Persistence of excitation contraction coupling in "slow" muscle fibres after a treatment that destroys transverse tubules in "twitch" fibres

Spread of the junction potential in the T-system in hagfish slow muscle fibres

The membrane constants of the slow muscle fibres from the Atlantic hagfish were determined by analyzing the responses to sinusoidal currents at different frequencies. The apparent low frequency membrane resistance was 20-50 komega-cm2 and was not significantly altered by replacing external chloride with methyl sulphate. The low frequency capacitance was 2.1-2.4 muF/cm2 and decreased to about 1 muF/cm2 at 100 Hz. A model of the T-system consisting of a number unbranched T-tubules was fitted to the impedance locus. About 15 tubules per sarcomere with a length approximately 8 times the fibre radius gave good fit to the observed impedance locus. The electrical characteristics of the T-system allow an efficient electrotonic spread of the junction potential through the fibre cross-section.

Absence of action potentials in frog slow muscle fibres paralysed by botulinum toxin

1. As shown previously, slow muscle fibres of the frog develop the ability to produce action potentials in 2 weeks after denervation.2. Normal transmission at the slow fibre neuromuscular junction gives a junction potential of 11 mV average (range 6-20 mV), caused by summation of potentials from several motor axons.3. Botulinum toxin injected intramuscularly into the iliofibularis blocks the neuromuscular junction of slow fibres in 6 days at room temperature. Single nerve stimuli give junction potentials of 0.05 mV average (range 0-1.6 mV). Contraction of slow fibres in response to tetanic stimulation is eliminated.4. No action potentials could be elicited in botulinum treated fibres, even 6(1/2) weeks after injection of toxin. This finding is discussed in relation to the possible involvement of a ;trophic' factor regulating the action potential mechanism in frog slow muscle fibres.

Differential effects of glycerol treatment on membrane capacity and excitation-contraction coupling in toad sartorius fibres

Changes in membrane capacity and excitation-contraction coupling caused by glycerol movements have been investigated in toad sartorius fibres using a standard glycerol-Ringer solution containing 400 mM glycerol.1. The rates of glycerol movement, in and out of fibres, were determined by measuring diameter changes in single fibres. Glycerol equilibrated across the surface membrane within 20-25 min after changes in extracellular glycerol concentration.2. The reduction in membrane capacity, which occurs when glycerol-loaded fibres are returned to Ringer solution, was slower than, and not dependent on, changes in fibre volume.3. To obtain the maximum reduction in membrane capacity, it was necessary to expose fibres to glycerol-Ringer for 50-60 min and then to return them to Ringer for at least 60 min. If preparations were not kept in Ringer for the full 50-60 min, the reduction in membrane capacity could be partially or completely reversed by returning the fibres to glycerol-Ringer.4. When fibres were exposed to glycerol-Ringer there was an initial transient contracture: twitches and K contractures were rapidly inhibited, and then slowly recovered over the next 40-50 min. In some preparations, eventual potentiation of twitches was seen.5. When returned to Ringer solution after 60 min in glycerol-Ringer, preparations lost twitches and K contractures within 5-10 min. The time course of this effect was very similar to the time course of the recovery of the normal volume after the initial swelling.6. The briefer the exposure to the glycerol-Ringer, the slower the loss of twitches and K contractures on return to Ringer. In contrast to the loss of membrane capacity, the lesion of excitation-contraction coupling was essentially irreversible. Exposure times as brief as 5 min were eventually effective in producing paralysed fibres which, however, still responded to caffeine.7. The differences in the glycerol load-times required to produce decoupling of excitation and contraction, and disconnexion of the transverse tubules, show that the two effects are separable and that the first does not depend on the second.8. It is proposed that the two lesions obtained in glycerol-treated fibres may be related to distension of the transverse tubular system. It is shown in an Appendix that the outward movement of glycerol from sarcoplasm to transverse tubules would be expected to produce some early swelling of the tubules and this is related to the loss of contraction. Furthermore, much greater swelling would occur if a slow-loading compartment (presumed to be the sarcoplasmic reticulum) dumped glycerol into the transverse tubules: it is thought that this is related to the disconnexion of the transverse tubules.

Electrical properties of white and red muscle fibres of the elasmobranch fish Scyliorhinus canicula

1. Standard electrophysiological techniques (including a method which controls the membrane potential at a point on a muscle fibre) were used to investigate the electrical properties of white and red muscle fibres of the elasmobranch fish Scyliorhinus canicula.2. The resting potential of the white fibres in the standard Ringer solution was - 85.2 +/- 0.4 mV. That of the red fibres was - 71.1 +/- 1.2 mV. The membrane resistance of white fibres was 1588 +/- 97 Omega cm(2) and that of red fibres was 5410 +/- 1070 Omega cm(2).3. White fibres always responded to direct stimulation with an action potential. It proved impossible, with two impaling micro-electrodes, to record action potentials from the red fibres, although on one occasion an abortive spike was seen.4. The resting membrane of the red fibres seemed less permeable to chloride than was the membrane of the white fibres. However, the resting potassium permeability showed the potential dependence called inward or anomalous rectification in both white and red fibres.5. White fibres responded to square depolarizing pulses with conductance changes to sodium and, subsequently, to potassium.6. All red fibres examined with the point voltage clamp showed a delayed increase in potassium conductance on depolarizing.7. Out of twenty-seven red fibres examined, six showed no sign of having any sodium conductance mechanism. Eight showed large sodium currents on depolarizing, and the remaining thirteen had small sodium currents. It seemed likely that the group of eight fibres might be able to propagate action potentials.

The volume of the T-system and its association with the sarcoplasmic reticulum in slow muscle fibres of the frog

1. A study has been made of the T-system and sarcoplasmic reticulum (SR) in slow muscle fibres of the frog, Rana temporaria.2. The size of the T-system was measured by an autoradiographic method, using tritium-labelled albumin as a marker. Its volume, expressed as a fraction of that of the fibre, was found to be 1.8 x 10(-3), as compared with a figure of 3.9 x 10(-3) for the T-system in a twitch fibre.3. The spatial distribution of the T-tubules, and their association with the SR, was studied with the electron microscope, employing ferritin and the enzyme peroxidase as markers. The observations show (a) the tubules form a three dimensional, rather than transverse, network and (b) the area of triadic (and diadic) contact with the SR is 5-10 x smaller than in a twitch fibre.4. The possibility that the T-system and SR of the slow fibre participate in linking membrane excitation with contraction is discussed in the light of these findings.

The maintenance of resting potentials in glycerol-treated muscle fibres

1. A modification of a previously published method for the disruption of the T-tubules of frog skeletal muscle is described. The modification permits the disruption of the T-tubules without the decline in resting potentials which was reported previously.2. The method for the disruption of the T-tubules involves the washout of glycerol following loading in a 400 mM glycerol Ringer solution. The modification consists of elevating the concentration of divalent cations in the Ringer used for glycerol washout.3. The optimum concentrations are 5 mM-Ca(2+) and 5 mM-Mg(2+) added as their chloride salts. Neither 10 mM-Ca(2+) nor 10 mM-Mg(2+) are as effective as the combination of each at 5 mM. Other concentrations gave less satisfactory results.4. The use of the modified technique provides a preparation which maintains 85-90 mV resting potentials for up to 6 or 8 hr but which will not contract in response to membrane depolarization.

A lesion of the transverse tubules of skeletal muscle

1. A method is described by which a selective lesion can be made in vitro in the transverse tubules of frog skeletal muscle.2. The method consists of exposing the muscle for 1 hr or more to a buffered salt solution made hypertonic by the inclusion of 400 mM glycerol and then returning the muscle to an isotonic salt solution. The lesion is induced during the washout of the glycerol.3. Electron micrographs reveal that the lesion consists of a rearrangement of the T-system membranes in such a way that the continuity of the tubules is lost. The membranes appear to coalesce into large vesicles scattered irregularly throughout the sarcoplasm.4. The glycerol treatment results in a depression of the resting potential of up to 30 mV. The treated fibres are depolarized by high concentrations of K as are normal muscle fibres.5. T-tubule lesioned fibres are unable to respond mechanically either to electrical stimulation or to elevated K but they do contract in the presence of caffeine and relax when the caffeine is removed.6. Problems concerning the variability of the procedure are presented and certain considerations concerning the mechanism of the effect are discussed.