THE EFFECTS OF SYMPATHOMIMETIC AMINES ON CHRONICALLY DENERVATED SKELETAL MUSCLES

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.

The positive inotropic effect of epinephrine on skeletal muscle: a brief review

Chronic and acute administration of epinephrine or related sympathomimetic agents are typically prescribed for the treatment of clinical disorders such as hypotension, anaphylactic and allergic reactions, and bronchial asthma. In addition to its effects on these infirmaties and on carbohydrate metabolism, epinephrine also exerts a positive inotropic effect on fast-contracting skeletal muscle in a variety of animal species. At present, the precise mechanisms responsible for the inotropic effect are not known. This communication reviews the positive inotropic effects of epinephrine on fast-contracting skeletal muscle and discusses possible mechanisms which might mediate this phenomenon. Epinephrine potentiates muscle twitches via the second messenger, cAMP, secondary to hormone binding to membrane-bound beta-receptors. Cyclic AMP then acts to increase carbohydrate metabolism, alter sodium/potassium exchange, phosphorylate myosin isozymes, and/or alter intracellular calcium exchange. Based on theoretical grounds, the first three mechanisms can be excluded. Therefore, it is tentatively hypothesized that the effect is due to cAMP-enhanced calcium exchange within the muscle fiber and/or to increased influx of extracellular calcium. This notion is consistent with the mechanism of the positive inotropic effects of epinephrine on cardiac tissue. If this hypothesis is correct, it would also suggest a role, at least under some conditions, for extracellular calcium in the process of skeletal muscle excitation-contraction coupling.

Increase in the amount of adenylate cyclase in rat gastrocnemius muscle after denervation

After section of the sciatic nerve, the basal adenylate cyclase (AC) activity in rat gastrocnemius muscle increased 6-7 times per membrane protein and about 2 times per whole muscle in the following 30 or 40 days. The AC activity in the muscle 30 days after denervation was increased about 4 times by forskolin. Calcitonin gene-related peptide (CGRP) also increased the adenylate cyclase activity in the denervated muscle. The binding of [3H]-forskolin (10nM) to cells isolated from gastrocnemius muscle was examined to determine the amount of AC molecules. Inhibition of [3H]-forskolin binding by increasing amounts of unlabeled forskolin gave a sigmoid curve with a IC50 value of 3 x 10(-7) M. Results showed that the number of [3H]-forskolin binding sites per cell was higher on the denervated side than on the control side, like the basal AC activity. The IC50 values for inhibition by unlabeled forskolin of binding of [3H]-forskolin were similar to muscles on the control and denervated sides. These results suggest that an increase in the AC activity induced by denervation was due to an increase in the numbers of AC molecules in the muscle.

Cyclic nucleotides and contractility of isolated soleus muscle

The effects of isoprenaline, terbutaline and forskolin were examined on cyclic nucleotide concentrations and contractile responses in guinea-pig isolated soleus muscles. Isoprenaline and terbutaline induced rapid, concentration-related reductions in the tension and degree of fusion of subtetanic contractions of the soleus muscle. These changes were associated with increases (about 2 fold) in the levels of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in the muscle cells. Propranolol competitively inhibited these responses. Forskolin failed to elicit a sympathomimetic response in the soleus muscles despite increasing (by about 20 fold) the intracellular concentration of cyclic AMP. Forskolin also failed to potentiate the effects induced by isoprenaline. The levels of cyclic GMP in the soleus were increased by isoprenaline (about 1.5 fold) and forskolin (about 2.5 fold). Terbutaline was without effect on cyclic GMP levels. These data suggest either that cyclic AMP is not involved as the mediator underlying beta-adrenoceptor-induced changes in contractility of slow contracting skeletal muscles or that forskolin does not stimulate the particular adenylate cyclase that leads to appropriate increases in cyclic AMP in those functional compartments associated with modulation of intracellular Ca2+ movements. Cyclic GMP is not involved in modifying changes in contractility of the soleus muscle.

Effect of denervation on glycogen metabolism in fast and slow muscle of rat

The postdenervation changes in glycogen metabolism were explored in fast (extensor digitorum longus, EDL), and slow (soleus, S) muscle of rat. During the first 12-14 hours after denervation, glycogen accumulates to a similar content in the two muscles, an effect reproduced by paralysis. Increase in the molecular weight of glycogen and decrease in the turnover rate of similar degree occur. During the ensuing 12-24 hours the glycogen concentration decreases in EDL (but not in S). This decrease is influenced by factors such as muscle type, longer or shorter nerve stump, age, and previous training of the animal. In both EDL and S, the rate of glucose incorporation into glycogen and the glycogen synthase [l/(l + D)] activity were depressed by denervation. The cAMP concentration increased 36 hours after denervation. This may have consequences on the muscle sensitivity to hormones. In fact, at this time, insulin did not increase glycogen synthase I activity or the glycogen content (with the exception of glycogen in S) and the effects of epinephrine on glycogen metabolism became more significant.

The effect of catecholamines on Na-K transport and membrane potential in rat soleus muscle

1. The action of catecholamines on the transport and the distribution of Na and K and the resting membrane potential (E(M)) has been investigated in soleus muscles isolated from fed rats.2. In a substrate-free Krebs-Ringer bicarbonate buffer adrenaline (ADR) (6 x 10(-6)M) increased (22)Na efflux by 83%, (42)K influx by 34%, and E(M) by 10%. Similar effects were exerted by noradrenaline (NA), phenylephrine, salbutamol and isoprenaline. The effects of ADR on Na-K transport and E(M) were suppressed by ouabain (10(-3)M) and propranolol (10(-5)M), but not by thymoxamine (10(-5)M) or tetracaine (10(-4)M).3. Following 90 min of incubation in the presence of ADR (6 x 10(-6)M), the intracellular K/Na-ratio was increased threefold. NA produced almost the same change, and both catecholamines seem to induce a new steady-state distribution of Na and K which can be maintained for several hours in vitro.4. The effect of ADR on (22)Na efflux and E(M) could be detected at concentrations down to 6 x 10(-9) and 6 x 10(-10)M, respectively, and half-maximum increase was obtained at around 2 x 10(-8)M. NA was at least one order of magnitude less potent.5. The effect of low concentrations of ADR on (22)Na efflux was potentiated by theophylline (2 mM). When added together, dibutyryl-cyclic AMP and theophylline mimicked the action of ADR on (22)Na efflux, (42)K influx, Na/K content and E(M). Ouabain (10(-3)M) also suppressed the effect of dibutyryl-cyclic AMP and theophylline on Na-K transport.6. Following the addition of ouabain (10(-3)M), E(M) rapidly dropped from a mean of -71 to -63 mV, and then showed a slow linear fall for up to 4hr.7. The hyperpolarization induced by ADR was associated with a decrease in membrane conductance, (22)Na influx and (42)K efflux. The time course and the response to ouabain suggests that all of these effects are secondary to stimulation of the active coupled transport of Na and K.8. It is concluded that in rat soleus muscle, the active Na-K transport is electrogenic and susceptible to stimulation by catecholamines via beta-adrenoceptors. This effect is mediated by adenyl cyclase activation and may account for the increase in E(M) and the intracellular K/Na ratio.

In vitro studies of skeletal muscle membranes. Adenylate cyclase of fast and slow twitch muscle and the effects of denervation

Sarcolemmal membranes were prepared from slow-twitch (red) and fast-twitch (white) skeletal muscle of the rat. A sensitive adenylate cyclase assay was used and basal, fluoride- and catecholamine-stimulated activities measured. The greater in vivo sensitivity of red muscle to the effects of catecholamines correlates, in the present study, with approximately a twofold stimulation of its sarcolemmal adenylate cyclase with isoproterenol (10 micronm). The white muscle enzyme, on the other hand, is only minimally stimulated (20%) at the same concentration of beta-adrenergic agonist. Fast-twitch muscle is known to be physiologically insensitive to catecholamine in vivo. A course of sciatic nerve denervation was followed to further distinguish these two metabolic types of skeletal muscle and their respective adenylate cyclases. The slow-twitch muscle enzyme activities were completely and permanently lost on denervation. The white muscle enzyme, however, recovered almost completely after an initial reduction in specific activity the first week. Interestingly, the NaF-stimulated activity lagged behind both the basal and hormone-stimulated activities of the white muscle enzyme, in returning to control levels. The activities of cyclic nucleotide phosphodiesterase were evaluated in homogenates of the two muscle types in innervated rats and following denervation, in order to further define the neural influence on skeletal muscle cyclic nucleotide metabolism. The results suggest that the motor nerve may regulate some of the metabolic properties of slow-twitch muscle (which may involve cyclic AMP) by controlling the responsiveness of its sarcolemmal-bound adenylate cyclase system.

The denervated cremaster muscle of the guinea-pig as a pharmacological preparation

The pharmacological responses of both denervated and innervated cremaster muscle preparations from the guinea-pig have been investigated and compared. Acetylcholine (ACh) dose-response curves were obtained in both preparations. The affinity constants for (+)-tubocurarine and for atropine were calculated and indicated that the ACh receptors in both preparations were nicotinic. Histamine dose-response curves could be obtained only in the denervated preparation. The response was unaltered by metiamide, and the affinity constant for mepyramine fitted in with those previously obtained by others on ileum and trachea, indicating that the histamine receptors are H1 in type. Sustained contractions were obtained to adrenaline and noradrenaline but not isoprenaline, with the denervated preparations. Schultz-Dale responses were obtained with denervated muscle in sensitized guinea-pigs. This preparation did not respond to 5-hydroxytryptamine, bradykinin, or the slow-reacting substance of anaphylaxis.

The effect of catecholamines on the influx of calcium and the development of tension in denervated mouse diaphragm muscle

1 The nature of the catecholamine-induced contracture of chronically denervated mouse diaphragm muscle has been investigated and compared with the contractural response evoked by acetylcholine. 2 The time course of onset of catecholamine-sensitivity in denervated diaphragm muscles was similar to the development of acetylcholine sensitivity. However, catecholamine contractures were absent in tissues denervated for periods longer than 90 days whereas acetylcholine-sensitivity was still evident several months after denervation. 3 The catecholamine-induced contracture of the denervated muscle was inhibited specifically by beta-receptor blocking drugs and was unaffected by alpha-receptor blocking drugs and cholinoceptor antagonists. 4 Catecholamine-induced contractures of denervated muscles, unlike contractures to acetylcholine, were dependent upon the presence of spontaneous fibrillation and the amplitude of spontaneous fibrillation was increased by catecholamines. Fibrillation was absent in the presence of tetrodotoxin (1 muM), 2,4-dinitrophenol (10 muM), potassium cyanide (10 muM), ouabain (100 muM), in lithium chloride Ringer solution and at low temperature. Under these conditions catecholamine-induced contractures, but not those to acetylcholine, were abolished. 5 Labelled calcium was found progressively to enter denervated muscle fibres and this entry of calcium was increased by catecholamines. It is suggested that this calcium entry may represent either an increased calcium permeability of denervated muscle fibres which is increased further by catecholamines or the presence of a calcium current that occurs during the fibrillatory potentials of denervated muscle.

Spontaneous activity in denervated mouse diaphragm muscle

Intracellular electrodes were used to study the discrete depolarizations which trigger fibrillation potentials in chronically denervated mouse diaphragm muscles. Provided that the muscles were perfused on both sides spontaneous activity was maintained in vitro. 2. Discrete spontaneous depolarizations, present only in the centre of the muscle, were recorded from the third day of denervation reaching a maximum in prevalence 9-12 days after sectioning the nerve. These potentials had random occurrence and nearly constant amplitude and frequency within a fibre, dependence of amplitude and frequency on membrane potential, and low temperature dependence. 3. The spontaneous activity was enhanced and could be initiated in previously quiescent fibres by lowering the external Ca concentration. The activity was reduced by increasing external Ca and was abolished at 15mM-[Ca] 0. Tetrodotoxin (10-(7)M) blocked spontaneous activity. 4. The spontaneous activity was enhanced by the catecholamines isoprenaline and adrenaline (0.5-10 mug/ml.). This effect of isoprenaline was accompanied by an increase in the rate of rise and the amount of overshoot of the action potential. 5. Ouabain (10-(6)-10-(4)M) of K+-free solutions reversibly blocked spontaneous activity. Ouabain (10-(4)M) reduced the rate of rise and the amount of overshoot of the action potential. 6. Detubulation of muscle fibres with glycerol of the presence of hypertonic solutions abolished spontaneous activity which could not be restarted by reducing Ca or by the addition of isoprenaline. 7. The results support the suggestion that the spontaneous discrete depolarizations which give rise to fibrillation potentials in denervated muscle result from regenerative sodium conductance increases within the transverse tubular system of the muscle fibres. Catecholamines and ouabain could affect this activity either directly, through an action on membrane excitability, or indirectly via the Na+-K+ pump.

The membrane potential of rat diaphragm muscle fibres and the effect of denervation

1. Resting membrane potentials of rat diaphragm muscles were measured in vitro after previous denervation for 0-10 days. In some experiments denervated muscles were incubated in vitro for 3 hr while in others they were cultured for 15-24 hr to allow adequate exposure to drugs before recording. 2. It was found that resting membrane potentials, within 2-5 mm of the site of nerve section were significantly lower, within 3 hr, than resting membrane potentials measured more than 9 mm away from site of nerve section. This difference could be reduced or abolished by bathing preparations in solutions containing adrenaline (10 muM), noradrenaline (10 muM) or isoprenaline (10 muM) or dibutyryl cyclic AMP (10 muM-0-25 mM in the presence of 2 mM theophylline). Cyclic AMP (0-5 mM) was ineffective. 3. Application of solutions containing dibutyryl cyclic AMP for 3 hr also raised the resting membrane potential of muscles denervated 4-5 days previously. Culture studies showed that this effect was sustained when the time of incubation was 24 hr. 4. Incubating freshly denervated preparations with cycloheximide (22 mug/ml.) or actinomycin D (1 mug/ml.) did not prevent the development of the early (3 hr) fall in resting membrane potential despite a concomitant inhibition of RNA or protein synthesis. Culturing freshly denervated muscles in solutions containing cycloheximide (10 or 25 mug/ml.) which blocked 93% of protein synthesis, did not prevent the expected drop in resting membrane potential after 15 or 24 hr. 5. It was found that exposure to ouabain (1 or 5 mM) produced a rapid (15 min) fall in resting membrane potential in innervated and denervated preparations treated with dibutyryl cyclic AMP but not denervated preparations. After 5 days denervation cyclic AMP levels in muscle were increased by about 40%. 6. It is suggested that upon denervation an electrogenic action of a NA+-pump is blocked and that dibutyryl cyclic AMP and catecholamines are capable of stimulating this pump.

Noradrenaline stimulation of a membrane pump in frog skeletal muscle

Resting membrane potentials were recorded from the frog sartorius before and during exposure to noradrenaline (NA; 10(-2) M) in the presence or absence of ouabain (10(-3) M) and during manipulation of the extracellular potassium concentration. In potassium-free Ringer, NA produced no significant change in the resting membrane potential. With the extracellular potassium concentration at 2.5 or 5mM, NA caused a hyperpolarization. This reversed to a depolarization when extracellular K+ was greater than 7 mM. In all cases NA-induced changes were blocked by ouabain. These effects suggest that NA stimulates a membrane cation pump, the Na : K coupling ratio of which is affected by the available ion concentrations.

The possible role of cyclic AMP in the neurotrophic control of skeletal muscle

1. Motoneurones provide trophic control of some of the functional characteristics of skeletal muscle fibres. This study has been designed to test whether the adenylate cyclase: cyclic AMP system may offer one potential mechanism for the mediation of neurotrophic regulation. 2. The concentration of cyclic AMP was measured at various intervals after muscle denervation. Muscle cyclic AMP concentration increases for the first 2 days after nerve section. It reaches a maximum value at 48 h and subsequently returns to the control value at 7 days. 3. Cyclic AMP concentration is unchanged by muscle disuse for the first 3 days following limb immobilization. Four days after immobilization, however, cyclic AMP increases in both the disused and contralateral control muscles. This phenomenon has been tentatively ascribed to some aspect of the inflammatory response. 4. Changing the level of nerve section, and therefore the length of the residual nerve stump, changes the temporal pattern of the increase in muscle cyclic AMP concentration. 5. Reinnervation of a denervated muscle produces a decrease in muscle cyclic AMP concentration. 6. It is concluded from the results that some aspect of nerve function provides trophic regulation of the muscle adenylate cyclase: cyclic AMP system. The mechanisms by which this regulation may be applied are considered in the Discussion.

Contractility and supersensitivity to adrenaline in dystrophic muscle

In the adductor pollicis muscle of patients with limb-girdle and facioscapulohumeral muscular dystrophies and possible carriers of Duchenne type muscular dystrophy, abnormal active state properties were found at the time when there was no alteration of needle electromyography and evoked muscle action potentials. Adrenaline induced a marked reduction of incomplete tetanus via beta receptors without change in neuromuscular transmission.

Beta-adrenoreceptors in denervated skeletal muscles of the cat

1. A number of sympathomimetic amines have been compared with (-)-isoprenaline for their ability to produce contractions in chronically denervated soleus and tibialis anterior muscles and to increase heart rate in bilaterally vagotomized anaesthetized cats. 2. The mean dose-ratios ((-)-isoprenaline equals 1) for adrenaline, orciprenaline and salbutamol were 2.9, 29.7 and 11.5 respectively in the tibialis anterior muscle and 3.5, 24.1 and 14.3 in the soleus muscle. The dose-ratios for noradrenaline were 21.1 in the tibialis anterior and 57.6 in the soleus muscle. 3. The dose-ratios ((-)-isoprenaline equals 1) obtained for the positive chronotropic effects of the drugs in bilaterally vagotomized cats were 14.5, 14.6, 29.6 and 24.2 for adrenaline, noradrenaline, orciprenaline and salbutamol respectively. 4. The beta2-receptor antagonists butoxamine and H35/25 antagonized the vasodepressor and skeletal muscle responses to (-)-isoprenaline to a greater extent than the cardiac responses, whereas the beta1-receptor selective antagonists practolol and H93/26 antagonized cardiac to a greater extent than vascular and skeletal muscle responses. 5. The results obtained suggest that the contractions of denervated skeletal muscle to sympathomimetic drugs result from stimulation of beta2-adrenoreceptors.

The effect of bradykinin on denervated tongue

1. The contractile response of the chronically denervated tongue of the cat to chorda stimulation, and to close arterial injections of bradykinin, acetylcholine (ACh) and other drugs was examined.2. Bradykinin in doses of 50 ng-20 mug injected close arterially always produced a contractile response of the denervated tongue. Sodium nitrite (1 mg i.a.) and isoprenaline (3-200 ng i.a.) also produced contracture; histamine (40-100 ng i.a.) evoked an increase in tension in only 2 out of 5 experiments.3. Tubocurarine in doses of 0.25-1 mg injected intra-arterially, produced a large and long-lasting contracture of the denervated tongue. When the contracture was over, the effect of bradykinin was reduced to about half; the effects of ACh and chordo-lingual nerve stimulation were markedly reduced (over 80%), and those of sodium nitrite and isoprenaline were transiently abolished. Gallamine only slightly reduced the effect of bradykinin.4. Close intra-arterial injection of physostigmine (100 mug) potentiated the effect of ACh and chordo-lingual nerve stimulation, but did not increase the response to bradykinin.5. Cocaine (1 mg/kg i.v.) deeply depressed the response to bradykinin, and moderately reduced the responses to ACh (41%) and to chorda stimulation (66%).6. In 2 out of 7 experiments, close arterial injections of bradykinin (100-500 ng) to the denervated tibialis anterior muscle of the cat, produced a contractile response. Bradykinin in small doses (200-250 ng) injected immediately before ACh potentiated its effect. On the other hand, the effect of ACh was depressed when given immediately after a big dose of bradykinin (10-15 mug).7. The possible mechanism of action of bradykinin and other substances on denervated muscle is discussed.

Some properties of the denervated anterior gracilis muscle of the rat

1. Microdrops of acetylcholine (ACh) were topically applied to denervated, mammalian skeletal muscle in vivo, and ACh-evoked depolarization and contracture were recorded simultaneously.2. Contracture tension was directly proportional to the degree of ACh-elicited depolarization.3. Atropine and (+)-tubocurarine increased the amount of ACh required to produce a given amount of depolarization, but these anticholinergic agents did not alter the relationship between the degree of ACh-evoked depolarization and contracture tension.4. Topically applied catecholamines did not produce either depolarization or contracture, despite the fact that parenterally administered catecholamines elicited both responses.

The neuromuscular blocking action of a series of bicyclic bis-oniu esters

1. Thirteen bicyclic dicholine esters have been tested on mammalian and avian skeletal muscle preparations.2. One of the compounds exhibited depolarizing activity in all the preparations.3. Three of the compounds exhibited depolarizing or dual-blocking activity in avian and denervated mammalian preparations, but exhibited non-depolarizing blocking activity in innervated mammalian preparations.4. The remaining compounds exhibited non-depolarizing blocking activity with evidence of an additional facilitatory action.5. The activity exhibited was dependent upon the onium substituents and the structure of the bicyclic ring.6. All the compounds exhibited a choline-reversible block in the rapidly stimulated rat diaphragm preparation and six of them exhibited a secondary choline-reversible block of the rapidly stimulated cat tibialis anterior muscle.