Calcium influx in contracting and paralyzed frog twitch muscle fibers

Toward the roles of store-operated Ca2+ entry in skeletal muscle

Store-operated Ca(2+) entry (SOCE) has been found to be a rapidly activated robust mechanism in skeletal muscle fibres. It is conducted across the junctional membranes by stromal interacting molecule 1 (STIM1) and Orai1, which are housed in the sarcoplasmic reticulum (SR) and tubular (t-) system, respectively. These molecules that conduct SOCE appear evenly distributed throughout the SR and t-system of skeletal muscle, allowing for rapid and local control in response to depletions of Ca(2+) from SR. The significant depletion of SR Ca(2+) required to reach the activation threshold for SOCE could only be achieved during prolonged bouts of excitation-contraction coupling (EC coupling) in a healthy skeletal muscle fibre, meaning that this mechanism is not responsible for refilling the SR with Ca(2+) during periods of fibre quiescence. While Ca(2+) in SR remains below the activation threshold for SOCE, a low-amplitude persistent Ca(2+) influx is provided to the junctional cleft. This article reviews the properties of SOCE in skeletal muscle and the proposed molecular mechanism, assesses its potential physiological roles during EC coupling, namely refilling the SR with Ca(2+) and simple balancing of Ca(2+) within the cell, and also proposes the possibility of SOCE as a potential regulator of t-system and SR membrane protein function.

Voltage-dependent mobilization of intracellular calcium in skeletal muscle

In skeletal muscle calcium is released from the sarcoplasmic reticulum (SR), an internal organelle, in response to changes in the voltage across the transverse tubule (T-tubule) membrane, an external membrane system that is distinct from the SR but in close proximity to it. For T-tubule voltage changes within the physiological range, calcium release can be turned on or off on a time scale of milliseconds. The control of calcium release from the SR appears to involve at least three functional components: a voltage sensor in the T-tubule membrane, a calcium channel in the SR, and a mechanism for coupling the voltage sensor to the channel. Movement of charged or dipolar molecules within the T-tubule membrane is thought to serve as the voltage sensor. Such intramembrane charge movement (Q) can be monitored electrically and can be compared with the rate of calcium release from the SR. Calcium release is calculated from cytosolic calcium transients measured with a metallochromic indicator. Comparison of Q and the rate of release in the same isolated muscle fibre indicates that this rate is directly proportional to the amount of charge displaced in excess of a 'threshold' amount. The nature of the coupling mechanism between T-tubules and SR remains to be established but present observations impose some restrictions on possible mechanisms.

Effects of diltiazem upon a rapidly exchanging calcium compartment related to repriming in frog skeletal muscle

Following spontaneous relaxation, fast skeletal muscle must first repolarize and then undergo a first-order repriming reaction before depolarization will result in maximal tension production. 45Ca exposure during repriming defined two Ca compartments during subsequent efflux, named Ca(fast) and Ca(fast). Ca(slow) had an average time constant of 112 +/- 17 min. On the basis of slow turnover and content determined by a variety of methods, I suggest Ca(slow) represents Ca within the sarcoplasmic reticulum. Ca(fast) contained 12 pmol Ca per fibre and resting exchange had a time constant of 5.1 +/- 0.4 min. A total of 12 pmol 45Ca within Ca(fast) was released during a maximal contracture. Most of the Ca released from Ca(fast) rapidly entered the extracellular space; however, 0.39 +/- 0.15 pmol Ca per fibre transferred from Ca(fast) into Ca(slow) when the muscle bundle contracted. When 1-10 microM diltiazem reduced contracture time-tension, release of Ca(fast) was reduced proportionally. When 10 microM diltiazem paralyzed excitation-contraction coupling, Ca(fast) was not released. Refilling of Ca(fast) was proportional to the extent of repriming during 45Ca exposure. Although release and refilling of Ca(fast) is related to contraction, its role in excitation-contraction coupling remains to be elucidated.

Contractile responses in rat extensor digitorum longus muscles at different times of postnatal development

Some contractile properties of small bundles (100-200 microns diameter) of muscle fibres isolated from the extensor digitorum longus muscle of rats at different times of development were compared. An increase of resting potential was observed in these muscles from -26.9 mV at 1 day of age to -72.6 mV at 3 months. Twitch tension and duration of postnatal muscles 1-7 days were diminished by reducing [Ca]o (substituted by Mg2+) or adding inorganic cations (Ni2+, Cd2+, La3+), unlike in the oldest animals (14 days-3 months postnatal) where twitch responses were unaffected. In the latter, potentiation of the twitch tension was even recorded in the presence of Ni2+ (0.5-1 mmol.1-1) and Cd2+ (0.5-2 mmol.1-1). Properties of activation and inactivation of the developed tension following elevation of [K]o to 15-200 mmol.1-1 were analysed at the same stages of postnatal development. In contrast to the tension-membrane potential curves for activation, which presented an average negative shift of -17.6 mV between 1 day postnatal and 3 months of age, a voltage dependence of inactivation similar to that encountered in adult extensor digitorum longus muscles, was already reached at 7 days of age. These results suggest an asynchronism in the maturation of the potential-dependent characteristics of the depolarization-contraction coupling mechanism. Furthermore, during the first week postnatal, in relation with poorly developed membrane systems and low [Ca]i-recycling capability, [Ca]o plays a fundamental role in maintaining contraction by replenishing the intracellular calcium pool.

Na/Ca exchange and excitation--contraction coupling in frog fast fibres

A 3 Na/Ca exchanger in the transverse tubular wall is modelled as the coupling mechanism between transverse tubular depolarization and Ca release from the sarcoplasmic reticulum. At rest, the Ca-occupied site faces the transverse tubular lumen. Upon depolarization, the difference in chemical potentials of Na and Ca gives a net inward force on Ca resulting in a reorientation of the exchanger so the Ca site now faces the myoplasm and releases Ca to stimulate Ca-induced Ca release from the sarcoplasmic reticulum. The rotation of the exchanger's asymmetrical charge could generate the 'charge movement' signal. As depolarization continues, the site is depleted of Ca and contraction ends spontaneously. Repolarization reorients the exchanger; the depleted Ca site now faces the transverse tubular lumen and slowly refills with Ca (repriming). A kinetic model is capable of controlling both twitch and contracture tension. The Na/Ca exchange blocker dichlorobenzamil (Merck) (10 microM), elevated external Na and low pH all slowed the rate of rise of potassium contracture tension. The ratios of rates of tension rise were dCB/control = 0.4 +/- 0.1, elevated external Na/Tris = 0.6 +/- 0.1, pH 6.3/control = 0.7 +/- 0.01. These results can be mimicked with the kinetic model by slowing the rate of 'rotation' (and hence charge movement) by 50%. Elevated internal Na increases the rate of rise of contracture tension; elevated internal Na/control 1.6 +/- 0.3. Dichlorobenzamil also slows the recovery following spontaneous relaxation; the time constant (68 s) of repriming is unchanged but shifted to longer recovery times. Reduced external Na and pH 6.3 also slow recovery in a similar manner, consistent with delayed rotation of the Ca-depleted site. These results suggest that Na/Ca exchange is a step in both the excitation contraction coupling chain and the repolarization-repriming sequence.

Voltage sensors of the frog skeletal muscle membrane require calcium to function in excitation-contraction coupling

1. Intramembrane charge movements and changes in intracellular Ca2+ concentration (Ca2+ transients) elicited by pulse depolarization were measured in frog fast twitch cut muscle fibres under voltage clamp. 2. Extracellular solutions with very low [Ca2+] and 2 mM-Mg2+ , shown in the previous paper to reduce Ca2+ release from the sarcoplasmic reticulum (SR), were found to cause two changes in charge movement: (a) a decrease (-12 nC/microF) in the charge that moves during depolarizing pulses from -90 to 0 mV, termed here 'charge 1'; (b) an increase (+7 nC/microF) in the charge moved by hyperpolarizing pulses from -90 to -180 mV, termed 'charge 2'. 3. The increase in charge moved by hyperpolarizing pulses was correlated (r = 0.64) with the decrease in charge moved by depolarizing pulses and both were correlated with the inhibition of Ca2+ release recorded in the same fibres. 4. The low Ca2+ solutions caused a shift to more negative voltages of the dependence relating charge movement and holding potential (VH). This shift is of similar magnitude (about 22 mV) and direction as the shift in the curve relating Ca2+ release flux to VH (previous paper). 5. In solutions with normal [Ca2+] a conditioning depolarization to 0 mV, of 2 s duration, placed 100 ms before a test pulse from -70 to 0 mV, reduced by 30% the amount of charge displaced by the test pulse. Conditioning pulses of 1 s or less caused potentiation of charge movement by up to 30%. 6. In low Ca2+ solutions, reduction of charge was observed at all durations of the conditioning pulse. The duration for half-inhibition was near 200 ms. 7. An extracellular solution with no metal cations caused a more radical inhibition than the low Ca2+ solutions that contained Mg2+. The inhibition of Ca2+ release was essentially complete (90-100%). The charge moved by a pulse to 0 mV was reduced by 20 nC/microF and the charge moved by a pulse to -170 mV increased 8 nC/microF. This shows that Mg2+ supports excitation-contraction (E-C) coupling to some extent. 8. A state model of the voltage sensor of E-C coupling explains qualitatively the observations in both papers.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of extracellular calcium on calcium movements of excitation-contraction coupling in frog skeletal muscle fibres

1. The effect of low extracellular free calcium ion concentration ([Ca2+]o) on the transient changes in cytoplasmic [Ca2+] associated with membrane depolarization (Ca2+ transients) was studied on single cut skeletal muscle fibres of the frog, voltage clamped in a double-Vaseline-gap chamber. The Ca2+ transients were monitored with the dye Antipyrylazo III diffused intracellularly. 2. The Ca2+ transients were substantially reduced in external salines with low [Ca2+] (10(-5) M or less and Mg2+ substituted for Ca2+). This decrease was more noticeable at late times during 100 ms or longer depolarizing pulses. 3. The rates of the processes that remove Ca2+ from the myoplasmic solution were not altered by the low [Ca2+]o. This implies that the input flux of Ca2+ into the myoplasm was reduced. 4. The Ca2+ input flux, equal to release flux from the sarcoplasmic reticulum (SR) plus Ca2+ influx via the T-tubule membrane Ca2+ channel, was derived from the Ca2+ transient. In low [Ca2+]o the peak input flux was reduced by 45% (n = 16 fibres) and decayed more rapidly during a depolarizing pulse. 5. The reduction in Ca2+ influx via the T-tubule membrane Ca2+ channel due to the reduced [Ca2+]o could not account for more than 5% of the reduction in Ca2- input flux, which was thus interpreted as an actual reduction of release from the SR. 6. The inward (T-tubular) Ca2+ current was not associated with this effect of extracellular Ca2+ as the effect was voltage independent at high intracellular voltages at which the Ca2+ inward current was strongly voltage dependent. 7. Low [Ca2+]o made Ca2+ release more readily inactivatable; the effect of low [Ca2-]o is best described as a left shift by 29 mV of the 'inactivation curve' of Ca2+ release, relating peak release flux to membrane holding potential. 8. The reduction of Ca2+ release by low [Ca2+]o was not accompanied by changes in the voltage dependence of Ca2+ release or in the threshold voltage for just-detectable release. 9. The results are consistent with a primary effect of Ca2+ on the T-tubular-membrane voltage sensor of excitation-contraction coupling.

Effect of Ca2+ channel blockers on K+ contractures in twitch fibres of the frog (Rana pipiens)

1. The effects of Ca2+ channel blockers (nifedipine, nitrendipine and diltiazem) were tested on K+ contractures in single muscle fibres of the frog, Rana pipiens. 2. Nifedipine (1 microM) reduced the area under K+ contractures to 24 +/- 9% (4) (100 mM-K+) and 34 +/- 24% (4) (190 mM-K+). Nitrendipine (0.1 microM) reduced the area to 30 +/- 10% (4) (120 mM-K+). The blockade of the contractures was reversible. 3. Diltiazem (1 microM) shortened the first 190 mM-K+ contracture without affecting the peak amplitude. The first contractures, performed at 15-20 min after the removal of diltiazem, were greatly reduced to 29 +/- 14% (4). This effect was reversed after three to five contractures in the absence of the drug. Similar results were obtained with 60 and 100 mM-K+. 4. The resting potential in control saline and after a brief exposure to 120 mM-K+ was not affected by the dihydropyridines and diltiazem. 5. Slow and fast Ca2+ currents were not modified by 1 microM-diltiazem at any stimulation rate or with pre-pulse depolarizations. Diltiazem (50 microM) did not affect the fast Ca2+ current and reduced the slow one to 48 +/- 10% (4). 7. The reduction of K+ contractures by Ca2+ channel blocking agents was not related to a blockade of Ca2+ currents. This can be tentatively explained by interactions of these compounds on membranes sites which regulate the coupling between membrane depolarization and contraction.

D600 prolongs inactivation of the contractile system in frog twitch fibres

(1) Single twitch fibres were dissected from tibialis muscles of Rana temporaria and used to study the effect of D600 (gallopamil) on potassium-induced contractures. (2) 95 mM K-Ringer's was applied for 8-15 s at intervals of generally 2.5-5 min; at temperatures of 6-8 degrees C and in the absence of D600 the amplitude of the contractures remained fairly constant. After pretreatment with D600 (30 microM) a single (conditioning) K-contracture was sufficient to 'paralyze' the fibres (cf. Eisenberg et al. 1983). (3) Complete paralysis could also be achieved at 18-20 degrees C. In three fibres a single conditioning K-application was sufficient; in two more fibres two or three conditioning K-applications were required. (4) D600-paralysis could not only be achieved with high K-concentrations but also by conditioning with sub- or suprathreshold K-concentrations (20-40 mM); the duration of the conditioning periods required to induce complete paralysis was approximately the same before and after D600-treatment. (5) Contractures were partially abolished by application of 20-40 mM K-Ringer's for short conditioning periods; after D600-treatment the degree of contracture loss was similar. (6) At low temperature the state of partial or complete paralysis induced by subthreshold K-concentrations and D600 was maintained for long periods of time. (7) The presence of 10 mM Ca2+ did not protect the fibres from being paralyzed by treatment with D600 and high K-Ringer's at low temperature; however, more than one conditioning K-application was required.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of resting membrane potential and intactness of the T-tubules on caffeine contractures in rat skeletal muscle

We have studied the effects of changes in the resting membrane potential (Vm) and T-tubules on caffeine contracture (25 mM) elicited in rat soleus muscle in vitro at 34 degrees C. In high [K]o (30-140 mM, [K]o X [Cl]o constant) caffeine contractures were reduced by about 40-50% and had a faster time course than in normal Krebs ([K]o = 5 mM). Detubulation of the muscles by an osmotic treatment produces a reduction of about 30% in the caffeine contracture tension. Our results with high K solutions suggest a reduced sensitivity of the myofibrils to calcium released by caffeine. The effects of detubulation on caffeine contracture suggest that caffeine may have a direct effect on sarcolemma in addition to its well known action on the sarcoplasmic reticulum (SR). However, a depletion of the calcium content in the SR of depolarized muscle fibres as well as an anatomical damage produced by the osmotic treatment can not be ruled out as an explanation for the reduced caffeine contracture.

Contractile inactivation in frog skeletal muscle fibers. The effects of low calcium, tetracaine, dantrolene, D-600, and nifedipine

Short muscle fibers (approximately 1.5 mm) of Rana pipiens were voltage-clamped with a two-microelectrode technique at a holding potential of -100 mV. Using conditioning depolarizing ramps, with slopes greater than 0.2 mV/s, partially inactivated responses are obtained at threshold values between -55 and -35 mV. With slopes equal to or slower than 0.1 mV/s, one inactivates contraction without ever activating it. When the membrane potential is brought slowly to values more positive than about -40 mV, test pulses, applied on top of the ramps, bringing the membrane potential to values up to +100 mV, are ineffective in eliciting contractile responses, which indicates complete inactivation. After inactivation, contractile threshold is shifted by perhaps 10 mV, to about -40 mV. The sensitivity of fibers to depolarizing ramps is increased by D-600 (50 microM), dantrolene (50 microM), tetracaine (100 microM), and low calcium (10(-8) M). In the presence of these agents, complete inactivation was obtained using ramp slopes of 1, 0.8, 0.4, and 0.2 mV/s, respectively. Nifedipine was less effective. With D-600, once inactivation had been induced, no repriming occurred after repolarization to -100 mV, and partial recovery occurred after washing out the drug. With low calcium, tetracaine, and nifedipine, the tension-voltage relationship was not affected, whereas the steady state inactivation curve (obtained in repriming experiments) was shifted by 10-25 mV toward more negative potentials. With D-600, the activation curve was not modified, whereas the inactivation curve could not be obtained, because of repriming failure. With dantrolene, the inactivation curve was not affected, whereas the activation curve was shifted toward less negative potentials and peak tension diminished, depending on the pulse duration. The results indicate that it is possible to induce complete inactivation without activation, and to differentiate activation and inactivation parameters pharmacologically, which suggests that the two are separate processes.

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.

Charge movement and depolarization-contraction coupling in arthropod vs. vertebrate skeletal muscle

Voltage-dependent charge movement has been characterized in arthropod skeletal muscle. Charge movement in scorpion (Centuroides sculpturatus) muscle is distinguishable from that in vertebrate skeletal muscle by criteria of kinetics, voltage dependence, and pharmacology. The function of scorpion charge movement is gating of calcium channels in the sarcolemma, and depolarization-contraction coupling relies on calcium influx through these channels.

Muscle stiffness and electrical activity in paramyotonia congenita

To investigate the pathomechanism of paramyotonic stiffness, the mechanogram of isometric finger force and the electromyogram of the flexor digitorum muscle were simultaneously recorded in five unrelated paramyotonia congenita patients. Cooling of the forearm provoked "spontaneous" electrical activity, but the accompanying force was less than 5% of the maximal voluntary isometric contraction amplitude. The relaxation of maximal voluntary contractions executed in the cold had a normal first phase and a very slow second phase. The force amplitude at the beginning of the slow phase was up to 80% of the maximal contraction amplitude; the duration of the slow phase was up to several minutes. It was concluded that the slowed muscle relaxation is more important as a factor contributing to paramyotonic stiffness than spontaneous force generation. Involuntary electrical activity recorded during the slow relaxation phase was too low to account for the force. Intercostal muscle biopsies obtained from four patients showed similar phases of slow relaxation when stimulated to give isometric twitches or tetani in the cold. Extracellular recording with electrodes designed to pick up all activity from the small bundles clearly showed that the slow relaxation phase was not caused by spontaneous action potentials. One possible explanation for the slowed relaxation is a long-lasting depolarization-induced contracture of the muscle fibers following activation in the cold.