Ion channels in the sarcoplasmic reticulum of striated muscle

This review provides a summary of current concepts about the structure and single-channel properties of ryanodine receptor calcium release channels and counter ion channels that facilitate Ca2+ release and reuptake by the sarcoplasmic reticulum. Some recent results, obtained with single ryanodine receptor ion channels incorporated into lipid bilayers from terminal cisternae vesicles of rabbit skeletal muscle and sheep ventricular myocardium, are described. The ryanodine receptor is the major Ca2+ release channel in skeletal and cardiac muscle and has been studied in far greater detail than other sarcoplasmic reticulum ion channel proteins. Several ryanodine receptor genes have been cloned and sequenced, and isoforms of the protein have been detected in muscle and in endoplasmic reticulum of brain and many other tissues from mammals, lower vertebrates, nematodes and drosophila. The proteins from all species are tetramers of a peptide with a molecular mass of approximately equal to 560 kDa, containing approximately equal to 5000 amino acids, with a similar maximum single-channel conductance of 500-800 row S for monovalent cations at 250mM. Results presented here include: Ca2+ activation and adaptation of activity in skeletal ryanodine receptors with rapid changes in [Ca2+] controlled by perfusion; activation by FK506 and regulation of cooperative gating of skeletal ryanodine receptor channel activity by FK506-binding proteins; activation and block of cardiac ryanodine receptors by addition of reactive disulphides and by bilayer voltage. Effects of phosphorylation, calmodulin, triadin, calsequestrin and interactions with the alpha 1 subunit of the dihydropyridine receptor on ryanodine receptor activity are summarized. Potassium and chloride channels in skeletal muscle sarcoplasmic reticulum, are described.

Characteristics of two types of chloride channel in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

A comparison is made of two types of chloride-selective channel in skeletal muscle sarcoplasmic reticulum (SR) vesicles incorporated into lipid bilayers. The I/V relationships of both channels, in 250/50 mM Cl- (cis/trans), were linear between -20 and +60 mV (cis potential,) reversed near Ecl and had slope conductances of approximately 250 pS for the big chloride (BCl) channel and approximately 70 pS for the novel, small chloride (SCl) channel. The protein composition of vesicles indicated that both channels originated from longitudinal SR and terminal cisternae. BCl and SCl channels responded differently to cis SO4(2-) (30-70 mM), 4,4'-diisothiocyanatostilbene 2,2'-disulfonic acid (8-80 microM) and to bilayer potential. The BCl channel open probability was high at all potentials, whereas SCl channels exhibited time-dependent activation and inactivation at negative potentials and deactivation at positive potentials. The duration and frequency of SCl channel openings were minimal at positive potentials and maximal at -40 mV, and were stationary during periods of activity. A substate analysis was performed using the Hidden Markov Model (S. H. Chung, J. B. Moore, L. Xia, L. S. Premkumar, and P. W. Gage, 1990, Phil. Trans. R. Soc. Lond. B., 329:265-285) and the algorithm EVPROC (evaluated here). SCl channels exhibited transitions between 5 and 7 conductance levels. BCl channels had 7-13 predominant levels plus many more short-lived substates. SCl channels have not been described in previous reports of Cl- channels in skeletal muscle SR.

Porin-type 1 proteins in sarcoplasmic reticulum and plasmalemma of striated muscle fibres

The location of porin-type 1 proteins in mammalian striated muscle has been assessed using immunogold electron microscopy with an anti-porin 31HL monoclonal antibody as the primary antibody. Gold particles were found on the mitochondrial outer membrane, the sarcoplasmic reticulum and plasmalemma in longitudinal sections of rat and rabbit skeletal muscle and rabbit and sheep cardiac muscle. The relative densities of gold particles in the mitochondrial outer membrane, sarcoplasmic reticulum and plasmalemma were 7:3:1 in white sternomastoid muscle, for example. Skeletal and cardiac sarcoplasmic reticulum vesicles, which had been fractionated by discontinuous sucrose density centrifugation, were subjected to SDS-polyacrylamide gel electrophoresis and Western blotting. The anti-porin 31HL monoclonal antibody detected a band of relative molecular mass (M(r)) 31,000 in all muscle sarcoplasmic reticulum vesicle fractions and also in liver mitochondria. The intensity of immunostaining of the sarcoplasmic reticulum fractions was 2.5-10% that of mitochondrial outer membranes per microgram of membrane protein blotted. Contamination of the sarcoplasmic reticulum fractions by mitochondrial outer membrane was < 0.75% as determined from the specific activity of monoamine oxidase. Thus, only a small part of the porin detected in sarcoplasmic reticulum vesicles can be attributed to mitochondrial contamination. These results show that porin-type1 immunoreactivity is not restricted to mitochondria but found in the sarcoplasmic reticulum and plasmalemma of both mammalian skeletal and cardiac muscle.

Cytoplasmic Ca2+ inhibits the ryanodine receptor from cardiac muscle

Ca(2+)-dependent inhibition of native and isolated ryanodine receptor (RyR) calcium release channels from sheep heart and rabbit skeletal muscle was investigated using the lipid bilayer technique. We found that cytoplasmic Ca2+ inhibited cardiac RyRs with an average Km = 15 mM, skeletal RyRs with Km = 0.7 mM and with Hill coefficients of 2 in both isoforms. This is consistent with measurements of Ca2+ release from the sarcoplasmic reticulum (SR) in skinned fibers and with [3H]-ryanodine binding to SR vesicles, but is contrary to previous bilayer studies which were unable to demonstrate Ca(2+)-inhibition in cardiac RyRs (Chu, Fill, Stefani & Entman (1993) J. Membrane Biol. 135, 49-59). Ryanodine prevented Ca2+ from inhibiting either cardiac or skeletal RyRs. Ca(2+)-inhibition in cardiac RyRs appeared to be the most fragile characteristic of channel function, being irreversibly disrupted by 500 mM Cs+, but not by 500 mM K+, in the cis bath or by solublization with the detergent CHAPS. These treatments had no effect on channel regulation by AMP-PNP, caffeine, ryanodine, ruthenium red, or Ca(2+)-activation. Ca(2+)-inhibition in skeletal RyRs was retained in the presence of 500 mM Cs+. Our results provide an explanation for previous findings in which cardiac RyRs in bilayers with 250 mM Cs+ in the solutions fail to demonstrate Ca(2+)-inhibition, while Ca(2+)-inhibition of Ca2+ release is observed in vesicle studies where K+ is the major cation. A comparison of open and closed probability distributions from individual RyRs suggested that the same gating mechanism mediates Ca(2+)-inhibition in skeletal RyRs and cardiac RyRs, with different Ca2+ affinities for inhibition. We conclude that differences in the Ca(2+)-inhibition in cardiac and skeletal channels depends on their Ca2+ binding properties.

High-frequency fatigue in rat skeletal muscle: role of extracellular ion concentrations

High-frequency fatigue (HFF), the decline of force during continuous tetanic stimulation (lasting 4-40 s), was studied in isolated bundles of rat skeletal muscle fibers. HFF was slower in slow-twitch soleus fibers than in fast-twitch red or white sternomastoid fibers; denervation accelerated fatigue in soleus. Maximal 200-mmol/L potassium contractures of normal amplitude were induced in fatigued fibers, suggesting that crossbridge cycling and the voltage activation of excitation-contraction coupling could still occur maximally, but that activation by action potentials was impaired. An increase in [Na+]o slowed HFF, while a small increase in [K+]o or reduction in [Cl(-)]o accelerated HFF. Increasing the tetanic stimulation frequency exacerbated fatigue. Recovery from HFF proceeded rapidly since force increased markedly within a few seconds when stimulation ceased. These results support the hypothesis that a redistribution of Na+, K+, and Cl- across the transverse tubular membranes during repeated action potential activity induces fatigue by reducing the amplitude and conduction of action potentials.

Single channel activity of the ryanodine receptor calcium release channel is modulated by FK-506

The immunosuppressant drug FK-506 (3-20 microM) increased the open probability of ryanodine receptor calcium release channels, formed by incorporation of terminal cisternae vesicles from rabbit skeletal muscle into lipid bilayers, with cis (cytoplasmic) calcium concentrations between 10(-7) M and 10(-3) M. FK-506 increased mean current and channel open time and induced long sojourns at subconductance levels that were between 28% and 38% of the maximum conductance and were distinct from the ryanodine-induced subconductance level at about 45% of the maximum conductance. FK-506 relieved the Ca2+ inactivation of the ryanodine receptor seen at 10(-3) M Ca2+. The results are consistent with FK-506 removal of FK-506 binding protein from the ryanodine receptor.

Beta-adrenoceptor activation shows high-frequency fatigue in skeletal muscle fibers of the rat

The effect of terbutaline (a beta 2-adrenergic agonist) on high-frequency fatigue (HFF) was studied in small bundles of rat soleus muscle fibers. HFF, the decline in force during continuous stimulation (50 Hz for 20 s), was reduced by 10-20% with 10 microM terbutaline. A similar reduction in HFF with 2 mM dibutyryl-adenosine 3',5'-cyclic monophosphate (DBcAMP) implicated adenosine 3',5'-cyclic monophosphate (cAMP) as the second messenger in the terbutaline effect. Sodium (Na-K)-pump inhibition with 1 mM ouabain depressed peak tetanic force but did not significantly alter either the subsequent fatigue or the effect of terbutaline on fatigue. This suggested that the pump was neither rate limiting in HFF nor involved in the terbutaline effect. Nevertheless, a significant hyperpolarization recorded with terbutaline implied that beta 2-adrenoceptor activation stimulated the Na-K pump at rest. Caffeine (1 mM) slowed HFF and prevented additional effects with terbutaline. Caffeine is known to potentiate Ca2+ release from the sarcoplasmic reticulum (SR), and we suggest that terbutaline, acting via cAMP, facilitates Ca2+ release from the SR to better maintain myoplasmic Ca2+ concentration during continuous tetanic stimulation.

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.

Monoclonal antibody to skeletal muscle ryanodine receptor detects a polypeptide in rat brain: comparison of immunogenic fragments after limited proteolysis

A polypeptide of high molecular mass has been detected in mammalian brain by a monoclonal antibody, 5C3, raised against skeletal muscle ryanodine receptor. 5C3 does not crossreact with the cardiac ryanodine receptor, the isoform which is believed to be located in many regions of the brain. Endogenous proteases in brain formed a prominent immunogenic fragment of 116 kDa whereas five immunostaining polypeptides greater than 200 kDa were observed in skeletal muscle. Mild trypsin digestion of brain microsomes resulted in fragments of approximately 400 and approximately 280 kDa, of similar mass to two peptides formed from the skeletal muscle ryanodine receptor. However a peptide of 28 kDa, resistant to trypsin, was observed in muscle but not in brain. The brain polypeptide recognised by 5C3 is therefore not identical to the skeletal muscle ryanodine receptor.

Beta-adrenergic potentiation of E-C coupling increases force in rat skeletal muscle

We examined the mechanism(s) which allow terbutaline, a beta 2-adrenergic agonist, to increase isometric force in bundles of normal and denervated rat soleus fibers. Terbutaline (10 mumol/L) potentiated tetanic contractions during exposure to 1 mmol/L ouabain, 10 mumol/L nifedipine, or 0.5 mmol/L iodoacetate. Terbutaline induced equivalent increases in submaximal potassium (K+) contracture and tetanic force: these effects were mimicked by 2 mmol/L dibutyryl-cyclic AMP. Therefore, terbutaline increased force by a cyclic AMP-dependent mechanism other than enhancement of sodium-pump activity, dihydropyridine sensitive Ca2+ currents, glycolysis, or action potentials. Pretreatment with 1 mmol/L caffeine induced submaximal potentiation of peak tetanic force but prevented further potentiation by terbutaline. This suggested that terbutaline did not influence the myofilaments, but acted on the sarcoplasmic reticulum (SR) to increase the myoplasmic Ca2+ concentration and hence force production. We speculate that force is potentiated following beta-adrenoceptor activation by a cyclic AMP-dependent phosphorylation of Ca2+ release channels to facilitate SR calcium release during tetanic stimulation.

The effects of beta-adrenoceptor activation on contraction in isolated fast- and slow-twitch skeletal muscle fibres of the rat

1. The aim of the experiments was to examined the effects of beta-adrenoceptor activation on twitch and tetanic contractions in fast- and slow-twitch mammalian skeletal muscle fibres. Isometric force was recorded from bundles of intact fibres isolated from the normal and denervated slow-twitch soleus and normal fast-twitch sternomastoid muscles of the rat. 2. Terbutaline (10 microM), a beta 2-adrenoceptor agonist, induced an average 15% potentiation of peak twitch and peak tetanic force in normal soleus fibres and abbreviated twitch and tetanic relaxation. In white- and red-sternomastoid fibres, 10 microM terbutaline potentiated peak twitch force by about 7% and slowed twitch relaxation. 3. The potentiation of twitches and tetani by terbutaline was quantitatively similar in normal and denervated soleus fibres. However, in contrast to the normal soleus, terbutaline slowed twitch relaxation and had no effect on tetanic relaxation in denervated soleus fibres. 4. Adrenaline (10 microM) increased peak tetanic force by about 7% in both normal and denervated soleus fibres. 5. Exposure to (+/-)-propranolol (0.1 microM), a general beta-adrenoceptor blocker, completely abolished the tetanus potentiation by terbutaline. 6. Dibutyryl-cyclic AMP (2 mM) mimicked the effects of 10 microM terbutaline on peak tetanic force and tetanic relaxation in normal and denervated soleus fibres. Dibutyryl-cyclic AMP also potentiated peak twitch force in denervated soleus fibres but only after a brief period of twitch depression: the twitch depression might be due to butyrate. 7. The results suggest that the increase in peak twitch and tetanic force and abbreviation of tetanic relaxation induced by terbutaline depend on the activation of beta-adrenoceptors and a consequent increase in the myoplasmic cyclic AMP concentration.

Immunogold labeling of calcium ATPase in sarcoplasmic reticulum of skeletal muscle: use of 1-nm, 5-nm, and 10-nm gold

We evaluated the use of immunogold electron microscopy to study the distribution of calcium ATPase in the sarcoplasmic reticulum membrane of skeletal muscle. We examined (a) 1-nm gold labeling, (b) the effect of gold size on immunolabeling, and (c) the densities of gold particles in areas of maximal labeling in fibers from rat extensor digitorum longus and pig gracilis muscles. The technique allowed unequivocal identification of the calcium ATPase. Gold particles of 1 nm were successfully visualized in unstained or lightly stained sections and the density of labeling was about 20 times greater than with 10-nm gold. The average densities in areas of intense labeling were 2878 +/- 139/microns 2 with 5-nm gold and 4310 +/- 276/microns 2 with 1-nm gold. These numbers are similar to the density of particles in freeze-fracture replicas of sarcoplasmic reticulum. The low density of 10-nm gold suggests that the large gold particles hinder binding of secondary to primary antibodies. The difference between 1- and 5-nm gold is explained by the amounts of gold conjugated to the immunoglobulin. The results suggest that there is a one-to-one relationship between secondary immunoglobulins (1-nm or 5-nm gold conjugates) and oligomeric complexes of calcium ATPase.

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.

Ultrastructure of sarcoballs on the surface of skinned amphibian skeletal muscle fibres

The formation of sarcoballs on the surface of skinned fibres from semitendinosus muscles of Xenopus laevis, and the sarcoplasmic reticulum content of the structures, have been studied using conventional electron microscopic techniques and immunoelectron microscopy. Examination of the fibres showed many membrane-bound blebs projecting from the surface in areas where vesicles of internal membranes (including sarcoplasmic reticulum, T-tubules and mitochondria) were clustered in interfilament spaces. The blebs varied in size from 1 micron to 150 microns and those with diameters > 10 microns are referred to as sarcoballs. Small blebs were often seen in close association with each other and might have fused during sarcoball formation. The interior of the sarcoball was filled with foam-like material made up of vesicles with diameters of 100 nm to 1.0 microns. The sarcoplasmic reticulum membrane content of the sarcoballs was evaluated using two monoclonal antibodies, one to the Ca2+ ATPase of the sarcoplasmic reticulum and the second to ryanodine receptor calcium release channels in the junctional-face membrane. The antibodies bound to some components of the surface and interior of the sarcoball, but not to mitochondrial-like structures and tubular vesicles. The results show that a large component of the sarcoball and its surface is derived from sarcoplasmic reticulum and suggest that mitochondria and T-tubules might also contribute membranes to the structures. Our hypothesis is that (a) blebs bud out from the surface of the skinned fibre following fusion of internal vesicles that are extruded along interfilament channels during unrestrained contractures, (b) blebs grow into sarcoballs by additional fusion of internal membrane vesicles and fusion of adjacent blebs, and (c) the sarcoball is a foam-like structure composed of bathing medium and membrane lipid (containing membrane proteins).

Extra-junctional ryanodine receptors in the terminal cisternae of mammalian skeletal muscle fibres

The distribution of ryanodine receptor calcium-release channels over the terminal cisternae (TC) membrane of skeletal muscle fibres was examined by using immunogold electron microscopy. Two monoclonal antibodies (5C3 and 8E2) that bound to monomers of the ryanodine receptor protein on Western blots of SDS-polyacrylamide gels were used to locate calcium-release channels in longitudinal sections of rat sternomastoid and diaphragm fibres. Up to 21% of 5C3 binding on TC membranes was extra-junctional, compared with 46% for 8E2. Binding of 8E2 to the fibres was less than half that of 5C3, possibly because of steric shielding of the 8E2 antigenic site at the junction. The distances between neighbouring particles in clusters was 20-40 nm, i.e. the distance between subunits of the ryanodine receptor or between neighbouring foot structures. We suggest that, during activation, extra-junctional ryanodine receptors may release Ca2+ directly into the myoplasm, rather than into the restricted space of the triad junction.

Actions of perchlorate ions on rat soleus muscle fibres

1. The effects of perchlorate (ClO4-) on contraction have been studied in rat soleus muscle fibres using (i) potassium (K+) contracture and (ii) two-microelectrode-point voltage clamp techniques. 2. Membrane potentials (Vm) at all external [K+] were 3-5 mV more negative in ClO4-. The hyperpolarization could not be attributed to a change in Na+, K+, or Cl- permeability, or to an effect on the Na(+)-K+ pump. 3. ClO4- shifts the voltage dependence of tension activation, and contraction threshold, to more negative membrane potentials without altering maximum tension. Consequently, twitches and submaximal K+ contractures were potentiated, whereas tetanic contractions and 200 mM-K+ contractures were unaltered. 4. The decay of K+ contractures during steady depolarization with ClO4- developed a slow exponential phase with an average time constant of 6.05 +/- 0.76 min at -38 mV, and 1.68 +/- 0.15 min at -19 mV. This slow component was (a) under the rapid control of the surface Vm and (b) did not depend on external Ca2+. 5. Inactivation of E-C coupling was measured with a test 200 mM-K+ depolarization following 3-10 min depolarizations in conditioning solutions containing 20-120 mM-K+. ClO4- induced a negative shift in the curve-relating test K+ contracture amplitude to conditioning Vm but did not alter the rate of repriming of tension upon repolarization. 6. The results suggest that ClO4- increases the amount of activator produced during depolarization and thus allows the slow inactivation step in excitation-contraction (E-C) coupling to be reflected in the decay of K+ contracture tension. 7. A 'perchlorate contracture', which did not depend on the activation of E-C coupling, was observed. The contracture depended on external Ca2+, but not on voltage-dependent Ca2+ channels or Na(+)-Ca2+ exchange.

Calcium ATPase in the sarcoplasmic reticulum of muscle from normal and malignant hyperthermia susceptible pigs

The density of Mg(2+)-dependent Ca2+ ATPase in the terminal cisternae of pig skeletal muscle fibers was investigated to discover whether a reduction in Ca2+ ATPase content impairs Ca2+ sequestration and contributes to the elevated myoplasmic Ca2+ concentration in malignant hyperthermia. Unexpectedly, immunogold electron microscopy showed an increase in Ca2+ ATPase, while densitometry of SDS-polyacrylamide gels suggested that the Ca2+ ATPase content of terminal cisternae vesicles did not change. The affinity of Ca2+ ATPase in vesicles for our monoclonal antibody was not altered. We suggest that the availability of antigenic sites in malignant hyperthermia increases after processing for electron microscopy, perhaps as a consequence of altered sarcoplasmic reticulum membrane properties.

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)

The rate of tetanic relaxation is correlated with the density of calcium ATPase in the terminal cisternae of thyrotoxic skeletal muscle

The density of calcium ATPase was measured in the terminal cisternae of extensor digitorum longus (EDL) and soleus muscles from normal and thyrotoxic rats. The experiments tested the hypothesis that the rate of relaxation of these muscles following contraction, at temperatures above 22 C, is correlated with the density of calcium ATPase in the sarcoplasmic reticular membrane. In soleus fibres there was a progressive increase in calcium ATPase density, measured with immuno-electronmicroscopic techniques, of more than two-fold after 3 weeks of daily injections with triiodothyronine (T3). There was a parallel decrease in the relaxation time (from 80% to 20% of peak tension) of the tetanus: the parameters were closely correlated (r = 0.998) during the 3-week period. The rate of relaxation of the twitch also doubled and was correlated with the increase in gold particle density at the end of the 3-week injection period. However, twitch relaxation slowed during the 1st week of T3 injection and was not correlated with gold particle density at that time. The changes in calcium ATPase density and relaxation times in EDL fibres were small and largely insignificant. In contrast to relaxation, an increase in the rate of rise of tension is soleus was complete after only 2 weeks of T3 injection. The results show that the relaxation of tetanic tension is closely correlated with the calcium uptake capacity of the sarcoplasmic reticulum and that thyroid hormone acts more rapidly on factors regulating the rate of rise of tension than on those regulating tension relaxation and the density of calcium ATPase in the terminal cisternae.

Effects of cobalt, magnesium, and cadmium on contraction of rat soleus muscle

The effects on isometric tension of three divalent ions that block calcium channels, magnesium, cobalt, and cadmium, were tested in small bundles of rat soleus fibers. Cobalt, at a concentration of 2 or 6 mM, reversibly depressed twitch and tetanic tension and the depression was much greater in solutions containing no added calcium ions. Magnesium caused much less depression of tension than cobalt. The depression of tension was not accompanied by membrane depolarization or a reduction in the amplitude of action potentials. A reduction caused by 6 mM cobalt in the amplitude of 40 or 80 mM potassium contractures was not accompanied by a comparable reduction in tension during 200 mM potassium contractures, and could be explained by a shift in the potassium contracture tension-voltage curve to more positive potentials (by +7 mV on average). Similar effects were not seen with 2 or 6 mM magnesium. At a concentration of 20 mM, both cobalt and magnesium depressed twitch and tetanic tension, cobalt having greater effect than magnesium. Both ions shifted the potassium contracture tension-voltage curve to the right by +5 to +10 mV, caused a small depression of maximum tension, and slowed the time course of potassium contractures. Cadmium (3 mM) depressed twitch, tetanic, and potassium contracture tension by more than 6 mM cobalt, but experiments were complicated by the gradual appearance of large contractures that became even larger, and sometimes oscillatory, when the solution containing cadmium was washed out. It was concluded that divalent cations affect both activation and inactivation of tension in a manner that cannot be completely explained by a change in surface charge.

Noninactivating tension in rat skeletal muscle. Effects of thyroid hormone

Inactivation of excitation-contraction coupling was examined in extensor digitorum longus (EDL) and soleus muscle fibers from rats injected daily with tri-iodothyronine (T3, 150 micrograms/kg) for 10-14 d. Steady-state activation and inactivation curves for contraction were obtained from measurements of peak potassium contracture tension at different surface membrane potentials. The experiments tested the hypothesis that noninactivating tension is a "window" tension caused by the overlap of the activation and inactivation curves. Changes in the amplitude and voltage dependence of noninactivating tension should be predicted by the changes in the activation and inactivation curves, if noninactivating tension arises from their overlap. After T3 treatment, the area of overlap increased in EDL fibers and decreased in soleus fibers and the overlap region was shifted to more negative potentials in both muscles. Noninactivating tension also appeared at more negative membrane potentials after T3 treatment in both EDL and soleus fibers. The effects of T3 treatment were confirmed with a two microelectrode voltage-clamp technique: at the resting membrane potential (-80 mV) contraction in response to a brief test pulse required less than normal depolarization in EDL, but more than normal depolarization in soleus fibers. After T3 treatment, the increase in contraction threshold at depolarized holding potentials (attributed to inactivation) occurred at more depolarized holding potentials in EDL, or less depolarized holding potentials in soleus. The changes in contraction threshold could be accounted for by the effects of T3 on the activation and inactivation curves. In conclusion, (a) T3 appeared to affect the expression of both activation and inactivation characteristics, but the activation effects could not be cleanly distinguished from T3 effects on the sarcoplasmic reticulum and contractile proteins, and (b) the experiments provided evidence for the hypothesis that the noninactivating tension is a steady-state "window" tension.

Feet, bridges, and pillars in triad junctions of mammalian skeletal muscle: their possible relationship to calcium buffers in terminal cisternae and T-tubules and to excitation-contraction coupling

The structure of the triad junction was examined in thin sections of mammalian fast-twitch skeletal muscle. The aims of the experiments were twofold: first, to examine relationships between the contents of the junctional gap and the terminal cisternae that could be significant in excitation-contraction coupling and, second, to look for structures in the transverse tubules that could support a calcium buffer system. Procedures known to stabilize cytoskeletal elements were used in an attempt to retain the original structure. "Feet," "pillars" and "bridges" were often seen side by side in the same junction. In one such junction, the average center-to-center spacing between four bridges was 30.9 +/- 1.7 nm and between five foot-like structures was 29.2 +/- 1.4 nm. The subunit structure of the feet could be seen in many sections. The lumen of the terminal cisternae was filled with a tetragonal network of calsequestrin which formed parallel strands near the junctional membrane, in register with the feet. The strands overlay the area occupied by "rods" seen in freeze-fracture replicas of terminal cisterna membrane. The contents of the transverse tubules were aggregated into bands, or "tethers," which extended across the short axis of the tubule at regular intervals of about 30 nm. The tethers consisted of flattened discs, stacked across the long axis of the tubule, aligned with the junctional feet. Lanthanum staining of the tethers indicated cationic binding sites that could buffer luminal calcium ion concentration in the vicinity of the voltage sensor for contraction. It is suggested (i) that the control of calcium concentration near the voltage sensor is necessary for normal activation, (ii) that feet, pillars and bridges are different images of a spanning structure, and (iii) that the regular alignment of tethers, feet and calsequestrin is functionally significant in excitation-contraction coupling.

Effects of extracellular calcium concentration and dihydropyridines on contraction in mammalian skeletal muscle

1. Twitches, tetanic contractions and potassium contractures were recorded isometrically from small bundles of rat soleus muscle fibres. 2. Solutions with reduced calcium concentrations (low-calcium solutions), whether buffered with EGTA (85 and 3 microM-Ca2+) or not (15 microM-Ca2+), caused an initial potentiation of contraction followed by depression. 3. The decay of potassium contractures (200 mM-potassium) was more rapid than normal in low-calcium solutions. 4. Recovery from the inactivation produced by a 200 mM-potassium contracture was slowed in low-calcium solutions but full recovery was seen within 10-15 min after return to a solution containing 2.5 mM-Ca2+. 5. Nifedipine (50 microM) in solutions containing 2.5 mM-Ca2+ potentiated contraction whereas, in low-calcium solutions, contraction was depressed and the depression was more pronounced the lower the Ca2+ concentration. 6. As with low-calcium solutions, potassium contractures decayed more rapidly in solutions containing nifedipine. Nifedipine slowed still further the rate of recovery from inactivation in low-calcium solutions. 7. (-) Bay K 8644 (50 microM) depressed contraction, increased the rate of decay of potassium contractures and slowed recovery from inactivation, like nifedipine. The racemate of Bay K 8644 was less effective. 8. In explanation of these and other observations, it is proposed that there is a dihydropyridine-binding molecule in the walls of the transverse tubular system that normally exists predominantly in a 'precursor' form at the resting membrane potential and is converted by membrane depolarization to an 'activator' form essential for excitation-contraction coupling. Conversion of the precursor to activator involves both conformational change and dissociation of calcium. Prolonged depolarization converts activator to an inactivated form by inducing further conformational change and dissociation of calcium. Recovery from inactivation requires reverse conformational changes and rebinding of calcium. The dihydropyridines affect contraction by reducing the affinity of the molecule for calcium.

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.