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PubMed1978

A potential- and time-dependent blockade of inward rectification in frog skeletal muscle fibres by barium and strontium ions.

1. A three-electrode voltage clamp method was used to investigate the effects of Ba and Sr ions on the inwardly rectifying K conductance of resting frog sartorius muscle fibres. 2. When Ba2+ (0.01-5 mM) was added to the control (115 mM-K+) solution the inward currents recorded during hyperpolarizing voltage steps turned off exponentially with time as the blockade by Ba2+ developed. Outward currents showed no time-dependence. 3. Ba2+ ions reduced both the instantaneous and the steady-state values of currents recorded on hyperpolarization. The blockade was potential-dependent, steady-state currents being increasingly reduced with increasing hyperpolarization. 4. The concentration-effect relation for the blockade of instantaneous currents by Ba2+ could be fitted assuming 1:1 binding of Ba2+ to a receptor, with the block being proportional to the number of Ba2+-filled receptors. The apparent dissociation constant at the holding potential (-5 mV) was 0.65 mM. Concentration-effect relations were shifted along the concentration axis to lower concentrations by hyperpolarization. The apparent dissociation constant was reduced e-fold for a 16.8 mV change in potential. 5. Increasing the [Ba]o increased the rate of onset of the blockade at a given potential. 6. The rate of onset of the blockade had a high temperature dependence (Q10 = 3.15 +/- 0.08). 7. When [K]o was doubled to 230 mM, under conditions where [K]i was also doubled, [Ba]o had to be raised approximately fourfold to produce the same degree and rate of onset of blockade. Similarly, when [K]o was decreased, the degree and rate of onset of blockade were increased for a given [Ba]o. 8. The blockade could be readily removed by removal of Ba2+ from the bathing solution. In addition the blockade which develops on hyperpolarization is removed exponentially on return to the holding potential. 9. The blockade which exists at the holding potential may be removed by a depolarizing prepulse. 10. Sr causes a similar potential-dependent blockade to that by Ba2+, but is around 400 times less effective. 11. The results have been fitted with a model assuming that the permeability mechanism is an aqueous pore with a site which binds one Ba2+ ion or two K+ ions. The site must have affinity for Ba2+ and a low affinity for K+.

Animals↗

PubMed1988

Analysis of quantal acetylcholine noise at end-plates of frog muscle during rapid transmitter secretion.

1. Using the theory of noise analysis an attempt was made to measure frequency and amplitude of miniature end-plate potentials (MEPPs) under conditions of vigorous transmitter release. Frog sartorius muscles were incubated in a depolarizing (32 mM-K+) medium which lacked Ca2+ to prevent transmitter release. Subsequently, when the membrane potential had become stable at about -40 mV, end-plates were superfused with 4 mM-Ca2+-containing medium for 1 min periods with 5 min intervals between the superfusions. 2. Most junctions ('fast' type) responded to Ca2+ with a relatively large, noisy depolarization (5.8-14.5 mV) which subsided rapidly during subsequent challenges with Ca2+. Other junctions ('slow' type) responded with only 1-1.6 mV depolarizations which were rather well sustained during the consecutive Ca2+ applications. 3. From the variance, E2, and the depolarization, V, caused by Ca2+ the frequency n and amplitude factor q of the MEPPs were calculated. Values of n were 3-4 x 10(4) and 0.1-1 x 10(4) s-1 in the fast- and slow-type junctions, respectively. The mean value of q was 0.16 mV; it remained more or less constant in the fast-type junctions, but tended to decline in the slow-type junctions. 4. As expected, cholinesterase inhibitors potentiated V and E2 as well as individual MEPPs. However, no advantage could be taken from this finding, since these drugs caused burst-like peaks superimposed on the voltage signal, precluding application of noise analysis. 5. The results strongly suggest that, at least in the fast-type junctions, K+ caused an extremely rapid depletion of the store of transmitter quanta, whose mean size did not change appreciably in the course of the experiment. However, in the slow-type junctions during prolonged incubation, it cannot be excluded that the gradual decline of q was due to the release of newly formed, unripe quanta.

Acetylcholine↗

PubMed2002

Relationship between wire EMG activity, muscle length, and torque of the hamstrings.

OBJECTIVE: To determine the effect of change of muscle length on the torque and wire electromyographic activity of six knee flexor muscles. DESIGN: Maximum isometric knee flexion torque and wire EMG data were collected at nine different positions. BACKGROUND: In vivo EMG-length-tension relationship is difficult to determine because of the interaction between muscle length and moment arm. The study of two-joint muscles allows the change of muscle length at one joint while preserving stable mechanical relationships at the other. This model facilitates understanding of length-tension and EMG-length relationship in vivo. METHODS: Nineteen subjects performed maximum isometric knee flexion contraction at nine positions of varying hip and knee angles. Wire EMG activity was recorded from semitendionsus, semimembranosus, long and short head of the biceps femoris, gracilis and sartorious muscles. RESULTS: As the two-joint hamstrings were lengthened, torque was significantly increased. Maximum isometric torque ranged from 257 to 716 kg cm. The ratio of the torque values to EMG activity of all muscles was increased at longer muscle lengths. A change in the muscle length of the two-joint hamstrings did not produce a consistent change of EMG activity. The short head of the biceps femoris and sartorius muscles increased their activity as the angle of knee flexion increased. CONCLUSIONS: Maximum torque of knee flexion occurs at the most lengthened position of the hamstrings. EMG activity did not consistently change with the change in muscle length. RELEVANCE: Understanding in vivo length-tension relationship and associated EMG activity is important for designing rehabilitation protocols, tendon lengthening and transfer and interpretation of EMG data.

Adult↗

PubMed1967

Nickel substitution for calcium in excitation-contraction coupling of skeletal muscle.

In 1962 Frank (22) reported that the addition of any one of a number of divalent cations, including Ni, to a Ca-free Ringer solution prevented the rapid loss of contractility seen in the absence of external Ca. To investigate further the Ni-Ca substitution, studies were made of (45)Ca and (63)Ni exchange during contraction and at rest using frog striated muscle. In contrast to (45)Ca, it was found that there is no increase of (63)Ni uptake associated with a K contracture of the sartorius muscle. The rates of loss of (63)Ni and (45)Ca from resting toe muscles previously bathed in the respective radioisotopes are not significantly different. Resting and action potentials, after 1 hr in a Ringer solution with Ni replacing Ca, closely resemble these potentials in normal Ca-Ringer's solution. Studies on the syneresis of isolated myofibrils indicate that Ni cannot replace Ca in activating this reaction. It is suggested that Ca is required for at least two steps in E-C coupling: one is the spread of excitation at the sarcolemma and transverse tubular system; the second is the activation of actomyosin ATPase. Conceivably Ni can substitute for Ca in the former but not in the latter.

Action Potentials↗

PubMed1979

Competitive interaction between foreign nerves innervating frog skeletal muscle.

1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back. 2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2--3 months; tetanic stimulation of the nerve elicited 9--100% of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60--100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80--100% of the maximal tension to direct stimulation. If one nerve was implanted 2--3 months before the other, the second, although usually less effective than the first, normally innervated 50--100% of the fibres, with approximately the same time course of innervation as a single s.m.n. 4. Mutual synaptic repression was seen on examination of twitch tensions. With either simultaneous or staggered innervation, stimulation of each s.m.n. resulted in a twitch of 30--50% of the total direct twitch tension, with little overlap between the fields driven by the two nerves. Intracellular recordings showed that the distribution of subthreshold and spike-producing e.p.p.s reflected the existence of separate twitch fields. Even if one s.m.n. was implanted several months before the other and had time to establish suprathreshold junctions on most muscle fibres, an s.m.n. implanted later was able to reduce sharply the effectiveness of many junctions from the earlier nerve while itself innervating most muscle fibres. 5. The subthreshold e.p.p.s had low quantal content, typically ten or fewer quanta/e.p.p. The min e.p.p. frequency was very low, while min e.p.p. amplitude appeared to be normal. 6. In the vast majority of muscle fibres, junctions from the two nerves were not within recording distance of each other. Hence, we infer that the competitive interaction was mediated somehow via the muscle fibre. 7. The preganglionic splanchnic nerve, which also successfully reinnervated frog skeletal muscle, competed with a foreign s.m.n. in ways which differ qualitatively from the competition by a second s.m.n. In the presence of a s.m.n., synapses of the s.p.n. were almost universally subthreshold. However, if the s.p.n. was implanted 2--3 months before the s.m.n., the s.m.n. was prevented for several months from innervating fibres driven by the s.p.n. This delay in s.m.n. reinnervation was greater than if the first nerve implanted was also an s.m.n. 8. After 6--8 months of dual innervation by s.m.n. and s.p.n., the s.m.n. became almost totally dominant. However, if the s.m.n. was then sectioned, the s.p.n. became as effective, within approximately 1 week, as it would have been in the absence of the s.m.n.

Animals↗

PubMed1996

Are there oxygen-deficient regions in resting skeletal muscle?

The purpose of this study was to determine whether there are local regions in resting skeletal muscle in which the oxygen delivery is insufficient to support oxidative metabolism. This hypothesis was tested by stopping the blood supply to the exteriorized cat sartorius muscle for 5 min while monitoring NADH fluorescence in localized tissue areas 15-25 microns in diameter. A rise in fluorescence was taken to indicate a shift to anaerobic metabolism. Tissue sites in the arteriolar and venular regions of the capillary network were selected for study. After flow stoppage, fluorescence did not change for an average of 48 +/- 22 (SD) s and then rose over a period of 61 +/- 27 s to an average value 55 +/- 19% above control for arteriolar and venular sites combined. Fluorescence began to rise within 5 s of flow stasis in only 1 of 61 sites and within 10 s in 2 sites. There was no difference in the time course or magnitude of fluorescence changes at arteriolar and venular sites. The data indicate that in resting skeletal muscle, oxygen supply appears sufficient to support oxidative metabolism in over 95% of the tissue.

Animals↗

PubMed1985

The reversible cholinesterase inhibitor physostigmine has channel-blocking and agonist effects on the acetylcholine receptor-ion channel complex.

The actions of the carbamate cholinesterase inhibitors, physostigmine (Phy) and physostigmine methiodide (MetPhy), were studied on the acetylcholine receptor-ion channel complex (AChR) of skeletal muscles. Low concentrations of these agents produced cholinesterase inhibition which resulted in potentiation of nerve-elicited muscle twitches and an increased peak amplitude and prolongation of the decay time constant (tau EPC) of endplate currents (EPCs) elicited in frog (Rana pipiens) sartorius muscles. However, increasing concentrations of Phy depressed the peak amplitude and shortened the decay phase of the EPC with an apparent loss in the voltage dependence of tau EPC. At higher concentrations and depolarized potentials, EPC decays were double exponential. The effects of both Phy and MetPhy on the postsynaptic AChR complex were also evident in preparations pretreated with diisopropylfluorophosphate. Under these conditions, a linear relationship between the reciprocal of tau EPC and the concentration of these agents was observed. Single channel studies revealed that Phy (20-600 microM) shortened channel lifetime and decreased channel conductance at very high concentrations. In addition, Phy (0.5 microM) induced the appearance of channel openings with conductance similar to that of acetylcholine. High concentrations (greater than 50 microM) of this agent activated channel openings with decreased conductance. Similar results were obtained with MetPhy. Thus, the reversible cholinesterase inhibitors Phy and MetPhy altered the properties of the AChR by interacting as agonists capable of inducing desensitization and blockade.

Acetylcholine↗

PubMed1985

Ultrastructural correlates of experimentally altered transmitter release efficacy in frog motor nerve terminals.

After experimentally inducing long term changes in transmitter release, a series of frog neuromuscular junctions were studied with intracellular recording and then semi-serially sectioned and examined in the electron microscope. Transmitter release per unit length of motor nerve terminal was well correlated with several measures of the length of individual presynaptic active zones and with the number of mitochondria per terminal. Total release from each terminal correlated with estimates of the total amount of active zone. This study of neuromuscular junctions in sartorius muscles of the frog Rana pipiens was undertaken to search for ultrastructural correlates of the increase in transmitter release efficacy that follows denervation of the contralateral sartorius. This treatment typically results in greatly enhanced release at some synapses while others appear unaffected. In the present study, nine identified junctions with known physiological properties were sectioned every 6 micron throughout much of their length to yield 40-105 cross-sectional profiles per junction. Overall, these 9 synapses showed a 33-fold range in quantal transmitter release and an 18-fold range in release per unit nerve terminal. Release correlated with estimates of active zone size. No correlations were found between release and the density of synaptic vesicles adjacent to active zones. Our results suggest that active zones in motor nerve terminals are plastic structures, and that changes in active zone size may be the structural basis of long term changes in transmitter release and synaptic efficacy.

Acetylcholine↗

PubMed1998

Estimation of cross-bridge stiffness from maximum thermodynamic efficiency.

In muscle, work is performed by myosin cross-bridges during interactions with actin filaments. The amount of work performed during each interaction can be related to the mechanical properties of the cross-bridge; work is the integral of the force produced with respect to the distance that the cross-bridge moves the actin filament, and force is determined by the stiffness of the attached cross-bridge. In this paper, cross-bridge stiffness in frog sartorius muscle was estimated from thermodynamic efficiency (work/free energy change) using a two-state cross-bridge model, assuming constant stiffness over the working range and tight-coupling between cross-bridge cycles and ATP use. This model accurately predicts mechanical efficiency (work/enthalpy output). A critical review of the literature indicates that a realistic value for maximum thermodynamic efficiency of frog sartorius is 0.45 under conditions commonly used in experiments on isolated muscle. Cross-bridge stiffness was estimated for a range of power stroke amplitudes. For realistic amplitudes (10-15 nm), estimated cross-bridge stiffness was between 1 and 2.2 pN nm-1. These values are similar to those estimated from quick-release experiments, taking into account compliance arising from structures other than cross-bridges, but are substantially higher than those from isolated protein studies. The effects on stiffness estimates of relaxing the tight-coupling requirement and of incorporating more force-producing cross-bridge states are also considered.

Actins↗

PubMed1990

Electrically stimulated sartorius neosphincter: canine model of activation and skeletal muscle transformation.

The sartorius muscle was transposed into the abdominal cavity of six dogs, passed around a Thiry-Vella loop and sutured to itself to form a neosphincter. The muscle was activated by electrical stimulation and on contraction the neosphincter stopped or reduced the flow of saline through the Thiry-Vella loop in all animals until the onset of muscle fatigue. Continuous low frequency stimulation was used to transform the skeletal muscle, and when studied after a mean of 8 weeks of stimulation (range 6-11 weeks) the neosphincter stopped the flow for a significantly longer period of time (P = 0.027). Associated with the improved neosphincter function was a significant decrease in the fusion frequency (P = 0.003) and prolongation of the stimulus-peak tension time as assessed by a strain gauge sutured to the neosphincter muscle (P = 0.002). The changes in the contraction properties of the skeletal muscle suggest that continuous low frequency stimulation transformed the muscle fibres from type 2 to type 1, resulting in improved fatigue resistance and potential for continuous sphincter activity.

Animals↗

PubMed1995

Effects of essential oil of Croton zehntneri, and of anethole and estragole on skeletal muscles.

Extracts of the essential oil of Croton zehntneri (CzEO) yield a mixture of substances in which the major components are anethole and estragole. The action of CzEO and of pure anethole and estragole (at 0.05-1.0 mg/ml) were studied in muscle preparations of the toad and rat. All three oils blocked twitches evoked by nerve stimulation. In the rat diaphragm direct stimulation resulted in enhanced twitches (CzEO), partially depressed twitches (estragole) and anethole did not induce a significant change. The response of toad rectus to acetylcholine was reduced by all three oils. Caffeine contractions of the rectus abdominis and sartorius muscles were enhanced by all oils. This enhancement in the case of the sartorius was blocked by procaine. Lowering the temperature to 4 degrees C in the presence of CzEO, anethole or estragole provoked a contraction which could be blocked by procaine. CzEO, anethole or estragole alone sometimes caused contractions. The occurrence of this response was increased by elevation of calcium (to 22 mM). Other divalent cations and La3+ were also effective, in order: La3+ > Ca2+ > Mn2+ > Ba2+ > Mg2+ except in the case of anethole where the order of Ca2+ and Mn2+ are interchanged. CzEO- and estragole-induced contractions were blocked by procaine. The data suggest that CzEO, anethole and estragole may have two sites of action on muscle fibers: the post junctional membrane, by blocking neuromuscular transmission, and on the sarcoplasmic reticulum (SR), by increasing myoplasmic calcium.

Allylbenzene Derivatives↗

PubMed1978

Comparison of efflux rates of hydrogen and lactate ions from isolated muscles in vitro.

Relative rates of efflux of hydrogen and lactate ions from skeletal muscle in vitro were determined on isolated rat diaphragms and frog satorius muscles. After a period of lacate accumulation by stimulation in vitro, muscles were suspended in a small volume of Ringer solution for different time periods lasting up to 60 min. The pH change in the solution was monitored continuously. After the predetermined time period, samples of the muscle and the Ringer solution were analysed for lactate content. Results showed that in both types of muscles the rate of efflux of hydrogen ions exceeded that of lactate ions by factors of about 14 and 50 in the case of diaphragm and sartorius muscles respectively. Because of this difference observed in the efflux kinetics of hydrogen and lactate ions, it is evident that the lactate content of a body compartment does not represent the absolute hydrogen ion load of the same compartment, particularly during the early phase of the efflux process.

Animals↗

PubMed1998

The effects of stretch parameters on eccentric exercise-induced damage to toad skeletal muscle.

Repeated contractions during which a muscle is stretched, known as eccentric contractions, have previously been shown to produce damage. This can be quantified by changes in various mechanical properties, of which reduction in tension and a shift in the optimum length for tension generation to longer lengths are examined here. The magnitude of these changes has been reported to depend strongly on the number of stretches, the amplitude of each stretch and the maximum tension reached. One proposed explanation of the changes predicts that muscle length should have a strong influence, but past reports have been contradictory on this point. Experiments were performed to test this hypothesis using whole toad sartorius muscles, which have the advantage of a relatively small passive tension, allowing a large range of lengths to be used. Initial length, amplitude of stretch and number of eccentric contractions were found by multiple linear regression to be the strong determinants of changes due to eccentric exercise. Velocity had a weak effect, and tension, varied only by varying the length of maximally activated muscles, was poorly correlated.

Animals↗

PubMed1975

The variation in thermoelasticity with sarcomere length in frog's striated muscle.

The effect of lowering temperature on the resting tension of frog's striated muscle was studied at various sarcomere lengths, using whole muscle, single-fibre and skinned-fibre preparations. And the temperature coefficient of tension, beta=(1/P)(dP/dT) (P, tension; T, temperature) was investigated. In whole sartorius muscles in Ringer solution, beta was negative over the whole range of sarcomere lengths investigated. In the presence of 2 mM procaine the rubber-like thermoelasticity (beta greater than 0) was revealed at medium lengths. At shorter (greater than 2.3 mum) and longer (greater 2.7 mum) lengths, beta stayed negative. In single muscle fibres suspended in Ringer solution with mM procaine, beta was positive over a wider range of lengths (2;3-3.4 mum), and the peak positive value of beta was obtained at the length of 2.6 mum. In skinned fibres suspended in a relaxing solution, beta stayed positive up to the length of 4.2 mum, and the peak value of beta was obtained at the length of 3.0 mum which was considerably longer than in single fibres. The results substantiated A. V. Hill's view on the thermoelasticity of the muscle; there are components with positive beta (contractile apparatus) and those with negative beta (connective tissue and sarcolemma), and at high elongations the positive coefficient is masked by the increasing importance of the negative coefficient.,

Animals↗

PubMed1970

Linear electrical properties of the transverse tubules and surface membrane of skeletal muscle fibers.

With the use of two intracellular microelectrodes and a circuit designed to compensate for the effects of stray capacitances around the electrodes, transfer impedance measurements were made at frequencies from 0.5 to 1000 c/s on frog sartorius muscle fibers bathed in 7.5 mM K Ringer solution. Complete AC cable analyses performed at 46, 100, 215, 464, and 1000 c/s showed that the fibers behaved as ideal one-dimensional cables having purely resistive internal impedances (R(i) = 102 +/- 11 Omega cm). Two circuits were considered for fiber inside-outside impedance, a four lumped parameter circuit and a parallel resistance and capacitance shunted by the input impedance of a lattice model for the T-system. Least squares fits to fiber input impedance phase angles were better with the latter circuit than with the former. With the use of the lattice model the specific capacitance of both the surface and transverse tubule membranes was found to be 1 microF/cm(2) and the internal resistivity of the tubules to be about 300 Omega cm.

Animals↗

PubMed1975

Optical diffraction study of muscle fibers.

Skeletal muscle fibers act as a one-dimensional grating to electromagnetic waves in the optical frequency range. Results are presented of the optical diffraction study of frog sartorius muscle fibers. 1. A simple theoretical consideration predicts that, at a sarcomere length near 3 mum, the second-order reflection is contributed only by the thin filaments due to the destructive interference between scattered rays from thick filaments. At that sarcomere length, both the first-order and the third-order reflections are contributed by both kinds of filaments. 2. Intensity measurements of diffraction lines of muscle fibers at rest at various sarcomere lengths confirmed the validity of our theoretical prediction.3. The isometric contraction of skinned fibers was realized by the phoretic injection of Ca2+ from a micropipette. During the Ca2+ injection intensity drops of the diffraction lines were observed. At a very narrow range of sarcomere lengths near 3 mum, the intensity drop of the second-order diffraction line became very small on Ca2+ injection, whereas the intensity drops of both the first-order and the third-order diffraction lines were very large. 4. A theoretical analysis concludes that the intensity drops on Ca2'njection are solely due to small random fluctuations of the position of thick filaments in the sarcomeres.

Animals↗

PubMed1989

Effect of Escherichia coli infection on growth and protein metabolism in broiler chicks (Gallus domesticus).

1. A controlled experimental Escherichia coli infection was developed in broiler chicks. 2. Infection with E. coli significantly reduced feed intake, altered growth of the whole body, eviscerated carcass, skeletal muscles, heart and liver. Organ weight and/or the proportions of organs within the body were affected. 3. Protein accumulation in the eviscerated carcass, extensor digitorum communis and sartorius muscles was severely inhibited by infection, and to a greater extent than body weight. 4. Failure of muscle tissue to accumulate protein was associated with a significant decline in protein synthesis, when measured in vitro (-48%; P less than 0.05) and in vivo (-42%; P less than 0.001). Protein degradation also declined (-28.7%), but to a smaller extent than protein synthesis. 5. Although the infected chicks showed no viable bacteria at day 12 after infection, chicks did not reach the same body weight as controls by day 30 after infection.

Animals↗

PubMed1990

Morphometric study of neuromuscular junction in rabbits subjected to shock by superior mesenteric artery occlusion.

The purpose of this study was to describe changes in the neuromuscular junction of rabbit sartorius muscle after shock by superior mesenteric artery occlusion in 18 rabbits. The results revealed that the numeric and volume densities of the acetylcholine (Ach) vesicles of the sartorius neuromuscular junction were decreased during shock when compared by electron microscopy with normal subjects (p less than .001). We also discovered that the release of lysosomes from hepatic cells, erythrocyte acetylcholinesterase (AchE) activity, and hepatic ATP values decreased during shock. Both scopolamine (0.023 mg/kg) and verapamil (0.74 mg/kg) blocked the release of Ach vesicles and lysosomes, and increased hepatic ATP values. Scopolamine also increased AchE activity. This study shows that the parasympathetic activation plays an important role in mesenteric artery occlusion shock.

Acetylcholinesterase↗