Ionic strength effects on ionic currents of *Shaker.* (A) *I*(*V*) relationships obtained from ramps of voltage (0.32 mV/ms) using *S* = 318 K-Asp intracellular K^{+} solution and switching to *S* = 12 mM intracellular solution. The external solution is pipet K. The continuous curves superimposed in the *I*(*V*) curves are fits to a open channel *I*(*V*) curve derived from a single barrier permeation model according to *I**V*=*G*_{a}K_{i}*e*^{−1−δzeoVkT}−K_{o}*e*^{δzeoVkT}, where [K]_{o} and [K]_{i} are the extracellular and intracellular potassium concentrations, respectively, δ is the electrical distance, and *z* is the valence; *e*_{o}, *k*, and *T* have their usual meanings. *G*_{a} has units of nanoamperes per millimolar. The values used in the fit are for *S* = 318 mM, δ = 0.7, and *G*_{a} = 0.37 nA/mM. For *S* = 12 mM, δ = 0.7 and *G*_{a}= 0.23 nA/mM. The value of *z* remained constant at unity. (B) The relative open probability as a function of voltage at three different ionic strengths. The continuous traces are obtained from the ramps in A by dividing by the open channel *I*(*V*) function. Boltzmann fits show that the lowest ionic strength solution (12 mM) produces a 10-mV shift in the hyperpolarizing direction. The shift at 24 mM is 6 mV. (C) Nonstationary noise analysis of currents at −20 mV recorded at 8 kHz bandwidth. The effect of the lower ionic strength is to increase the single channel conductance by a small factor that accounts for the increased variance, while the change in the open probability and the number of channels is also small. The estimated values from a fitted parabola (continuous curves) are as follows: at 318 mM, *i* = −0.24, pA, *N* = 5,624, and *p*_{max}= 0.81. At 12 mM, *i* = −0.3 pA, *N* = 5,605, and *p*_{max} = 0.67.

## PubMed Commons