THE EFFECTS OF THYROXINE TREATMENT ON SLOW- AND FAST-CONTRACTING SKELETAL MUSCLE CONTRACTIONS OF THE CAT AND THEIR CYCLIC AMP LEVEL

1. The effects of thyroxine treatment on soleus and extensor digitorum longus (EDL) muscle contractions and their cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels were examined in anaesthetized cats. 2. Thyroxine treatment decreased the tension of incomplete tetanic contractions of the soleus as well as the EDL muscles. The effect on tension of these muscles was not associated with an increase in the cyclic AMP level of the muscle as is the case with a beta 2-adrenoceptor agonist effect. 3. The results do not support the involvement of cyclic AMP in the tension depressant effect of thyroxine on contractions of skeletal muscle. 4. It is suggested that the muscle weakness and tremor observed in thyrotoxicosis and during administration of beta 2-adrenoceptor agonists are mediated by different mechanisms.     

Pentobarbitone and skeletal muscle contractions: on the interaction with the effect elicited by the beta-adrenoceptor agonist, terbutaline.

The soleus, a slow-contracting muscle, and the extensor digitorum longus (EDL), a fast-contracting muscle from guinea-pig were prepared for isometric recording in vitro. Subtetanic contractions were evoked by transmural field-stimulation. Pentobarbitone increased the force of contraction in both muscles. In the soleus it shifted the stimulation frequency-response curve to the left. Terbutaline caused a decrease in the force of subtetanic contractions of the soleus, an effect which was dependent on the stimulation frequency. In the presence of pentobarbitone, the stimulation frequency had to be lowered by about 2 HZ in order to maintain the optimum response to terbutaline. The EDL responded to terbutaline with an increased force of contraction. In this case the stimulation frequency was less critical and the effects were the same in the presence and in the absence of pentobarbitone. Experiments with alpha-chloralose yielded results similar to those obtained with pentobarbitone.     

Pentobarbitone and skeletal muscle contractions: on the interaction with the effect elicited by the β-adrenoceptor agonist,terbutaline

The soleus, a slow-contracting muscle, and the extensor digitorum longus (EDL), a fast-contracting muscle from guinea-pig were prepared for isometric recording in vitro. Sub-tetanic contractions were evoked by transmural field-stimulation. Pentobarbitone increased the force of contraction in both muscles. In the soleus it shifted the stimulation frequency-response curve to the left. Terbutaline caused a decrease in the force of subtetanic contractions of the soleus, an effect which was dependent on the stimulation frequency. In the presence of pentobarbitone, the stimulation frequency had to be lowered by about 2 Hz in order to maintain the optimum response to terbutaline. The EDL responded to terbutaline with an increased force of contraction. In this case the stimulation frequency was less critical and the effects were the same in the presence and in the absence of pentobarbitone. Experiments with α-chloralose yielded results similar to those obtained with pentobarbitone.     

Effects of isoprenaline on contractions of directly stimulated fast and slow skeletal muscles of the guinea-pig

1. The actions of isoprenaline on the contraction and the resting potential of the isolated extensor digitorum longus (EDL), a fast contracting muscle, and the soleus, a slow contracting muscle, of the guinea-pig were investigated. Twitch tension was elicited by direct supramaximal stimulation and recorded isometrically.2. The twitch tension of EDL elicited by pulses of 0.5-10 ms duration was increased in the presence of isoprenaline (1 mug/ml). Isoprenaline increased the twitch tension of the soleus elicited by a pulse of more than 5 ms duration, but decreased it when elicited by a pulse of less than 1 millisecond. These effects were blocked by propranolol (1-3 mug/ml) but not by phentolamine (1-5 mug/ml).3. In EDL, isoprenaline prolonged the time to peak tension and the half-relaxation time. The twitch of the soleus was shortened by isoprenaline due to an acceleration of relaxation. These findings were independent of stimulus duration.4. The potentiating effects of isoprenaline on the twitch tension of EDL and the soleus were not observed in K(+)-free Krebs solution and were abolished by ouabain (1 mug/ml) and by reduction of the temperature from 33 degrees to 18 degrees C. The effects of isoprenaline on the relaxation proces were not affected by these treatments.5. In EDL, the resting potential increased from 77.3 mV to 78.5 mV after isoprenaline, whereas in the soleus it increased from 69.1 to 74.7 mV. These effects were blocked by propranolol, K(+)-deficiency, ouabain, and cooling to 18 degrees C. Hyperpolarization by isoprenaline was increased by substitution of isethionate for the external chloride.6. There was a good correlation between the potentiation of the mechanical response and the hyperpolarization of the membrane by isoprenaline. The hyperpolarization seems to be due to activation of the Na(+)-K(+) pump.     

EFFECTS OF CATECHOLAMINES, CYCLIC NUCLEOTIDES AND PHOSPHODI-ESTERASE INHIBITORS ON CONTRACTIONS OF SKELETAL MUSCLES IN ANAESTHETIZED CATS

SUMMARY 1. The actions and interactions of compounds with phosphodiesterase-in-hibiting activity and of sympathomimetic amines have been studied on contractions of skeletal muscles in chloralose-anaesthetized cats treated with bethanidine and in which the adrenals were excluded from the circulation.
2. Compounds with phosphodiesterase-inhibiting activity, ICI 63,197, ICI 58,301, papaverine, theophylline, and dipyridamole, potentiated isoprenaline in its depressant effect on tension and fusion of incomplete tetanic contractions of the slow-contracting soleus muscle, the order of potency being as listed. ICI 63,197 and theophylline also potentiated adrenaline and salbutamol in depressing contractions.
3. ICI 63,197 potentiated isoprenaline in its enhancing effect on tension and degree of fusion of incomplete tetanic contractions of the fast-contracting tibialis anterior and flexor digitorum longus muscles.
4. The results are compatible with the hypothesis that the effects of sympathomimetic amines on muscle contractility are mediated by cyclic adenosine-3',5'-monophosphate, although this nucleotide and its dibutyryl analogue, injected intra-arterially, were themselves without any consistent effect.
5. High doses of ICI 63,197, ICI 58,301, papaverine and dipyridamole, themselves produced isoprenaline-like effects on the soleus muscle. These effects were partially antagonized by (±)-propranolol, (+)-propranolol, or sotalol. The (+)-isomer of propranolol was only about ten times less potent than racemic propranolol in this respect. This antagonistic action of propranolol and sotalol appeared to be independent of β-adrenoceptor blockade.     

Dissociation between the effects of some xanthine derivatives on the tracheal smooth muscle and on the skeletal muscle

The ability of some xanthine derivatives to relax the trachea, contracted by pilocarpine, and to increase the force of contraction of directly stimulated skeletal muscles from the guinea-pig was studied in vitro. No relationship was found between these two effects. Relaxation of the trachea occurred at lower concentrations and with a different order of potency as compared with the effects on the slow-contracting soleus muscle or on the fast-contracting extensor digitorum longus. One of the compounds, IBMX, 1-methyl-3-isobutyl-xanthine, showed an isoprenaline-like effect on the soleus muscle i.e. it depressed the force and fusion of subtetanic contractions. The relaxing effect of theophylline and IBMX on the trachea was additive to that of terbutaline but no clear potentiation was observed. The depression of the contraction of the soleus muscle elicited by terbutaline was reinforced by IBMX but not by theophylline. Theophylline in concentrations which used alone enhanced the contractions of the soleus muscle inhibited the effect of terbutaline. We conclude that the relative contribution of the various effects of xanthine derivatives differs from compound to compound.     

Correlation between the effects of salbutamol on contractions and cyclic AMP content of isolated fast — and slow — contracting muscles of the guinea pig

Summary The effects of isoprenaline and salbutamol on incomplete tetanic contractions of the isolated soleus (slow contracting) and extensor digitorum longus (EDL-fast-contracting) muscles of the guinea pig were studied and an attempt made to correlate these effects on contractility with changes in cyclic AMP concentrations. Salbutamol was 10–12 times less potent than (±)isoprenaline in decreasing the force of subtetanic contractions in the soleus and between 5–6 times less potent in increasing the force of subtetanic contractions in the EDL.This observation plus the lack of activity of both the selective ₁-adrenoceptor antagonist (atenolol) and the selective ₁ agonist (H 133/22) in the EDL implies involvement of ₂-adrenoceptors in these responses of the muscles to isoprenaline and salbutamol. The soleus muscle was about 6–12 times more sensitive to effects of -adrenoceptor agonists than the EDL. In concentrations which produced effects on muscle contractility, salbutamol significantly elevated cyclic AMP concentrations in both types of muscle. These effects were antagonised by propranolol. It seems clear that the contrasting effects of sympathomimetic amines on slow — and fast contracting muscle are mediated through a common mechanism — elevation of cyclic AMP. Possible explanations of this apparent paradox are discussed.     

SELECTIVE DEVELOPMENT OF TOLERANCE TO β-ADRENOCEPTOR AGONISTS IN SKELETAL MUSCLE AS COMPARED WITH AIRWAY SMOOTH MUSCLE FROM THE GUINEA-PIG

1. Guinea-pig were fed with a diet containing terbutaline or placebo for 4--5 days. The trachea, soleus muscle and the extensor digitorum longus (EDL) from these animals were prepared for recording of isometric contractions in vitro. 2. After treatment with terbutaline in vivo, the response of the pilocarpine-contracted trachea to terbutaline and isoprenaline was slightly suppressed with no change in maximum relaxation. 3. After treatment with terbutaline in vivo the maximum depression of the incomplete tetanic contractions of the soleus muscle brought about by terbutaline or isoprenaline was diminished by about 70%. The response of the EDL was also attenuated after previous treatment with terbutaline in vivo. 4. These data indicate a selective development of tolerance to the effects of beta-adrenoceptor agonists in skeletal muscle as compared with tracheal smooth muscle. 5. The present results provide an experimental analogue to the clinical observation that patients being treated with beta-adrenoceptor agonists become tolerant to the tremorogenic rather than to the bronchodilating effect.     

EFFECTS OF ISOPRENALINE, LEVODOPA AND A PHOSPHODIESTERASE INHIBITOR (ICI 63,197) ON CYCLIC AMP LEVELS AND CONTRACTIONS OF SOLEUS MUSCLES IN ANAESTHETIZED CATS

1. Cyclic AMP levels have been determined in the soleus muscles of anaesthetized cats in the absence of drugs, and during depression of incomplete tetanic contractions produced by (-)-isoprenaline, ICI 63,197 (a phosphodiesterase inhibitor) or levodopa. 2. Cyclic AMP levels were elevated at the peak of tension depression produced by isoprenaline. Effects of isoprenaline on cyclic AMP and on contractions were dose dependent and statistically significantly related one to the other. Both effects were blocked by propranolol. 3. ICI 63,197 and levodopa produced isoprenaline-like effects on contractions but times to peak effect and recovery were longer. Cyclic AMP levels estimated during the depressant action were elevated. 4. The results support the involvement of cyclic AMP in the depressant effect of beta-adrenoreceptor agonists on slow-contracting mammalian skeletal muscle.     

EFFECTS OF THYROXINE TREATMENT ON CONTRACTIONS OF SOLEUS MUSCLES OF ANAESTHETIZED CATS

The effects of chronic thyroxine treatment on cat soleus muscle contractions were studied. Maximum twitch tension, contraction time, half relaxation time and tension-time integral of maximal twitches of the soleus muscles of thyroxine treated cats were significantly decreased. Consequently, there was a decrease in tension and degree of fusion of incomplete tetanic contractions of the soleus muscle. The maximum tetanic tension was not statistically significantly changed, suggesting that the effects may be due to a decrease in the duration of the active state of the muscle. Isoprenaline given intravenously during incomplete tetanic contractions of the soleus muscle caused a statistically significant depression of tension in the control group but not in the thyroxine treated group. This further suggests reduction in the duration of the active state of soleus muscles of thyroxine treated cats. Propranolol injected chronically with thyroxine reversed or prevented the depression of tension caused by thyroxine treatment, suggesting the involvement of beta-adrenoceptors in these effects. The decrease in tension and degree of fusion during incomplete tetanic contractions of the thyroxine treated soleus could be responsible, at least partly, for the muscle weakness and tremor of thyrotoxicosis. Cyclic AMP may possibly be the mediator of these effects.     

Effects of creatine loading and depletion on rat skeletal muscle contraction

1. In humans, the effects of dietary creatine supplementation are controversial, with some studies showing increased muscle force and fatigue resistance and others reporting no effect on exercise performance. Little is known about the effects of creatine on muscle contractile properties. 2. Rats were fed a standard diet, creatine for 10 days or beta-guanidinopropionate, which depletes muscle creatine, for 7 days. Contractile properties were measured in isolated extensor digitorum longus and sternohyoid muscle as representative limb and upper airway dilator muscles, respectively. 3. Creatine had no effect on specific twitch and tetanic tension, contractile kinetics, twitch/tetanus tension ratio, the tension-frequency relationship or fatigue in both muscles. beta-Guanidinopropionate had no effect on the twitch and tetanic tension, contractile kinetics, twitch/tetanus tension ratio or tension-frequency relationship, but significantly increased (P < 0.05, anova) fatigue in both muscles. 4. Therefore, although creatine depletion increases fatigue, creatine loading has no effects on extensor digitorum longus and sternohyoid muscle contractile properties.     

Labelled decamethonium in cat muscle.

1 Tritium-labelled decamethonium was infused intravenously in 12 cats at final rates of 1.3-4.2 nmol kg-1 min-1 to produce a steady plasma concentration which ranged between 0.21-1.3 mumol/l in different experiments. Muscle contractions were elicited by nerve stimulation and the potential at the end-plate regions of superficial fibres was recorded by extracellular electrodes. 2 Under these conditions, it was not possible to obtain a steady degree of neuromuscular block. The initial decrease in muscle contractions was followed by recovery towards the original value although the concentration of decamethonium in the plasma remained constant, or in some cases rose. The initial depolarization of the end-plate region also waned during the constant infusion of the drug. 3 Once the twitch tension had returned to control values during infusion of the drug, prolongation of the infusion for a total of four hours did not produce a secondary neuromuscular block. 4 Scintillation counting showed that during infusion of labelled decamethonium the radioactivity of the muscles increased progressively with time. The uptake was less in the soleus muscle than in the fast-contracting flexor longus digitorum and extensor longus digitorum muscles. Muscles which had been denervated 12-13 days previously showed a greater uptake of labelled drug than control muscles from the contralateral limb. 5 The labelled drug was localized by autoradiography of frozen sections of leg muscles following intra-arterial injection of decamethonium. Grain counts in individual fibres showed that small amounts of decamethonium had entered the muscle fibres along their entire length, and there was increased uptake of the drug into the cell in the region of the end-plate. 6 The mechanisms underlying the waning of the pharmacological response during constant application of depolarizing drugs are discussed.     

Comparative actions of substances affecting cyclic AMP metabolism on the isometric contractions of fast and slow skeletal muscle during single-pulse and subtetanic stimulation

Increasing concentrations of isoprenaline (0.5-4 muM) produced a dose-dependent increase in both TD and dT/dtmax during direct single-pulse stimulation of hemidiaphragm of the rat. The same drug during the same type of stimulation produced an insignificant change in these parameters of the isometric contraction of the isolated guinea-pig soleus muscle. On the contrary, isoprenaline produced a dose-dependent decrease of the isometric contraction during subtetanic stimulation of the soleus muscle. Contrary to the results obtained on hemidiaphragm, there was no interaction between halothane and aminophylline on the soleus muscle. In the soleus muscle, aminophylline (0.3-3.2 mM) produced a dose-dependent increase in TD and dT/dtmax during single-pulse stimulation, whereas isoprenaline failed to do so under the same experimental conditions, in spite of the fact that both substances are activators of cyclic AMP system. The beta2-selective adrenoceptor agonist terbutaline acted in the same way as isoprenaline. During subtetanic stimulation aminophylline (0.3-3.2 mM) produced a dose-dependent decrease of both parameters of the isometric contraction of hemidiaphragm, which is opposite to the results obtained during single-pulse stimulation. It is concluded that various types of electrical stimulation can produce different responses in slow and fast-contracting muscles, depending on the fundamental biochemical differences of two types of muscle, but some of these responses are the same irrespective of the method of muscle activation.     

A comparison between the effects of a tetanus and the effects of sympathomimetic amines on fast- and slow-contracting mammalian muscles

The effects of adrenaline and isoprenaline on the tension and time-course of the contractions of the tibialis anterior and soleus muscles of cats and rabbits have been compared with the effects of previous high-frequency stimulation. Like a tetanus, adrenaline possessed a facilitating action on neuromuscular transmission and an action exerted directly on the muscle fibres. Isoprenaline possessed only the second of these two actions. The effect of adrenaline and isoprenaline on the muscle fibres was blocked by dichloroisoprenaline and by 1-(2 naphthyl)-2-isopropylaminoethanol, whereas the effect of adrenaline on neuromuscular transmission was blocked by phentolamine and by phenoxybenzamine. In the soleus muscle, both the catechol amines and a previous tetanus caused similar decreases in maximal twitch tension and in the times to peak tension and to half-relaxation. The muscle action potentials were unaltered or slightly increased in amplitude. In the tibialis anterior muscle, a previous tetanus and the catechol amines caused an increase in twitch tension and an increase in the overall duration of the twitch. The muscle action potentials were either unchanged or were slightly decreased in amplitude. In this muscle the effect of a tetanus differed from that of the catechol amines in that the large post-tetanic change was associated with a marked increase in the rate of rise of twitch tension.     

The effects of Anemonia sulcata toxin II on vertebrate skeletal muscle.

Some effects of the sea anemone toxin, ATX-II, on vertebrate skeletal muscle have been described. At a concentration of 1 X 10(-7)-1 X 10(-6)M, ATX-II caused a sodium-dependent depolarization of the muscle fibres of the rat soleus and extensor digitorum longus, of the mouse soleus and extensor digitorum longus and of the chicken posterior latissimus dorsi. The muscle fibres of the frog sartorius were insensitive to the toxin. Action potentials generated by direct stimulation were prolonged by ATX-II, but the degree of prolongation was variable. Chicken posterior latissimus dorsi muscle fibres were most sensitive in this regard, and mouse extensor digitorum longus were least sensitive. Both denervated and immature muscle fibres were more sensitive to ATX-II than mature innervated muscle fibres. The sensitivity to ATX-II declined rapidly as muscle fibres matured. In some muscles, the prolongation of the action potential was enhanced by repetitive stimulation, but not by the passive depolarization or hyperpolarization of the muscle fibres. The actions of ATX-II could be reversed by washing in all but the innervated soleus of the mature rat.     

The action of dantrolene sodium on individual fast and slow motor units of the rat anaesthetized with urethane

Rats, anaesthetized with urethane, were injected intravenously with dantrolene sodium in a vehicle of 5% mannitol taken to pH 10 with NaOH. The muscle relaxant action of dantrolene sodium was measured from the contractions of individual motor units of the extensor digitorum longus (EDL), soleus (SOL) and segmental tail (ST) muscles. Data were also collected from their parent whole muscle preparations. The depressant action of dantrolene sodium on the percentage-normalized amplitude of contraction of the individual motor units was greater than its effect on the whole muscle twitch amplitude, in all three muscles. The twitch amplitude of fast contracting motor units was significantly more reduced (P less than 0.001) by dantrolene sodium than was that of slow contracting motor units. Dantrolene sodium reduced the contraction time of all motor units. The effect of the drug on half-relaxation time varied within and between groups of motor units studied. The drug was confirmed to have a greater depressant action on the twitch contraction than on the fused tetanus of whole muscle. This was true also for single motor units. With tetanic stimulation the effect of dantrolene sodium was also dependent upon the motor unit type, fast or slow. A maximum depression of contractile tension occurred at a stimulation frequency of 64 Hz for fast EDL motor units whereas the maximum depression for ST slow units, the slowest units tested, was at a stimulation frequency of 14 Hz.     

Effect of hindlimb suspension and clenbuterol treatment on polyamine levels in skeletal muscle

Polyamines are unbiquitous, naturally occurring small aliphatic, polycationic, endogenous compounds. They are involved in many cellular processes and may serve as secondary or tertiary messengers to hormonal regulation. The relationship of polyamines and skeletal muscle mass of adductor longus, extensor digitorum longus, and gastrocnemius under unloading (hindlimb suspension) conditions was investigated. Unloading significantly affected skeletal muscle polyamine levels in a fiber-type-specific fashion. Under loading conditions, clenbuterol treatment increased all polyamine levels, whereas under unloading conditions, only the spermidine levels were consistently increased. Unloading attenuated the anabolic effects of clenbuterol in predominately slow-twitch muscles (adductor longus), but had little impact on clenbuterol's action as a countermeasure in fast- twitch muscles such as the extensor digitorum longus. Spermidine appeared to be the primary polyamine involved in skeletal muscle atrophy/hypertrophy.     

Fatigue and contractile responses of rat hindlimb muscles following excessive dexamethasone administration

Muscle weight, protein content and contractile performance (tetanic tension, fatigue and recovery) of extensor digitorum longus and soleus were investigated in rat following systemic administration of Dexamethasone (DX), 5 mg/kg/day for ten days. These animals showed marked reduction in food intake during the course of DX treatment. As a control, a group of food restricted (FR) rats receiving equal amount of food consumed by the DX treated rats was also studied along with the saline control group, to differentiate the effect of DX on muscle from that of dietary deficiency. There was a greater degree of atrophy (reduced muscle mass and protein content) of extensor digitorum longus in DX treated rats as compared to that of the FR rats. In-situ isometric tetanic tension per gram of muscle and per unit weight of protein was similar in both the muscles in the DX treated and the FR rats. There was increased fatiguability with reduced post fatigue recovery in both the muscles of DX treated rats as compared to the FR rats. The results indicate that besides atrophy of fast twitch muscles, DX increases the fatiguability and decreases the postfatigue recovery in both fast and slow muscles.     

ACTIONS OF DANTROLENE SODIUM ON CONTRACTIONS OF THE TIBIALIS ANTERIOR AND SOLEUS MUSCLES OF CATS UNDER CHLORALOSE ANAESTHESIA

SUMMARY 1. Dantrolene depresses the tension of directly or indirectly elicited twitches and tetaní of the tíbíalís anteríor and soleus muscles of cats under chloralose anaesthesia, without affecting the gross muscle action potentials.
2. The decrease in twitch tension is associated with slowed rates of rise of tension and of relaxation, but there is no change in the time to peak tension. The decrease in maximal tetanic tension is associated with a slowed rate of rise of tension, but there is no change in the rate of relaxation. It is concluded that dantrolene decreases the intensity of the active state, possibly by impairing the release of Ca²⁺ from the sarcoplasmic reticulum.
3. The soleus muscle is slightly more resistant than the tibialis anterior muscle to the action of dantrolene, and maximal tetani of both muscles are much more resistant than twitches.
4. Adrenaline, theophylline and quazodine produce effects on the dantrolene-depressed twitches that are proportionately the same as those produced on the control twitches; there is no evidence of a specific antagonistic effect of any of these drugs.
5. Tetanic stimulation of the tibialis anterior muscle but not the soleus muscle causes a temporary relief of the twitch depression produced by dantrolene.     

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.(ABSTRACT TRUNCATED AT 250 WORDS)