The Action Potential in Chara corallina IV.* Activation Enthalpies of the Hodgkin-Huxley Gates

Abstract

Voltage-clamp experiments were made to determine the effect of temperature on the Hodgkin-Huxley parameters describing the excitation of the plasmalemma of C. covallina. In these cells two activated and inactivated transients in addition to a potential-dependent leak conductance occur during excitation. The activation and inactivation time constants for both the Cl- and X (¿ Ca²+?) channels decreased with increasing temperature. The relative changes with temperature, however, were the same at all potentials. That is, the thermodynamic activation enthalpies for activation and inactivation were independent of membrane potential. This suggests that these parts of the gating processes do not involve movement, normal to the membrane, of charged moieties (other than the ions carrying the clamp currents). For clamps beyond the threshold, both gated current transients occur only after a delay, the latter being strongly dependent on the clamp potential. The delays ¿*Cl and ¿*x in the Cl- transient and X (¿ Ca²+?) transients, respectively, both decreased with increasing temperature, having values ¿*Cl ¿ 0.8 s and ¿*x ¿ 2.5 s at - 110 mV and 20°C. Apparently activation/inactivation commenced only after the resting-state channels had been transformed into some precursor state for excitation. The activation enthalpy for this transformation varied linearly with membrane clamp potential at a rate of ~ - 390 kJ per mol per volt and ~ -960 kJ per mol per volt for the CI- and X (¿ Ca²+?) channels respectively. The transformation to the precursor state therefore required the same energy as the separation (positive charges outwards, negative charges inwards), normal to the plasmalemma, of ~4 and ~10 charges per molecule for the Cl- and X channels, respectively.