Permeability and single channel conductance of human homomeric rho 1 GABA(C) receptors

1. Homomeric human rho 1 GABA(C) receptors were expressed in Xenopus oocyte s and in human embryonic kidney cells (HEK293) in order to examine their co nductance and permeability. 2. Reversal potentials of currents elicited by gamma-aminobutyric acid (GAB A) were measured in extracellular solutions of various ionic composition to determine relative permeability of homomeric rho 1 receptors. The rank ord er of anionic permeability was: SCN- > I- > NO3- > Br- > Cl- > formate (For (-)) > HCO3- > acetate (Ac-) approximate to proprionate (Prop(-)) approxima te to isethionate (Ise(-)) approximate to F- approximate to PO4-. 3. In the oocyte expression system, relative permeabilities to SCN-, I-, NO 3-, Br- and HCO3- were higher for rho 1 GABA(C) receptors than alpha 1 beta 2 gamma 2L GABA(A) receptors. 4. Expression of rho 1 GABA(C) receptors in Xenopus oocytes and in HEK293 c ells gave similar relative permeabilities for selected anions, suggesting t hat the expression system does not significantly alter permeation propertie s. 5. The pore diameter of the homomeric rho 1 GABA(C) receptor expressed in o ocytes was estimated to be 0.61 nm, which is somewhat larger than the 0.56 nm pore diameter estimated for alpha 1 beta 2 gamma 2L GABA(A) receptors. 6. Homomeric rho 1 GABA receptors expressed in oocytes had a single channel chord conductance of 0.65 +/- 0.04 pS (mean +/- S.E.M.) when the internal chloride concentration ([Cl-](i)) was 20 mM. With a [Cl-](i) of 100 mM, the single channel chord conductance was 1.59 +/- 0.24 pS. 7. The mean open time directly measured from 43 GABA-induced channel openin gs in six: patches was 3.2 +/- 0.8 s. The mean open time in the presence of 100 mu M picrotoxin was 0.07 +/- 0.01 s (77 openings from 3 patches). 8. The differences observed in ionic permeabilities, pore size, single chan nel conductance and mean open time suggest that the rho 1 homomeric recepto r may not be the native retinal GABA(C), receptor reported previously.