Noninactivating tension in rat skeletal muscle. Effects of thyroid hormone

Functional roles of the gamma subunit of the skeletal muscle DHP-receptor

In excitation-contraction coupling (EC coupling) of skeletal muscle, large and rapid changes of the myoplasmic Ca2+ concentration mediate the activation and termination of force. The L-type Ca2+ channel (dihydropyridine receptor, DHP receptor) is a central component of the EC coupling process. Its predominant role is to provide the Ca2+ release channels of the sarcoplasmic reticulum (SR) with the sensitivity to cell membrane voltage. The DHP receptor consists of five different proteins (alpha1S, beta1, gamma1, delta and alpha2) whose tasks and functional characteristics are still incompletely understood. This short review summarizes progress made in studying the physiology of the gamma1 subunit, a membrane polypeptide that is highly specific for skeletal muscle. The focus is on recent results obtained from muscle of gamma1-deficient mice.

Altered inactivation of Ca2+ current and Ca2+ release in mouse muscle fibers deficient in the DHP receptor gamma1 subunit

Functional impacts of the skeletal muscle-specific Ca2+ channel subunit gamma1 have previously been studied using coexpression with the cardiac alpha1C polypeptide in nonmuscle cells and primary-cultured myotubes of gamma1-deficient mice. Data from single adult muscle fibers of gamma-/- mice are not yet available. In the present study, we performed voltage clamp experiments on enzymatically isolated mature muscle fibers of the m. interosseus obtained from gamma+/+ and gamma-/- mice. We measured L-type Ca2+ inward currents and intracellular Ca2+ transients during 100-ms step depolarizations from a holding potential of -80 mV. Ratiometric Ca2+ transients were analyzed with a removal model fit approach to calculate the flux of Ca2+ from the sarcoplasmic reticulum. Ca2+ current density, Ca2+ release flux, and the voltage dependence of activation of both Ca2+ current and Ca2+ release were not significantly different. By varying the holding potential and recording Ca2+ current and Ca2+ release flux induced by 100-ms test depolarizations to +20 mV, we studied quasi-steady-state properties of slow voltage-dependent inactivation. For the Ca2+ current, these experiments showed a right-shifted voltage dependence of inactivation. Importantly, we could demonstrate that a very similar shift occurred also in the inactivation curve of Ca2+ release. Voltages of half maximal inactivation were altered by 16 (current) and 14 mV (release), respectively. Muscle fiber bundles, activated by elevated potassium concentration (120 mM), developed about threefold larger contracture force in gamma-/- compared with gamma+/+. This difference was independent of the presence of extracellular Ca2+ and likely results from the lower sensitivity to voltage-dependent inactivation of Ca2+ release. These results demonstrate a specific alteration of voltage-dependent inactivation of both Ca2+ entry and Ca2+ release by the gamma1 subunit of the dihydropyridine receptor in mature muscle fibers of the mouse.

Effects of membrane potential on just detectable movement in rat skeletal muscle: effects of denervation

The potential, Vt, at which a brief test depolarization first elicited movement was determined using two-microelectrode point voltage clamp. We expected that inactivation of excitation-contraction coupling at conditioning potentials between -60 and 0 mV would shift Vt to more positive potentials, and that fibers would become inactivatable with less conditioning depolarization in EDL than soleus. The curve relating Vt to conditioning potential had a negative slope (which was insensitive to addition of 1 mM cobalt or replacement of calcium with 20 mM CaEGTA) between -60 and -35 mV and a steep positive slope with further depolarization Unexpectedly, fibers became inactivatable with less conditioning depolarization in soleus than in EDL when Vt was measured with 50 msec test pulses. However, the positive shift in Vt became less steep as test pulse duration lengthened in soleus fibers. When Vt obtained with test pulses approaching rheobase (10 msec in EDL and 500 msec in soleus) was compared, EDL fibers became inactive with less conditioning depolarization than soleus fibers. The increase in Vt became steeper with 1 mM cobalt or 20 mM CaEGTA and was shifted to more positive potentials by denervation in soleus fibers. We conclude that inactivation (i) does not strongly influence threshold contractions at conditioning potentials between -60 and -40 mV and (ii) influences Vt between -40 and 0 mV in a manner that depends on test pulse duration.

Do independent processes control the activation and inactivation of potassium contracture tension in rat skeletal muscle?

Potassium (K+) contracture tension, measured in small bundles of rat soleus muscle fibers during maintained depolarization, increases to a peak value and then decays either to the baseline or to a pedestal level. We have tested the hypothesis that the rise and fall of tension are determined by independent activation and inactivation processes. If the "Independence" hypothesis is correct, tension during the decay of K+ contractures should equal tension predicted from the product of the activation and inactivation parameters determined from the same K+ contractures. Both the measured and predicted tensions decayed to a pedestal level that was increased in amplitude in the presence of perchlorate ions. However, the measured tensions in normal solutions and in the presence of perchlorate were three to five times smaller than the predicted tensions. This result indicates that the activation and inactivation of processes controlling the rise and decay of K+ contracture tension are not independent.

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.

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.

Activation and inactivation of excitation-contraction coupling in rat soleus muscle

1. Potassium (K+) contractures have been used to characterize the processes of activation and inactivation of excitation-contraction coupling during prolonged depolarization of fibres in small bundles dissected from rat soleus muscles at 23 degrees C. 2. The smallest measurable K+ contracture tension was recorded with depolarization to -40 mV in 30 mM-K+ and maximum tension was achieved between -26 mV in 80 mM-K+ and -19 mV in 120 mM-K+. 3. The rate of inactivation of K+ contracture tension was voltage dependent. Tension decayed from 80 to 20% of the peak amplitude within 44.0 +/- 2.2 s at -26 mV (in 80 mM-K+), compared with 66.7 +/- 4.8 s at -35 mV (in 40 mM-K+). Results are given as mean +/- 1 S.E.M. 4. The effect of inactivation on maximum tension was determined using a two pulse protocol in which a 'conditioning' depolarization in solutions containing 20-120 mM-K+ was applied for 0.5-10 min before a 'test' depolarization to -8 mV in 200 mM-K+. The amplitude of the test contracture was compared with the mean amplitude of 'control' 200 mM-K+ contractures elicited in normally polarized fibres immediately before and after the two pulse protocol. Conditioning depolarization to -47 mV (in 20 mM-K+) did not reduce test 200 mM-K+ contracture tension. Significant inactivation was seen with further conditioning depolarization to more positive potentials: after 10 min at -40 mV (in 30 mM-K+), or -35 mV (in 40 mM-K+), test 200 mM-K+ contracture tension was reduced by 33 and 70% respectively. 5. In contrast to amphibian muscle, where maximum tension falls to zero within a few minutes of depolarization to potentials positive to -50 mV, test 200 mM-K+ contracture tension in rat soleus fibres fell initially rapidly and then slowly, but was not reduced to zero, even after 10 min at -19 mV in 120 mM-K+. 6. The fast phase of inactivation of test 200 mM-K+ contracture tension occurred during the decay of the conditioning K+ contracture. The slow phase of inactivation reached completion after 10 min of conditioning depolarization and occurred during the period when conditioning tension was reduced to zero or to a plateau level. Both phases of inactivation in rat soleus fibres are slow compared with fast and slow inactivation times of 5-100 s respectively reported for amphibian muscle. 7. When repolarized after prolonged depolarization, the muscle fibres were initially refractory, i.e. unable to produce tension in response to electrical stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute effects of thyroid hormone on sodium currents in neonatal myocytes

Sodium currents and action potentials were recorded from myocytes of neonatal rats during acute exposure to thyroid hormone (5-20 nM). One to 5 minutes after addition of thyroid hormone to the bath, decay from peak Na current was slowed, with the fractional current flowing 20 ms after onset (relative to peak current) increasing from 6 +/- 5% to 17 +/- 13% (p less than 0.01, n = 12). Action potential durations were increased from 55 +/- 14 to 86 +/- 36 msec (p less than 0.05, n = 6). The effects of thyroid hormone were partially reversed by lidocaine (60 microM, n = 5), a specific blocker of a slow sub-population of Na channels. Thus thyroid hormone interacts directly with myocyte membrane, probably by slowing of inactivation of Na channels.