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PubMed1984

The morphological changes in the sarcomeres of frog sartorius. II. Muscles contracting against light loads.

The morphological changes in the sarcomeres of frog sartorii lightly loaded with 25 times muscle weight and fixed in the inactive and in the actively contracting states were studied in longitudinal electron micrographs. The sarcomere in actively contracting muscles, but not in the inactive ones, assumed a distinctive double-concave shape which was due to the bowing-out of the thick myofilaments distal ends at the contraction bands. The drop in tension in extremely shortened muscles was explained on the basis of this behavior.

Animals↗

PubMed1984

The morphological changes in the sarcomeres of frog sartorius. I. Muscles contracting against heavy loads.

The morphological changes in the sarcomeres of frog sartorii heavily loaded with 600 times muscle weight and fixed in the inactive and in the actively contracting states were studied in longitudinal electron micrographs. The sarcomere in actively contracting muscles, but not in the inactive ones, assumed a distinctive barrel shape which was referrable to the interaction of thick myofilaments distal ends, normally beyond interaction range because of their tapered structure, with thin myofilaments in an attempt to generate maximal force.

Animals↗

PubMed1978

Modified K-channel gating by exhaustion and the block by internally applied TEA+ and 4-aminopyridine in muscle.

Voltage clamp experiments on frog sartorius muscle fibres suggest that the large increase in resting potassium conductance during metabolic exhaustion is due to a change in the gating of activated potassium channels to a permanently open state. gK in exhausted fibres is less sensitive to externally applied blockers as Zn2+ and 4-aminopyridine (4-AP) while TEA+, Rb+ and Ba2+ act similarly, both in normal and exhausted fibres. In addition, injection experiments provide evidence that TEA+ and 4-AP applied internally to normal fibres effectively block potassium outward current, whereas in exhausted fibres the block seems to be smaller. These results suggest modifications in the structure of the potassium channels during metabolic exhaustion.

Animals↗

PubMed1975

The independence of membrane potential and potassium activation of the sodium pump in muscle.

The membrane potential (Em) of sartorius muscle fibers was made insensitive to [K+] by equilibration in a 95 mM K+, 120 mM Na+ Ringer solution. Under these conditions a potassium-activated, ouabain-sensitive sodium efflux was observed which had characteristics similar to those seen in muscles with Em sensitive to [K+]. In addition, in the presence of 10 mM K+, these muscles were able to produce a net sodium extrusion against an electrochemical gradient which was also inhibited by 10- minus 4 M oubain. This suggests that the membrane potential does not play a major role in the potassium activation of the sodium pump in muscles.

Animals↗

PubMed1978

A new method for excitation-contraction uncoupling in frog skeletal muscle.

The mechanical activity of frog sartorius muscle fibers can be uncoupled from the electrical activity of their surface membranes by immersing the preparation in Ringer solution containing either 1.5 or 2.0 M of formamide for 15--20 min. This uncoupling is not reversed when the muscle is transferred to normal frog Ringer solution. Formamide does not affect the electrical activity of the sciatic nerve branch, and both endplate potentials and miniature endplate potentials may be recorded from the uncoupled muscles. Prolonged exposure to formamide, beyond the time needed to paralyze, causes neuromuscular block.

Animals↗

PubMed1979

The deficit of the isometric tetanic tension redeveloped after a release of frog muscle at a constant velocity.

Frog sartorius muscles tetanized isometrically were released at a constant velocity from lengths lL to lS (delta l = lL -lS; Ls greater than lO). The tension PS redeveloped after the release was lower than the isometric tension PS at LS, and higher than the isometric tension PL at lL. The tension deficit D is defined as the difference PS-PS. The timing of the release during the tetanus did not influence D. D/PO was proportional to delta l/lO. The proportionality constant k was equal to 1.35 +/- 0.19 (n = 8) when the velocity of release was 2.5 mm/s. When the muscles were released the same delta l, D was found to be an exponential decreasing function of the velocity. The tension deficit was also found in experiments performed in the region lS less than lO. The proportionality constant k was smaller, but the influence of the velocity of the release on D was not modified. When the velocity of the release was changed during the release, D changed accordingly, showing that the effects of delta l and V are multiplicative. These facts suggest a working hypothesis based on the concept that the actin filaments which enter the overlap region during a release are strained by the tetanic stress and therefore unable to make normal cross-bridges.

Animals↗

PubMed1968

The release of adenosine triphosphate from frog skeletal muscle in vitro.

1. Active frog sartorius muscle in vitro liberates a substance into the bathing solution which has a pronounced stimulatory action on the frog heart.2. The stimulatory effect is not due to an increase in the K(+) concentration of the bathing solution, nor is it due to the liberation of catecholamines.3. In a molecular sieve chromatography procedure the stimulatory substance can be eluted in a single fraction which shows a maximum absorption of U.V. light at a wave-length of 265 nm, indicative of the presence of substances containing a purine ring.4. Low concentrations (10(-7)-10(-8) g/ml.) of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and uridine triphosphate (UTP) have a marked stimulatory action on the frog heart. The action of ATP and ADP on the heart is qualitatively very similar to that of the muscle bathing solution, while the action of UTP is distinctly different. The triphosphates of inosine, cytidine and guanosine stimulate the heart when in high concentration only. Adenosine and adenosine monophosphate do not stimulate the heart.5. Incubation of the muscle bathing solution and of solutions of ATP with the enzyme apyrase for the same time produces a similar marked reduction in the stimulatory action of both on the heart. Apyrase catalyses the break-down of nucleotide triphosphates to monophosphates.6. The elution behaviour of the stimulatory substance determined by molecular sieve chromatography is the same as that for ATP.7. The muscle bathing solution causes light to be emitted from firefly lantern extract, the pattern of light emission being similar to that produced by nucleotide triphosphates.8. The concentrations of ATP having the same quantitative action on the frog heart and on firefly extract as a given muscle bathing solution are almost identical, whereas the matching concentrations of ADP and UTP in the two methods of assay are widely different.9. It is concluded that ATP is released from active frog skeletal muscle in vitro. This release may play an important part in the reactive hyperaemia of muscular exercise since ATP has a powerful vasodilator action.

Adenosine Triphosphate↗

PubMed1970

Slow changes in potassium permeability in skeletal muscle.

1. Voltage clamp experiments on sartorius muscle fibres at 3 degrees C showed that the potassium current is divisible into three components, namely:(a) Current in the delayed rectifier channel, which reached a maximum in about 0.1 sec at -30 mV, and declined with a time constant of about 4 msec when the fibre was repolarized to -100 mV; this component had an approximately linear instantaneous current-voltage relation and an equilibrium potential E(1) at 10-15 mV positive to the resting potential.(b) A slow component which reached a maximum in about 3 sec at -30 mV, and declined with a time constant of about 0.5 sec when the fibre was repolarized to -100 mV; this component had an approximately linear instantaneous current-voltage relation and a mean equilibrium potential E(2) at -83 mV in fibres where E(1) averaged -75 mV.(c) Current in the inward rectifier channel which decreased with a time constant of about 0.25 sec when the fibre was hyperpolarized to -150 mV. This component had an equilibrium potential close to the resting potential and an instantaneous current-voltage relation which was that of an inward rectifier.2. The general characteristics of the late after-potential in muscles in hypertonic solutions at 3 degrees C are consistent with those of the slow conductance change. The sign of the late after-potentials was reversed by depolarizing below -80 mV.3. The decline of current during a maintained hyperpolarization cannot be attributed solely to a decrease in tubular potassium concentration, since there may be a large decrease in current without much alteration of equilibrium potential. The negative slope conductance often seen at -150 mV is also difficult to reconcile with the tubular depletion hypothesis.4. Replacement of 10 mM-K by 10 mM-Rb abolished inward rectification but had less effect on the fast and slow components of the potassium conductance.

Action Potentials↗

PubMed1978

Effects of fatigue and recovery on contractile properties of frog muscle.

The contractile properties of frog sartorius muscles were evaluated during development of fatigue and recovery. Muscles were stimulated under anaerobic conditions at a rate of 30 stimuli/min. Twitch tension (Pt) decreased 59% and tetanic tension (Po) decreased 52% during 15 min of stimulation. Contraction time and one-half relaxation time increased markedly during the first 6 min and then leveld off. The peak rate of twitch tension development (dP/dt) progressively declined to 33% of its initial value. Maximum shortening velocity did not change significantly. Large increases in Pt and twitch dP/dt occurred during the first 15 s of recovery while Po did not increase significantly in this time. Thereafter, Pt, Po, and, dP/dt all progressively returned towards normal over 30 min. These results show that the initial rapid recovery in Pt is not due to an increase in the capacity to generate force. Po is a better indicator of the force-generating capacity of a muscle because, unlike Pt, it is not affected by changes in the rate of tension development.

Action Potentials↗

PubMed1980

Intracellular chloride concentration and evidence for the existence of a chloride pump in frog skeletal muscle.

Intracellular chloride concentration in frog sartorius muscle was determined at 20 degrees C utilizing ionic permeability change of the membrane for an ion-selective electrode. The membrane potential and external potassium concentration relation was obtained with changes in pH. Using the constant field equation with a condition given by the crossing point in the two relations obtained with the two different pH, the intracellular chloride concentration was estimated to be 3.7 +/- 0.18 mM (mean +/- S.E.M.) with a 0.76 of the activity coefficient for chloride in Ringer's solution. The chloride potential (-88.5 +/- 1.26 mV) was significantly positive to the resting potential (-94.8 +/- 1.09 mV), suggesting that a chloride pump may be working in the frog skeletal muscle. The existence of the chloride pump was supported further by the fact that the depolarization induced by the pH increase became smaller with cooling and finally disappeared when cooled below 5 degrees C.

Animals↗

PubMed1983

The effect of different alkali metal ions on the release of muscle potassium during glycerol treatment.

Frog sartorius muscles were pretreated in 11.6 M (87%) ion-free glycerol. Then half of them were put in 5.8 M glycerol solutions containing 55 mM LiCl, NaCl, RbCl or CsCl and their pairs in ion-free 5.8 M glycerol solutions at 1:40 Vol. muscle:Vol. solution ratio. The release of potassium during exposure to the 5.8 M glycerol solution was significantly faster in the presence of Li or Na, but significantly slower in the presence of Rb than in the ion-free circumstance. The effect of Rb can be related to a cooperative phenomenon that implies existence of a structured phase of K-protein complexes. Some postulates of Ling's association-induction hypothesis are used for interpretation of two phases and their phase transitions in this type of bound-K structure in muscle. The phase transitions in this system could have an important role in the fast processes of excitation in muscle.

Animals↗

PubMed1976

[Tetanic fatigue and proximate post-tetanic recuperation in sartorius and flexor carpi radialis muscles of the male frog. Effects of iodoacetic acid (author's transl)].

The time-course of the isometric tension output, at 20 degrees C, during a long tetanus and after a short period of rest, was investigated in two isolated frog muscles : the sartorius and flexor carpi radialis muscles. To prevent aerobie and glycolytic recovery processes, some muscles were poisoned with 0,4 mM iodoacetic acid (IAA) and nitrogen, for 20 or 40 min. 1. For the unpoisoned sartorius muscle, tetanic tension declined quickly, but after a 0,8 sec period of rest, the muscle was able to develop high tension. Poisoning with IAA-N2 increased fatigue without suppressing the property of a proximate post-tetanic recuperation. 2. In the flexor carpi radialis muscle resistance to fatigue was very large before poisoning and diminished after poisoning. Proximate recuperation was very weak. 3. The results show that the recovery processes are not a primary factor of the development of the short-term fatigue ; they enhance the hypothesis that a failure of the electromechanical coupling can explain the rate of the tension fall in tetanized sartorius muscles.

Anaerobiosis↗

PubMed1995

The afterdepolarization in Rana temporaria muscle fibres following osmotic shock.

Rana temporaria sartorius muscle fibres were exposed to varied sequences of solution and temperature changes that have been employed hitherto in procedures that sought to decouple the transverse tubules from the surface membrane. The incidence of such detubulation was assessed in large numbers of fibres through demonstrating a loss or otherwise of the after-depolarization that normally reflects successful tubular propagation of the surface action potential. This criterion yielded assessments of the existing detubulation techniques in agreement with earlier results. The experiments then developed an improved detubulation procedure that required only brief (15 min) exposures to glycerol, its replacement in a single step by a Ca2+/Mg(2+)-Ringer solution for 30 min, and rapid cooling from room temperature (19-21 degrees C) to 6-10 degrees C prior to final restoration of the normal Ringer solution. This sequence of steps yielded an optimal incidence (98%) of detubulation in viable surface fibres that were amenable to electrophysiological studies. Studies that systematically modified the detubulation procedure demonstrated that the omission of any one step in the protocol significantly reduced the incidence of detubulation with or without accompanying deteriorations in fibre resting potentials. Successful detubulation accordingly required an initial exposure to an optimal glycerol concentration that lasted for a minimal duration and for its abrupt withdrawal. Inclusion of a cooling step within 30 min after glycerol withdrawal was coincident with, and critical to, optimal tubular isolation. Thus, cooling steps that either preceded, or that followed the glycerol withdrawal step by more than 60 min, resulted in a sharp reduction in the incidence of detubulation. Similarly, a critical period of exposure to Ca2+/Mg2+ Ringer solution also promoted detubulation without compromising the recovery of stable and satisfactory resting potentials. The findings reported here remain consistent with a primarily osmotic mechanism for detubulation. However, they demonstrated additional and important influences of temperature and of divalent cation concentration on the extent of tubular detachment when such factors were modified during the time course of the expected volume changes that followed each adjustment in osmotic condition.

Action Potentials↗

PubMed1988

Characteristics of Schwann-cell miniature end-plate currents in denervated frog muscle.

End-plates in denervated frog sartorius muscles were investigated, under voltage clamp, to determine the characteristics of miniature endplate currents (mepc) caused by the release of acetylcholine (ACh)-quanta from Schwann-cells. Staining for acetylcholinesterase ascertained that in most cases Schwann-cell mepc had been recorded focally from the end-plate. The mean amplitude of Schwann-cell mepc (about 1 nA) was smaller than the amplitude (3-4nA) of normal nerve evoked mepc. The amplitude distribution of Schwann-cell mepc was skewed, as were also the distributions of decay time constants and of rise times. Some Schwann-cell mepc appeared to be of composite nature, as shown by an inflexion on either the rising of falling phase. The decay time constant tau of both fast and slowly decaying Schwann-cell mepc increased with membrane hyperpolarization suggesting an exponential correlation. The potential dependence was 118 +/- 37 mV (means +/- SD from 6 experiments) for an e-fold change of tau, which does not differ significantly from that for neural mepc (98 mV). The cholinesterase inhibitor neostigmine increased the amplitude and prolonged the time course of Schwann-cell mepc to a similar degree after short and long term denervation, which indicates that cholinesterase was still functional after 3-4 months of denervation. The results show that the characteristics of Schwann-cell mepc resemble in many respects those of neural mepc but are more variable in shape and amplitude.

Acetylcholine↗

PubMed1983

Heterogeneity of capillary diameters in skeletal muscle of the frog.

Capillary diameters in sartorius muscle of frogs were measured in vivo by means of a new computer video method, based on the passage of red blood cells (RBCs) through the capillary (C. Ellis, R. Sanfranyos, and A. Groom (1983), Microvasc. Res. 26, 139-150). The distribution of capillary diameters from 21 frogs was represented by a histogram with a mean +/- SD of 16.7 +/- 4.4 microns (N = 83). The measured dimensions (mean +/- SD) of frog RBCs, which have a flattened ellipsoidal shape, were: major axis = 24.1 +/- 2.6 microns (N = 149); minor axis = 16.5 +/- 1.5 microns (N = 158); thickness at center = 5.4 +/- 0.8 microns (N = 32). Frog RBCs travel through capillaries with their major axes predominantly parallel to the direction of flow; therefore, RBCs pass through capillaries without deformation provided that the diameter of the capillary is larger than the minor axis of the cell. By standardizing the measured values of capillary diameter in terms of mean minor dimension of the RBCs (ratio of means for frog being 1.0, approx), we were able to compare the diameter distribution in an amphibian with that in a mammal (rat). If RBC size alone mattered, both standardized distributions should superimpose; however, that for frog was shifted to the right of that for rat, indicating that frog RBCs are less deformable than RBCs of rat. This highlights the necessity, in the microcirculation, for matching capillary diameter to both size and deformability of the red cell.

Animals↗

PubMed1956

The effect of changes of environment on the electrical and ionic pattern of muscle.

The resting and action potentials of sartorius muscles of the toad, Bufo marinus, have been measured under varying conditions of external environment. At the same time, analyses for Na(+) and K(+) content were carried out. There was a slight elevation of 2 mv. when the measurements were made in phosphate-Ringer instead of in bicarbonate-Ringer. The R.P. was independent of the hydrogen ion concentration between pH 6.5 and 8.5, although at these pH's there was marked alteration in the level of Na(+) and K(+) in the muscle. Alteration of the external K(+) level between 0 and 50 m.eq./liter has little influence on the internal K(+) concentration. When the log of the external K(+) concentration is plotted against the R.P. there is not a linear relationship until the external K(+) is raised above 12 m.eq./liter, at which point the cell is unexcitable. Above this value a straight line with a slope of 58 mv. per ten-fold change in concentration is obtained, but the absolute values at any point are about 35 per cent higher than those which would be given by the Nernst equation. Alteration of the external Na(+) level within a range of 45 to 650 m.eq./liter resulted in marked changes in the internal Na(+) content, without, however, having any effect on the ratio Na(+) (out)/Na(+) (in). This ratio has remained at about 3 in spite of marked fluctuations in the absolute value of the internal and external Na(+) levels. When the Na(+) level is lowered there is a decrease in the height of the action potential although there is no alteration in the ratio Na(+) (out)/Na(+) (in). As the Na(+) level is raised the height of the action potential is not affected even in the presence of a fivefold increase in Na(+) in the Ringer. The results do not support the conclusion that the bioelectric potentials can be calculated from the ionic ratios by means of simple physical chemical hypotheses such as the Nernst or Goldman equations. The maintenance of the normal K(+) content of the cell cannot be accounted for by a Donnan mechanism. No definite evidence has been produced to explain the mechanism of a Na(+) "pump." In other words, the concept of a Na(+) pump requires that there shall be a physico- or organochemical mechanism which will distinguish between Na(+) and K(+) (or other) ions. There is evidence that Na(+) can be extruded against a concentration gradient. On the other hand the cell is able to maintain a constant ratio of external to internal Na(+) even when the cell has been severely damaged by very high external Na(+) levels.

Action Potentials↗

PubMed1963

THE INFLUENCE OF SODIUM-FREE SOLUTIONS ON THE MEMBRANE POTENTIAL OF FROG MUSCLE FIBERS.

The membrane potential of frog sartorius muscle fibers in a Cl- and Na-free Ringer's solution when sucrose replaces NaCl is about the same as that in normal Ringer's solution. The K(+) efflux is also about the same in the two solutions but muscles lose K and PO(4) in sucrose Ringer's solutions. The membrane potential in sucrose Ringer's solution is equal to that given by the Nernst equation for a K(+) electrode, when corrections are made for the activity coefficients for K(+) inside and outside the fiber. For a muscle in normal Ringer's solution, the measured membrane potential is within a few millivolts of E(K). This finding is incompatible with a 1:1 coupled Na-K pump. It is consistent with either no coupling of Na efflux to K influx, or a coupling ratio of 3 or greater.

Animals↗

PubMed1965

THE CONTROL OF THE MEMBRANE POTENTIAL OF MUSCLE FIBERS BY THE SODIUM PUMP.

Frog sartorius muscles were made Na-rich by immersion in K-free sulfate Ringer's solution in the cold. The muscles were then loaded with Na(24) and the extracellular space cleared of radioactivity. When such Na-rich muscles were transferred to lithium sulfate Ringer's solution at 20 degrees C, Na efflux was observed to increase with time, to reach a maximum about 15 minutes after the transfer of the muscles to Li(2)SO(4), and then to decline. The decline in efflux from these muscles was proportional to ([Na](i))(8) over a considerable range of [Na](i). The membrane potential of Na-rich muscles was about -48 mv in K-free sulfate Ringer's at 4 degrees C but changed to -76 mv in the same solution at 20 degrees C and to -98 mv in Li(2)SO(4) Ringer's at 20 degrees C. By contrast, muscles with a normal [Na](i) showed a fall in membrane potential when transferred from K-free sulfate Ringer's to Li(2)SO(4) Ringer's solution. The general conclusions from this study are (a) that Na extrusion is capable of generating an electrical potential, and (b) that increases in [Na](i) lead to reversible increases in P(Na) of muscle fibers.

Animals↗