Figure 4.

GABA-induced steady-state inhibition is mediated by Gα_o. (A) Time course of Ca²⁺ current during intracellular application of Gα_o-GTPγS (0.2 nM, ○, n = 6; 20 nM, □, n = 8), Gα_o-GDP (20 nM, •, n = 3), or control internal solution without Gα_o (“rundown,” ⋄, n = 4). Data plotted as means ± SEM; one-sided error bars are plotted for clarity. (B) Three superimposed current traces evoked by 10-ms step depolarizations to 0 mV from a holding potential of −80 mV. Shown are the relative proportions of kinetic slowing and steady-state inhibition produced by 20 nM intracellular Gα_o-GTPγS in control cell (Left) and cell exposed to 100 μM extracellular genistein (Right). Control currents (C) were taken within 20 s of achieving whole cell access (before Gα_o-GTPγS inhibited the current). Inhibited currents (measured after 5 min of exposure to intracellular solution containing Gα_o-GTPγS) were evoked either without (α_o-GTPγS) or with (∗) a depolarizing conditioning pulse (not shown) to reverse kinetic slowing. Calibration: 0.75 nA (Left), 1 nA (Right), 5 ms. (C) Mean inhibition of Ca²⁺ current produced by intracellular application of recombinant Gα_o-GTPγS at the concentrations indicated on abscissa; kinetic slowing (empty bars) and steady-state inhibition (filled bars) in control cells or cells exposed to 100 μM genistein in the bath (as indicated). (D) Mean inhibition of calcium current produced by 100 μM GABA or NE in control cells (C) or in cells exposed to 100 μM intracellular βγ-binding peptide G (G) or peptide A (A) of the same length but without βγ binding activity. Errors represent SEM in all panels (n in parentheses).