J. Gen. Physiol., Volume 112, Number 2, August 1, 1998 223-242

Voltage Gating of Shaker K⁺ Channels
The Effect of Temperature on Ionic and Gating Currents

Beatriz M. Rodríguez, Daniel Sigg,^* and Francisco Bezanilla^*

From the ^* Departments of Physiology and Anesthesiology, University of California at Los Angeles, Los Angeles, California 90095; and  Instituto Venezolano de Investigaciones Científicas, Centro de Biofísica y Bioquímica, Laboratorio de Fisiología Renal, Caracas 1020A, Venezuela

Ionic (I_i) and gating currents (I_g) from noninactivating Shaker H4 K⁺ channels were recorded with the cut-open oocyte voltage clamp and macropatch techniques. Steady state and kinetic properties were studied in the temperature range 2-22°C. The time course of I_i elicited by large depolarizations consists of an initial delay followed by an exponential rise with two kinetic components. The main I_i component is highly temperature dependent (Q₁₀ > 4) and mildly voltage dependent, having a valence times the fraction of electric field (z) of 0.2-0.3 e_o. The I_g On response obtained between 60 and 20 mV consists of a rising phase followed by a decay with fast and slow kinetic components. The main I_g component of decay is highly temperature dependent (Q₁₀ > 4) and has a z between 1.6 and 2.8 e_o in the voltage range from 60 to 10 mV, and ~0.45 e_o at more depolarized potentials. After a pulse to 0 mV, a variable recovery period at 50 mV reactivates the gating charge with a high temperature dependence (Q₁₀ > 4). In contrast, the reactivation occurring between 90 and 50 mV has a Q₁₀ = 1.2. Fluctuation analysis of ionic currents reveals that the open probability decreases 20% between 18 and 8°C and the unitary conductance has a low temperature dependence with a Q₁₀ of 1.44. Plots of conductance and gating charge displacement are displaced to the left along the voltage axis when the temperature is decreased. The temperature data suggests that activation consists of a series of early steps with low enthalpic and negative entropic changes, followed by at least one step with high enthalpic and positive entropic changes, leading to final transition to the open state, which has a negative entropic change.

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