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J. Gen. Physiol.,
Volume 112, Number 2, August 1, 1998 223-242
From the * Departments of Physiology and Anesthesiology, University of California at Los Angeles, Los Angeles, California 90095; and Ionic (Ii) and gating currents (Ig) 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 Ii elicited by large depolarizations consists of an initial delay
followed by an exponential rise with two kinetic components. The main Ii 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
eo. The Ig On response obtained between Instituto Venezolano de Investigaciones Científicas, Centro de Biofísica y Bioquímica, Laboratorio de Fisiología Renal, Caracas 1020A,
Venezuela
60 and 20 mV consists of a rising phase followed by a decay with fast
and slow kinetic components. The main Ig component of decay is highly temperature dependent (Q10 > 4) and
has a z between 1.6 and 2.8 eo in the voltage range from 60 to 10 mV, and ~0.45 eo 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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