Membrane potential and contractures in segments cut from rat fast and slow twitch muscles

Abstract

Contractile responses associated with depolarization caused by an increase in [K]₀ or by voltage-clamp steps were compared for fast and slow mammalian twitch muscles. The contractions and contractures of isolated mammalian muscle fibres cut into 0.5–1.0 cm lenghts are similar to those obtained from intact cells. The slope of the relationship between the membrane potential and the log [K]₀ is similar in slow (46 mV±0.8) and in fast fibres (48 mV±1.1). This relation is not significantly modified in sodium-free or Cl-free solution. The K-contractures of cut sections of slow and fast fibres are transient and a full repriming of the response is only observed when the [K]·[Cl] product is kept constant. The contractile threshold in fast fibres is at 20–30 mM [K]₀ (−52 to −43 mV) and in slow muscle at 10–15 mM [K]₀ (−62 to −55 mV). Under voltage-clamp conditions, the contractile responses of both types of muscle show two components. In Na-free solution or in presence of TTX (5×10⁻⁷ g/ml) the first component is abolished and the second remaining component is similar to that developing during K-contractures. In iliacus fibres, the contracture threshold is between −50.5 mV and −40.5 mV and in soleus fibres between −66 mV and −56 mV; these values are close to those obtained with K-rich depolarizing fluids. The S-shaped curve of the contracture vs membrane potential relation is similar to that found in frog muscles except that the contractile responses are graded over a larger range of membrane potentials (−50 to + 30 mV in fast and −55 to +10 mV in slow muscle).