19 s, n = 4; hypertrophied 128 +/- 42 s, n = 6). In the absence of external K, aiNa increased to levels that were not significantly different between both groups. It was concluded that, in quiescent preparations, steady-state aiNa was not modified by the hypertrophic process. However, pressure overload induced a modification of aiNa regulation by a possible alteration of the sarcolemmal Na/Ca exchange, although other mechanisms, such as mitochondrial Ca transport, could be involved in the differential response to Na0 removal.

Dual effects of charged amphiphiles on depolarization-contraction coupling in denervated rat soleus muscle

Ionic currents and contraction were recorded under voltage clamp conditions in single fibres isolated from rat soleus muscles denervated for more than 20 days. The effects of amphiphiles on depolarization-contraction (d.c.) coupling in Na-free TEA-containing solutions were analyzed. An anionic amphiphile, sodium dodecyl sulfate (1-10 microM), caused a dose-dependent reduction of the contractile response at all amplitudes of depolarization while a cationic amphiphile, dodecyltrimethylamine (1-10 microM), increased the maximum developed tension with a shift in the contractile threshold. A neutral amphiphile, lauryl acetate (20 microM), induced no significant variation. The effects of charged amphiphiles were found to be strongly dependent on the external calcium concentration and on membrane potential. The effect of sodium dodecyl sulfate to decrease tension was reduced or changed to positive inotropy following hyperpolarization of the membrane by, respectively, +10 and +20 mV. In hyperpolarized (+20 mV) cells, dodecyltrimethylamine reduced the amplitude of the contraction. The results demonstrated that changes in Ca-binding properties of surface membrane modified d.c. coupling in denervated slow twitch skeletal muscle.

Na+-Ca2+ exchange in limb muscles of dystrophic (C57 BL/6J dy2J/dy2J) mice

Isometric tension of Na+-withdrawal contractures, membrane potential and intracellular Na+ activity (aiNa) have been measured in vitro under conditions which modify the activity of the Na+-Ca2+ exchanger of dystrophic limb muscles of (dy2J/dy2J) C57 BL/6J mice. In dystrophic soleus (Sol) and extensor digitorum longus (EDL) muscles exposed to Na+-free medium, there was a decrease in aiNa associated with a hyperpolarization of the membrane and the generation of a transient contracture. By contrast, exposing normal or denervated EDL muscles to Na+-free solution induced no change in aiNa nor in the resting tension. This study shows that Na+-Ca2+ exchange is still present in dystrophic Sol muscle. However from thermodynamic considerations, it could be suggested that the higher resting aiNa compared with that in normal muscle, may decrease the efficacy of the exchange.

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.

Potassium and caffeine contractures in limb muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

The strength of contractures, produced by 15 to 146 mM [K]0 (as L-glutamate), was measured in isolated small bundles of muscle fibers from the fast-twitch extensor digitorum longus and from the slow-twitch soleus of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice. The analysis of the relation between the maximal amplitude of the contracture vs the membrane potential and the time constant of relaxation of the K-contractures has shown that dystrophy induced an attenuation of the differences between fast- and slow-twitch muscles. The repriming of K-contractures was more affected by changes in [Ca]0 in normal soleus than in normal extensor digitorum longus and this difference was unaffected by dystrophy. For both types of muscles, the ability of caffeine to produce contractures was reduced in dystrophic muscle and this modification was not related to a change in the fiber typing.

Sodium withdrawal contractures in rat slow twitch skeletal muscle

Contractile responses due to alterations in [Na+]o have been investigated in fast (iliacus) and slow (soleus) twitch muscles of the rat. On exposure to a Na-free solution, the soleus in contrast to the iliacus cells, generated contractile responses without depolarizing the surface membrane. Following glycerol treatment, the twitch and a part of the Na-withdrawal contracture were abolished. The amplitude of the remaining contracture was between 5 and 50% of the original response and the time to peak was 0.4 to 2 times longer. In intact and detubulated preparations, the amplitude of the zero-Na contracture was modified by changes in [Ca2+]o and a linear relationship was found if the reciprocal of tension was plotted against 1/square root[Ca2+]o. In intact and detubulated fibres, a steep dependence of the Na-withdrawal contracture on [Na+]o was found and [Na+]o which induced the half maximal response at each [Ca2+]o was the same, the responses were inhibited by Mg-ions in a competitive way. It is assumed that the activator Ca is triggered at the tubular and sarcolemmal membrane level by lowering [Na+]o and that a calcium-induced calcium release mechanism at the S.R. level may also be involved.

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.

A comparative study of contractile responses in diaphragm muscles of normal and dystrophic (C57BL/6J dy2J/dy2J) mice

Potassium and caffeine contractures of isolated small bundles (100 to 200 micron diameter) of muscle fibers isolated from the diaphragm of normal and dystrophic (C57BL/6J dy2J/dy2J) mice were compared. In diaphragms of pathologic mice (3 to 5 months old) the resting potential, the characteristics of the twitch, and some histological examinations were typical of dystrophic muscles. The slopes of the relationships between the steady membrane potential and log [K]0 were similar for the two types of cells. In 110 mM and 146 mM K there were no significant differences in the time course of the contractures and reduction in [Ca]0 decreased the time to peak and the time constant of relaxation to the same extent; the relative efficiency of [Mg]0 compared with [Ca]0 was equivalent. Repriming of K contractures at different external calcium concentrations indicated that the normal diaphragm did not have any special advantage. The exposure of isolated strips to a solution containing caffeine resulted in a similar increase of the strength of the regularly evoked twitch responses. However, the contractures elicited by 1.25 to 20 mM caffeine showed a subsensitivity of the dystrophic diaphragm (KmDys = 9.3 KmN) and the rate of relaxation was significantly slower than in normal muscle (in 20 mM caffeine, 50% decay time for normal muscle was 25.2 +/- 7.6 s and for dystrophic muscle 54.8 +/- 11.2 s. These results suggest an absence of major alterations in the mechanism of excitation-contraction coupling associated with dystrophy, except for a change in the specific element of the sarcoplasmic reticulum where caffeine acts.

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

Contractile responses associated with depolarization caused by an increase in [K]0 or by voltage-clamp steps were compared for fast and slow mammalian twitch muscles. The contractions and contractures of isolated mammalian muscle fibres cut into 0.5-1.0 cm lengths are similar to those obtained from intact cells. The slope of the relationship between the membrane potential and the log [K]0 is similar in slow (46 mV +/- 0.8) and in fast fibres (48 mV +/- 1.1). This relation is not significantly modified in sodium-free or Cl-free solution. The K-contractures of cut sections of slow and fast fibres are transient and a full repriming of the response is only observed when the [K] x [Cl] product is kept constant. The contractile threshold in fast fibres is at 20-30 mM [K]0 (-52 to -43 mV) and in slow muscle at 10-15 mM [K]0 (-62 to -55 mV). Under voltage-clamp conditions, the contractile responses of both types of muscle show two components. In Na-free solution or in presence of TTX (5 x 10(-7) g/ml) the first component is abolished and the second remaining component is similar to that developing during K-contractures. In iliacus fibres, the contracture threshold is between -50.5 mV and -40.5 mV and in soleus fibres between -66 mV and -56 mV; these values are close to those obtained with K-rich depolarizing fluids. The S-shaped curve of the contracture vs membrane potential relation is similar to that found in frog muscles except that the contractile responses are graded over a larger range of membrane potentials (-50 to + 30 mV in fast and -55 to + 10 mV in slow muscle).

Comparison between the delayed outward current in slow and fast twitch skeletal muscle in the rat

1. A comparison of the delayed outward current of isolated fibres from rat soleus and iliacus muscle has been made using a double sucrose-gap voltage-clamp method. 2. The fast and slow components of the outward current were separated using time constants of the tail currents. The results indicate that in both iliacus and soleus fibres there is a shift in reversal potential which depends on the quantity of current that flows during depolarization. 3. The shift is larger in iliacus than in soleus; it is absent in glycerol-treated muscles. 4. The results obtained in normal and in detubulated fibres show that the shift is due to an accumulation process of potassium ions in the lumen of the T-tubules. 5. In detubulated soleus fibres the outward current is composed of a fast and a slow component, each with the same reversal potential; in detubulated iliacus the slow component is absent. 6. In both types of muscles TEA produces a dose-dependent block of the total outward current. 4-aminopyridine has different effects; it inhibits the total outward current in iliacus fibres and only the fast component in soleus fibres. 7. These results show that in soleus fibres a fast and a slow component participate in the potassium outward current, while only a fast component is present in iliacus muscle.

Ionic currents in slow twitch skeletal muscle in the rat

1. The ionic currents in slow fibres isolated from rat soleus muscle have been studied under voltage-clamp conditions with a double sucrose-gap method and the results are compared to those obtained from fast fibres isolated from the iliacus muscle. 2. The mean value of the resting potential in slow fibres is -70 mV. a value 8 mV more positive that the mean resting potential of fast fibres (-78 mV). 3. In slow muscle, a fast inward current which is blocked by tetrodotoxin and which depends on external sodium concentration is presumed to be carried by sodium ions. The characteristics of this current, which are time- and voltage-dependent, are similar to those of the iliacus fibres. From a holding potential at -86 mV, this inward current is maximal (2.6 mA/cm2 +/- 0.3) at +49.1 mV +/- 1.5 (mean +/- S.E. of mean), reverses at +127.3 mV +/- 2.2 (mean +/- S.E. of mean), and its half inactivation occurs at +23.2 mV +/- 0.8 (mean +/- S.E. of mean). 4. The delayed outward current in slow fibres is unchanged by exposure to chloride free solution and has a time course very different from that found in fast fibres. This current reaches an initial peak in 5-10 msec and a second peak or steady level after 40-150 msec. The decay of the outward current is also very different, being ten times slower than that in fast fibres (1500-3000 msec). 5. Analysis of the tail currents reveals the existence of two components of delayed current in slow fibres. The faster component reverses at a potential of 11.3 mV +/- 0.9 (mean +/- S.E. of mean) positive to the holding potential (equivalent to a membrane potential of about -75 mV), in contrast to a reversal potential of 35.4 mV +/- 2.5 (mean +/- S.E. of mean) positive to the holding potential for the slower component (equivalent to a membrane potential of about -51 mV. 6. In L-glutamate solution the characteristics of the inward-going rectification are the same in the two types of muscle.

Ionic currents in mammalian fast skeletal muscle

1. The double sucrose-gap technique has been applied to rat skeletal muscle fibres to study the ionic currents under voltage-clamp conditions. 2. The iliacus muscle was found to be of 'fast' type according to the characteristics of the twitch generated by an action potential. 3. Micro-electrode measurements have shown that the intracellular potential is under good control even when an inward current develops. 4. The components of an equivalent circuit with two time constants have been estimated from the records of the capacitive current. 5. In rat muscle, between 15 and 21 degrees C, inward and outward currents are similar to sodium and potassium currents found in frog muscle at lower temperature (1--3 degrees C). 6. The inward current which depends on [Na]o and is abolished by tetrodotoxin is carried by sodium ions. Related to the mean value for the holding potential (-90.5 mV) this current reaches its maximum amplitude a +40 and +50 mV, reverses between +130 and +150 mV and its half inactivation occurs between +14 and +22 mV. The effect of low doses of tetrodotoxin suggests that two components participate in the sodium current. 7. The delayed outward current which shows inactivation is divided in two components: (i) the fast has a linear instantaneous current-voltage relation and differs from the fast component of frog muscle in that its equilibrium potential is more negative than the resting potential; (ii) the slow has a linear instantaneous current-voltage relation and the mean value for its equilibrium potential is 26 mV less negative than the resting potential. 8. Inward-going rectification is present in rat muscle.