Stimulation of 36Cl- permeation in the in----out direction across the Dieters' neuron membrane by GABA on its cytoplasmic side: effect of different ionic conditions.

The permeation of labelled Cl- ions across single plasma membranes microdissected from rabbit Deiters' neuron was studied in a microchamber system. In particular, we studied 36Cl- in----out permeation and its stimulation by 10(-6) M GABA on the cytoplasmic side under different ionic conditions on both sides of the membrane. Three main results were found: a) The GABA effect turns up at a cytoplasmic side [Cl-], 21 mM, in the range of the normal steady state intracellular Cl- concentration. It disappears both at tracer level of Cl- on the cytoplasmic side and when [Cl-] is high there (140 mM). b) The increase in ionic strength due to the equinormal substitution of monovalent anions (Cl- and acetate) with the trivalent impermeant anion citrate on both sides of the membrane erases the GABA effect even if Cl- is at the optimal cytoplasmic side concentration, 21 mM. c) Citrate ions reduce to the limit of significance the GABA effect even when they are only on the membrane extracellular side and [Cl-] is at the optimal level on the other side. These results confirm that GABA stimulated Cl- permeation in the in----out direction is via Cl- channels exposing positive charges at their cytoplasmic mouth. In addition, they point out that such GABA activated channels are endowed with a partial electrical positivity at their extracellular exit.