Voltage-clamp analysis of the early current in frog skeletal muscle fibre using the double sucrose-gap method

Abstract

1. The early membrane currents in skeletal muscle fibres have been studied with the double sucrose-gap method, according to the experimental procedure used for the study of the giant axon of the squid.2. The earliest observable current is a capacitative transient. An equivalent circuit with two capacity-resistance systems can account for such a current. The components of the electrical circuit are estimated from the records.3. The capacitative transient current is followed by an initial current, the time and voltage dependence of which are analogous to those of nerve fibres. During this phase of current, the membrane behaves like a sodium electrode for modifications of the external sodium concentration.4. Using tetrodotoxin, a specific inhibitor of the sodium permeability, it is possible to separate two current components, (i) a transient component, suppressed by tetrodotoxin, which reflects the modifications, with time and voltage, of the membrane permeability towards sodium ions; the larger the depolarization, the shorter the time to reach peak current. This current is inward for depolarizations (V) of about +30 mV to about +130 mV (V(Na)). There was some variation in these values between individual fibres. Beyond V(Na), the current is outward. This current shows a small rectification - inward going - for high depolarizations which is comparable to the behaviour of the Ranvier node membrane. (ii) A quasi-instantaneous outward component, only voltage dependent. The current-voltage relation of this current shows an outward going rectification for high depolarizations. This current can be considered as a ;leak' current, in which chloride ions could play an important role.5. A sodium tail current is observed when the imposed potential fall occurs during the time course of the initial current. It corresponds to the deactivation of the sodium current previously activated. Because of the large contemporary capacitative currents, it is difficult to measure accurately its maximum amplitude and its time course: yet, the instantaneous current-voltage relation seems to show a slight rectification for the largest depolarizations, as observed on the Ranvier node membrane.6. The inactivation properties of the sodium current are studied in experiments similar to those described by Hodgkin & Huxley. As in nerve fibres the ability of the membrane to undergo an increase of the sodium permeability depends on the membrane potential. In our experimental conditions, the availability of the sodium carrying system is 90 per cent at the resting potential; the half-inactivation occurs for V = +11 mV. This potential value corresponds to the smallest value of tau(h) (-1), in agreement with the equations proposed by Hodgkin & Huxley (1952d) for the inactivation process.7. The activation parameters (m(infinity), tau(m)) are determined by a fitting method, taking account of the values of tau(h) previously established. The potential of half-activation is near V = +40 mV.