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A F Dulhunty

Publications and source records attributed to A F Dulhunty.

At least 19 recordsLinked to original sources
PubMed1992

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.

Animals↗
PubMed1992

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).

Animals↗
PubMed1992

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.

Animals↗
PubMed1991

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.

Animals↗
PubMed1991

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)

Animals↗
PubMed1990

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.

Animals↗
PubMed1989

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.

Animals↗
PubMed1989

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.

Action Potentials↗
PubMed1989

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.

Animals↗
PubMed1988

Paralysis of skeletal muscle by butanedione monoxime, a chemical phosphatase.

The chemical phosphatase butanedione monoxime (BDM) reversibly inhibited twitches and tetanic contractions in bundles of rat soleus fibres in a dose-dependent manner (2-20 mM) but had no effect on the amplitude or time course of action potentials. In addition, BDM reversibly reduced the amplitude of potassium contractures demonstrating a depressant effect on contraction not mediated by action potentials. BDM had no effect on asymmetric charge movement but depressed calcium currents across the surface membrane in voltage-clamped fibres. The most significant effect of BDM on excitation-contraction coupling was a reduction in the amplitude of the calcium transient associated with contraction in aequorin-injected fibres. While these experiments do not eliminate the possibility of a direct effect of BDM on contractile filaments, reduction of calcium release from the sarcoplasmic reticulum, at least at low concentrations of BDM (below 2 mM), would seem to be the main mechanism for the inhibition of contractions in rat skeletal muscle.

Aequorin↗
PubMed1988

Internal citrate ions reduce the membrane potential for contraction threshold in mammalian skeletal muscle fibers.

An effect of internal citrate ions on excitation-contraction coupling in skeletal muscle is described. The threshold for contraction was measured in rat extensor digitorum longus, (EDL), and soleus muscle fibers using a two microelectrode voltage clamp technique with either KCl-filled or K3 citrate-filled current electrodes. Contraction thresholds were stable for many minutes with KCl current electrodes. In contrast, thresholds fell progressively towards the resting membrane potential, by as much as -15 mV over a period of 10 to 20 min of voltage-clamp with citrate current electrodes. In addition, prepulse inhibition was suppressed, subthreshold activation enhanced and steady-state inactivation shifted to more negative potentials. Fibers recovered slowly from these effects when the citrate electrode was withdrawn and replaced with a KCl electrode. The changes in contraction threshold suggest that citrate ions act on the muscle activation system at an intracellular site, since the citrate permeability of the surface membrane is probably very low. An internal citrate concentration of 5 mM was calculated to result from citrate diffusion out of the microelectrode into the recording area for 20 min. 5 mM citrate added to an artificial cell lowered the free calcium concentration from 240 to 31 microM. It is suggested that citrate modifies excitation-contraction coupling either by acting upon an anion-dependent step in activation or by reducing the free calcium and/or free magnesium concentration in the myoplasm.

Animals↗
PubMed1988

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.

Animals↗
PubMed1988

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.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

The effect of diazepam on potassium contractures, contraction threshold, and resting tension in rat skeletal muscles.

The effects of diazepam on potassium contractures, contraction threshold, and resting tension have been examined in rat soleus muscle fibres. Two actions of the drug were defined that could not be attributed to changes in the resting membrane potential or depolarization in high potassium solutions. The major effect was an increase in the amplitude of submaximal tension during either twitches or potassium contractures and an increase in resting tension. At 400 microM diazepam, there was (a) a fourfold increase in 40 mM potassium contracture tension, (b) a negative shift of 8 mV in the membrane potential for half maximum tension estimated from the best fit of a Boltzmann-type equation to average potassium contracture data, (c) a negative shift of 8 mV in the threshold for contraction measured under voltage clamp conditions, and (d) a contracture of variable amplitude to a level that was occasionally equivalent to maximum tetanic tension. These potentiating actions of diazepam depended on drug concentration within the range of 100-800 microM. In contrast, the second effect of diazepam, depression of maximum tension by 10-15%, was independent of drug concentration between 100 and 400 microM. The results support the idea that diazepam produces an increase in resting myoplasmic calcium concentrations.

Animals↗
PubMed1987

Potassium contractures and asymmetric charge movement in extensor digitorum longus and soleus muscles from thyrotoxic rats.

Potassium contractures and asymmetric charge movement were recorded from extensor digitorum longus (EDL) and soleus muscle from normal rats and rats that had been made thyrotoxic by daily intraperitoneal injections of triiodothyronine (150 micrograms kg-1) for two to three weeks. Potassium contracture tension (relative to tetanic tension) in thyrotoxic rats was greater in EDL muscles and smaller in soleus muscles than in normal rats. As the relationship between membrane potential and potassium concentration was unaltered by thyroid treatment, it was concluded that the changes in potassium contracture tension were due to changes in excitation-contraction coupling. In thyrotoxic rats there was an average negative shift of -5 mV in the voltage sensitivity of tension in EDL fibres and a positive shift of 5 mV in soleus. As a result, the tension-membrane potential curves for EDL and soleus fibres essentially coincided. There was a corresponding average negative shift of -4 mV in the voltage sensitivity of asymmetric charge in EDL fibres, and a positive shift of 4 mV in soleus fibres from thyrotoxic rats. The dependence of asymmetric charge movement on membrane potential became essentially the same in EDL and soleus fibres from thyrotoxic rats. The maximum asymmetric charge in soleus fibres increased from an average value of 6.5 nC microF-1 in normal rats (n = 33) to 8.9 nC microF-1 in thyrotoxic rats (n = 32; p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Distribution of calcium ATPase in the sarcoplasmic reticulum of fast- and slow-twitch muscles determined with monoclonal antibodies.

Four monoclonal antibodies against the calcium ATPase in sarcoplasmic reticulum (SR) of rabbit fast-twitch skeletal muscle were characterized using SDS-PAGE, Western blots and immunofluorescence. The ultrastructural distribution of the antigens was determined using post-embedding immunolabeling. The antibodies recognized the calcium ATPase in the SR but not in transverse (T-) tubule or plasma membranes. The antibody, D12, had the same binding affinity for the calcium ATPase from fast-twitch (rabbit sternomastoid) and slow-twitch (rabbit soleus) fibers and the affinity fell by 30% after fixation for electron microscopy in both types of muscle fiber. Ultrastructural studies revealed that the density of D12 antibody binding to the terminal cisternae membrane of extensor digitorum longus (edl) and sternomastoid fibers was on average seven times greater than in the slow-twitch soleus and semimembranosus fibers. Since the affinity of the ATPase for the antibody was the same in SR from fast- and slow-twitch muscles, the concentration of calcium ATPase in the terminal cisternae membrane of fast-twitch fibers was seven times greater than in slow-twitch fibers. This conclusion was supported by the fact that the concentration of calcium ATPase in light SR membranes was six times greater in SR from fast-twitch fibers than in SR from slow-twitch fibers. The results provide strong evidence that the different calcium accumulation rates in mammalian fast- and slow-twitch muscles are due to different concentrations of calcium ATPase molecules in the SR membrane.

Animals↗
PubMed1987

Diazepam reveals different rate-limiting processes in rat skeletal muscle contraction.

The action of the tranquilizer diazepam on rat skeletal muscle showed that relaxation of isometric twitches is controlled by different processes in extensor digitorum longus (fast-twitch) and soleus (slow-twitch) muscles. Diazepam caused an increase in the amplitude of twitches in fibres from both muscles but increased the twitch duration only in soleus. The amplitude of fused tetani were reduced in both muscles and the rate of relaxation after the tetanus slowed by as much as 34% when the amplitude of the tetanus was reduced by only 11%. The slower tetanic relaxation indicated that calcium uptake by the sarcoplasmic reticulum was slower than normal in slow- and fast-twitch fibres. We conclude therefore that calcium uptake by the sarcoplasmic reticulum is rate limiting for twitch relaxation in slow-twitch but not fast-twitch fibres and suggest that calcium binding to parvalbumin controls relaxation in the fast fibres.

Animals↗