Effects of baclofen on stably expressed human Ca_v2.1 (α_1A-2) channels in the presence of transiently expressed human GABA_BRs (Ca_v2.1/GABA_BR cells). (A) Baclofen-inhibition of I_Ba in the presence of 0.5 mM EGTA in the intracellular recording solution. 50 µM baclofen reversibly inhibited I_Ba by 38.5 ± 3.9% (n = 5). (Left) Representative currents in the absence (control) and presence of baclofen, elicited by voltage ramps to +50 mV from an HP of −80 mV at 0.1 Hz. Dotted line represents zero-current level. (Middle) I-V relationships in the absence and presence of baclofen. Current amplitudes were determined from voltage ramps at selected membrane potentials (V_m). Solid lines are fits of the modified Boltzmann equation to normalized I-V relationships (see Materials and methods). (Right) Voltage dependence of activation determined from G-V relationships. Relative conductance (G/G_max) was calculated as I_Ba/(V_m − V_rev), where V_rev is the reversal potential of the whole-cell current and plotted as a function of V_m. The normalized G-V relationships were fitted with a Boltzmann function, G = G_max/(1 + exp((V_0.5,act − V_m)/k)), where V_0.5,act is the potential at which the conductance is half-maximally activated, and k is the slope factor. (B) Similar experimental procedures and data representation as shown in A, with 10 mM EGTA in the intracellular recording solution. Baclofen reversibly inhibited I_Ba by 41.8 ± 4.7% (n = 6). See Table S1 for V_0.5,act (voltage for half-maximal current activation) and k (slope factor) values resulting from experiments shown in A and B.