Temperature experiments on nerve and muscle membranes of frogs

The influence of temperature changes in the range of 25°C to –6°C on the time constants of Na activation (m) and inactivation (h) was studied in twitch muscle fibers and the node of Ranvier under voltage-clamp conditions. Arrhenius plots of m and h exhibit a change in activation enthalpy at temperatures below 10°C. Cooling and subsequent heating induce a hystersis in the temperature dependence of m and h Ni²⁺ and UO₂²⁺ increase the hysteresis width. With fast temperature changes the gating kinetics relax to their new values more slowly than the temperature change. Hence, temperature must be changed more slowly than 5°C/min if an additional apparent hysteresis due simply to this relaxation is to be avoided. The data are explained by the hypothesis of a phase transition in the membrane lipids. This conception is favoured over a temperature-induced change in protein conformation, since the neutral local anaesthetic benzocaine shows use-dependent block as if low temperature restricted the access of the drug through the lipid phase to its receptor.Supported by grant NS 08174 from the U.S. Public Health Service and SFB 38 from Deutsche Forschungsgemeinschaft