Temperature Effects on Membrane Chloride Conductance and Electrical Excitability of Lizard Skeletal Muscle Fibres

1. Intracellular recording techniques were used to study the effects of temperature on resting membrane conductances, electrical excitability and synaptic efficacy in fast-glycolytic (FG) skeletal muscle fibres from the lizard Dipsosaurus dorsalis. 2. The conductance of the resting muscle membrane to chloride ions (gci) increased from 488/iScm" at 15°C (pH7-8) to 730/iScm~ at 45°C (pH7-4), yielding a temperature coefficient (thermal ratio, Rio) of 1-14. Resting potassium conductance (gic) increased from 84/iScm~ at 15 °C to 236 ^S cm" at 45 °C (R10=l-41). 3. Fibres bathed in Cl -free Ringer's solution were hyperexcitable, and produced repetitive action potentials both during and following intracellular current injection. At the preferred body temperature of Dipsosaurus (near 40°C) the fibres also fired repetitively in response to single nerve shock. 4. The electrical excitability of Dipsosaurus fibres decreased with increasing temperature. Threshold current, measured at endplate regions of fibres bathed in normal Ringer's solution, was 146 nA at 15°C and 353 nA at 45°C (R1 0= 1-34). 5. Despite the temperature-dependent change in threshold current, at both 15 and 45 °C all fibres examined had suprathreshold neuromuscular transmission in response to single nerve shock. 6. The relative thermal independence of ga in Dipsosaurus fibres may be an adaptation that contributes to a large safety factor for neuromuscular transmission at the high body temperatures preferred by this lizard species.