The effects of external anions (SCN−, NO 3 − , I−, Br−, F−, glutamate, and aspartate) on gating of Ca2+-dependent Cl− channels from rat parotid acinar cells were studied using the whole-cell configuration of the patch-clamp technique. Shifts in the reversal potential of the current induced by replacement of external Cl− with foreign anions, gave the following selectivity sequence based on permeability ratios (P x/P Cl): SCN−>I−>NO 3 − >Br−>Cl−>F−>aspartate>glutamate. Using a continuum electrostatic model we calculated that this lyotropic sequence resulted from the interaction between anions and a polarizable tunnel with an effective dielectric constant of ∼23. Our data revealed that anions with P x/PCl > 1 accelerated activation kinetics in a voltage-independent manner and slowed deactivation kinetics. Moreover, permeant anions enhanced whole-cell conductance (g, an index of the apparent open probability) in a voltage-dependent manner, and shifted leftward the membrane potential-g curves. All of these effects were produced by the anions with an effectiveness that followed the selectivity sequence. To explain the effects of permeant anions on activation kinetics and g Cl we propose that there are 2 different anion-binding sites in the channel. One site is located outside the electrical field and controls channel activation kinetics, while a second site is located within the pore and controls whole-cell conductance. Thus, interactions of permeant anions with these two sites hinder the closing mechanism and stabilize the channel in the open state.
The Journal of Membrane Biology – Springer Journals
Published: Jan 1, 2004
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