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1. Muscle fibres from goats with myotonia congenita show characteristic responses to stimulation with intracellular currents (Adrian & Bryant, 1974). To test whether the reduced surface chloride conductance can account for these myotonic discharges, we have calculated responses of a model 'muscle fibre' to intracellular current of long duration (greater than 100 msec), assuming that the current is applied at the end of the fibre, that the fibre is of finite length, that a regenerative action potential occurs in the transverse tubular system as well as the surface, and that the potassium current in the wall of the transverse tubular system raises the potassium in the tubular lumen. In the absence of information about the kinetic parameters of the ionic currents in mammalian muscle we have used numerical values from frog muscle (Adrian, Chandler & Hodgkin, 1970). 2. In calculations with a normal surface chloride conductance a long maintained current gives only one action potential. Reduction of the chloride conductance to a half produces repetitive firing during the current; reduction to a tenth produces repetitive firing during and a small number of action potentials after the end of the current. Elimination of the tubular potassium accumulation from the calculation reduces the number but does not eliminate action potentials arising after the end of the applied current. 3. With a tenth of the normal chloride conductance calculated responses show maintained firing following a constant current if the deactivating rate of the sodium channels (betam) is reduced by 25%. As before, eliminating potassium accumulation reduces the number of post‐stimulus action potentials, but it does not eliminate them altogether. 4. We conclude that in the absence of a surface chloride conductance tubular potassium accumulation could certainly contribute to the instability of the membrane, but it is clear that potassium accumulation is not the only reason for the instability of myotonic muscle fibres. The kinetics of the sodium channels are important and we do not know that they are the same in normal and myotonic fibres. Nevertheless the presence of a surface chloride conductance does stabilize the response of a fibre to constant current or to repetitive stimulation, and its absence could be a sufficient condition for myotonic behaviour.

The middle of the fibre voltage‐clamp technique was used to measure ionic currents and non‐linear charge movements in intact, organ‐cultured (in vitro denervated) mammalian fast‐twitch (rat extensor digitorum longus) muscle fibres. Muscle fibres organ cultured for 4 days can be used as electrophysiological and morphological models for muscles in vivo denervated for the same length of time. Sodium currents in organ‐cultured muscle fibres are similar to innervated fibres except that in the temperature range 0‐20 degrees C (a) in the steady state, the voltage distribution of inactivation in cultured fibres is shifted negatively some 20 mV; (b) at the same temperature and membrane potential, the time constant of inactivation in cultured fibres is about twice that of innervated fibres. Potassium currents in innervated and cultured fibres at 15 degrees C can be fitted with the Hodgkin‐Huxley n variable raised to the second power. Despite the large range we would estimate that the maximum value of the steady‐state potassium conductance of cultured fibres is about one‐half that of innervated fibres. The estimated maximum amount of charge moved in cultured fibre is about one‐third that in innervated fibres. Compared to innervated fibres, culturing doubles the kinetics of the decay phase of charge movement. The possibility of a negative shift of the voltage distribution of charge movements in cultured fibres is discussed.

1. Some electrophysiological and morphological properties of ‘fast’ (singly innervated) and ‘slow’ (multiply innervated) muscle fibres were studied in normal and denervated posterior and anterior latissimus dorsi muscles of the young chickens. 2. Normal singly and multiply innervated muscle fibres are capable of generating action potentials which in all qualitative respects are similar. 3. The action potentials of multiply innervated muscle fibres are of lower amplitude and slower maximum rate of rise than action potentials in singly innervated muscle fibres. 4. Denervation causes the resting membrane potential and the maximum rate of rise of the action potential to fall. The changes are greater in singly innervated than in multiply innervated fibres, but in neither case are as great as in mammalian skeletal muscle fibres after surgical denervation. 5. In neither singly nor multiply innervated muscle fibres do the action potentials generate any ‘resistance’ to tetrodotoxin as a result of denervation. 6. The diameter of multiply innervated fibres is increased after denervation, but it is reduced in singly innervated fibres. The number of myofilaments increases in multiply innervated fibres, but decreases in single innervated fibres. In both types of muscle fibre the volume fraction of myofibrils is decreased. 7. In the singly innervated muscle fibres there is an increase in the volume fraction of mitochondria. 8. In the singly innervated muscle fibres, there is some rearrangement of the membrane systems in that some of the transversely orientated triads are replaced by longitudinally orientated dyads.

1. A method is described which allows the approximate computation of membrane current from measurements with three electrodes in the mid‐region of a muscle fibre.

1. The middle of the fibre voltage‐‐clamp technique (Adrian & Marshall, 1977), modified where necessary for electrically short muscle fibres, has been used to measure non‐linear charge movements in mammalian fast twitch (rat extensor digitorum longus), mammalian slow twitch (rat soleus) and frog (sartorius) muscles. 2. The maximum amount of charge moved in mammalian fast twitch muscle at 2 degrees C in hypertonic solution, was 3‐‐5 times greater than in slow twitch muscle. The voltage distribution of fast twitch charge was 10‐‐15 mV more positive when compared to slow twitch. 3. In both mammalian muscle types hypertonic Ringer solution negatively shifted the voltage distribution of charge some 6 mV. The steepness of charge moved around mechanical threshold was unaffected by hypertonicity. 4. The amount of charge in frog sartorius fibres at 2 degrees C in hypertonic solution was about half of that in rat fast twitch muscle; the voltage distribution of the frog charge was similar to rat soleus muscle. 5. Warming between 2 and 15 degrees C had no effect on either the amount of steady‐state distribution of charge in mammalian or frog muscles. 6. At 2 degrees C, the kinetics of charge movement in fast and slow twitch mammalian muscles were similar and 2‐‐3 times faster than frog muscle at the same temperature. In fast and slow mammalian fibres at 2 degrees C similar times were taken to shift the same fractions of the total amount of charge. The Q10 of charge movement kinetics was between 1.2 and 2.0 in the three muscles studied.

1. Single twitch fibres were isolated from frog muscle, then mounted in a chamber which was positioned on an optical bench. The fibres were immobilized by high stretch (sarcomere spacing 3·9‐4·3 μm) and by placement on a pedestal. Their optical properties were determined by illuminating a 35‐65 μm diameter spot with quasimonochromatic light of intensity I0and measuring the intensity Iof the transmitted light. Since the main purpose of the experiments was to draw inferences from the absorbance spectra of different indicator dyes injected into the fibres, all results were expressed in terms of absorbance Acalculated from the equation [Formula: see text]. Changes in absorbance ΔAwere calculated from the differential form of the equation [Formula: see text].

1. Absorbance changes were measured following stimulation of single muscle fibres injected with the metallochromic indicator dye Arsenazo III. Two dye‐related signals can be clearly resolved: (1) an early, transient isotropic signal that appears to be due to the formation of Ca2+:dye complex and (2) a slower, transient signal that is ‘dichroic’ in nature. The dichroic signal is obtained by taking the difference between absorbance changes measured with light plane polarized along the fibre axis (0° light) and at right angles to the axis (90° light).

1. Changes in transmission of quasi‐monochromatic light were measured in singly dissected, dye‐injected twitch fibres following a single propagated action potential. The records, after correction for the intrinsic transmission signal, indicate changes in dye‐related absorbance, ΔA. This paper describes the different components of dye‐related signals in fibres injected with either Arsenazo III, Antipyrylazo III or Dichlorophosphonazo III.

Single twitch fibres, dissected from frog muscle, were injected with the metallochromic dye Arsenazo III. Changes in dye‐related absorbance measured at 650 or 660 nm were used to estimate the time course of myoplasmic free [Ca2+] following either action potential stimulation or voltage‐clamp depolarization (temperature, 15‐17 degrees C). The amplitude of the Ca2+ transient decreased when fibres were stretched to sarcomere spacings approaching 4 microns. The effect appeared to be less marked in H2O Ringer than in D2O Ringer, where a reduction of about 40% was observed in going from 3.0 microns to 3.7‐3.9 microns. In fibres heavily injected with dye (1.5‐2.2 mM‐dye) at least 0.1 mM‐Ca2+ was complexed with Arsenazo III following a single action potential, implying that at least 0.1 mM‐Ca2+ was released from the sarcoplasmic reticulum (s.r.) into the myoplasm. Computer simulations were carried out to estimate the flux of Ca2+ between the s.r. and myoplasm (in fibres containing no more that 0.8 mM‐dye). The amounts and time courses of Ca2+ bound to the Ca2+‐regulatory sites on troponin and to the Ca2+, Mg2+ sites on parvalbumin were estimated from the free [Ca2+] wave form and the law of mass action. In the computations the total myoplasmic [Ca2+] was taken as the total amount of Ca2+ existing either as free ion or as ion complexed with dye, troponin or parvalbumin. The time derivative of total myoplasmic [Ca2+] was used as an estimate of net Ca2+ flux (release minus uptake) from the s.r. into myoplasm. Rate constants for formation of cation: receptor complex were taken from published values. For the Ca2+‐regulatory sites on troponin, three sets of rate constants, corresponding to two values of dissociation constant (0.2 and 2 microM) were used. Each set of three simulations was carried out both with and without parvalbumin. The simulations show that following action potential stimulation, 0.2‐0.3 mM‐Ca2+ enters the myoplasm from the s.r. The wave form of s.r. Ca2+ release is early and brief compared with the wave form of free [Ca2+]. Neither the selection of troponin rate constants nor the inclusion of parvalbumin has much effect on the shape of the release wave form; the main effect of varying these parameters is to change the magnitude. After the initial, rapid phase of Ca2+ release from the s.r. there is a longer, maintained period of Ca2+ uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

Single twitch fibres were dissected from frog muscle, stretched to a sarcomere spacing greater than or equal to 3.9 micron, then mounted for optical recording. The experiments were carried out at 15‐17 degrees C. In some cases D2O Ringer solution was used instead of H2O Ringer solution to reduce movement and any related optical artifacts. Following action potential stimulation, both the amplitude and time course of the change in intrinsic retardation (Baylor & Oetliker's second component, 1975) were found to be approximately independent of wavelength between 480 and 750 nm (D2O Ringer solution). Fibres were injected with the Ca2+‐sensitive dye Arsenazo III so that changes in myoplasmic free [Ca2+] could be estimated by measuring changes in dye‐related absorbance at 660 nm. The time course of free [Ca2+] was compared with the time course of two other optical signals which have been previously suggested to monitor s.r. (sarcoplasmic reticulum) membrane potential, intrinsic retardation and Nile Blue A fluorescence (Bezanilla & Horowicz, 1975). In D2O Ringer solution the retardation time course was closely similar to that of free [Ca2+] whereas the fluorescence time course was considerably slower. Thus, it is possible that either the retardation signal or Nile Blue A fluorescence (or both) monitors free [Ca2+] rather than s.r. potential. If so, the underlying mechanism which senses Ca2+ must do so very rapidly in the case of retardation and with a delay in the case of Nile Blue A. Changes in Nile Blue A fluorescence were measured in a voltage‐clamped fibre (H2O Ringer solution). Only small changes were observed during hyperpolarization or small depolarization whereas relatively large changes were observed near mechanical threshold. These increased e‐fold in magnitude every 4‐5 mV. This steep voltage dependence, similar to that already shown for intrinsic retardation and [Ca2+], provides additional evidence that Nile Blue A fluorescence monitors a step in excitation‐contraction coupling. Theoretical waveforms of s.r. membrane potential were computed using a typical waveform of s.r. Ca2+ release (from Baylor, Chandler & Marshall, 1983) under the assumption that Ca2+ crosses the s.r. membrane as electrical current. Voltage waveforms were calculated using several combinations of electrical parameters for the s.r. membrane. Only certain combinations gave theoretical potential changes similar to the intrinsic retardation or Nile Blue A fluorescence signal.(ABSTRACT TRUNCATED AT 400 WORDS)

1. Ruthenium Red (RR) blocks sarcoplasmic reticulum (SR) Ca2+ release in disrupted muscle preparations and this block has been used as a marker for the physiological Ca2+ release pathway. To investigate whether RR can also affect SR Ca2+ release in living muscle, optical signals reflecting Ca2+ release have been measured in intact single frog twitch fibres microinjected with RR. 2. The total myoplasmic concentration of RR, [RRT], was obtained from measurements of RR‐related absorbance and apparent diffusion constant, Dapp, of RR in myoplasm was estimated. The value of Dapp was about 1/30 of that expected and can be explained if the majority of RR (approximately 97%) was bound in myoplasm and free [RR] was only 1/30 of [RRT]. 3. Sarcoplasmic reticulum Ca2+ release following action potential stimulation was assessed from a Ca2+‐related change in intrinsic birefringence. The birefringence signal was blocked in the presence of RR and the degree of block was clearly dependent upon [RRT]. At 16 degrees C the estimated [RRT] for half‐block of the birefringence signal was 23 +/‐ 4 microM (+/‐ S.E. of mean; n = 3), and for half‐block of the Ca2+ release process itself was 72 +/‐ 14 microM. The estimated free [RR] for half‐block is then 0.8 +/‐ 0.1 and 2.4 +/‐ 0.5 microM, respectively. In the cold (6‐8 degrees C), the half‐blocking concentration of RR, referred to [RRT], appeared to be about 3‐fold smaller than that observed at 16 degrees C. 4. The values estimated for the free [RR] which caused half‐block of Ca2+ release in intact muscle fibres are in the range reported for RR's action in disrupted preparations, thus supporting the conclusion that the RR‐blockable channel observed in disrupted muscle is the physiologically important Ca2+ release channel. 5. Intramembrane charge movements in skeletal muscle are thought to underlie the dependence of SR Ca2+ release on transverse tubular membrane potential. Charge movements were measured in RR‐injected fibres at 4‐6 degrees C using a three‐microelectrode, middle‐of‐the‐fibre voltage‐clamp technique. Injected fibres did not survive well in solutions made hypertonic to prevent fibre movement and allow measurement of suprathreshold charge; therefore charge movements below contraction threshold were studied in isotonic solution.(ABSTRACT TRUNCATED AT 400 WORDS)

1. A study has been made of some passive and active membrane properties of myotubes of different ages obtained in culture from explants of chick embryo thigh muscle.

1. Anodal break action potentials were found to be a feature of denervated rat skeletal muscle fibres.