Temperature Sensitivity of the K+ Channel of Chara. A Thermodynamic Analysis

Temperature sensitivity of the K⁺ channel of Chara cytoplasmicdroplets has been characterized by means of the patch-clamptechnique. The activity of the channel was recorded in inside-outpatches over a range of temperatures (3°C to 25°C).An increment in the unitary channel conductance and a decreasein the probability of channel opening was found as temperatureaugmented. This could be explained by the combined effect ofa reduction in the mean open duration and an increase in theclosed times (Q₁₀ values of 0.7 and 1.1–1.6 respectively).Eyring's transition state theory was applied to the thermodynamicanalysis of conductance and kinetics of the K⁺ channel. Thevalues obtained for the activation enthalpy and entropy werecompared with, and found to be similar to, those reported forvoltage-dependent K⁺ channels in animal cells. The relativeinsensitivity of channel conductance to temperature (activationenthalpy of 2.4 kcal mol^–1) suggests that ions traversethe pore by diffusion. Channel closure appears to have the highestenergetic requirements (activation enthalpy of 6.4 kcal mol^–1).The channel closing rate, , exhibits a less negative entropicchange (–22.54 cal K^–1 mol^–1), which wouldprovide the driving force for stabilizing the closed configurationof the channel as the temperature increases. (Received September 16, 1993; Accepted December 3, 1993)