FIGURE 1.

Membrane potential regulates μ receptor‐induced K_ir3.X currents in HEK293T cells. (a) Representative recording (out of n = 6) of inward K⁺ currents in HEK293T cells expressing wild‐type μ receptors (MOP‐wt) and K_ir3.X channels is shown. K_ir3.X currents were evoked by 4.5 nM morphine (Mor) and 30 nM DAMGO and measured at −90 and −20 mV (voltage protocol is depicted at the bottom; note that depolarization to −20 mV shifts both basal and active currents due to the current–voltage relationship [see Figure S1] of the channel). Levels that were used for calculation of maximum responses are shown as light grey line (−90 mV) or dark grey line (−20 mV), and amplitude height is shown by boxes (morphine response: filled box; maximum DAMGO response: empty box). (b) K_ir3.X current responses evoked by non‐saturating morphine concentrations were normalized to the maximum response (evoked by a saturating DAMGO concentration) at respective membrane potentials (n = 6, for further explanation of evaluation, see Section 2). (c) Representative recording (out of n = 7) of inward K⁺ currents. K_ir3.X currents were evoked by 3 nM morphine and 30 nM DAMGO and measured and evaluated as explained in (a) (non‐saturating DAMGO response: filled box; saturating response: empty box). (d) K_ir3.X current responses evoked by non‐saturating DAMGO concentrations were normalized to the maximum response (evoked by a saturating DAMGO concentration) at respective membrane potentials (n = 7, for further explanation of evaluation, see Section 2). Responses at −90 and −20 mV were compared in the same recording. *P < 0.05, significantly different as indicated; paired two‐sample Wilcoxon test. (e) K_ir3.X current responses evoked by non‐saturating Met‐enkephalin concentrations were normalized to the maximum response (evoked by a saturating DAMGO concentration) at respective membrane potentials (n = 6, for further explanation of evaluation, see Section 2). (f) Representative recording (out of n = 5) of inward K⁺ currents. K_ir3.X currents were evoked by 10 nM fentanyl and 30 nM DAMGO and measured at −90 and −20 mV and measured and evaluated as explained in (a) (fentanyl response: filled box; saturating DAMGO response: empty box). (g) K_ir3.X current responses evoked by non‐saturating fentanyl concentrations were normalized to the maximum response (evoked by a saturating DAMGO concentration) at respective membrane potentials (n = 5, for further explanation of evaluation, see Section 2). (a, c, f) K_ir3.X channels were blocked with barium (500 μM) at several time points as indicated. (b, e, g) As responses at −90 and −20 mV were compared in the same recording, statistics were performed using a paired, two‐tailed t‐test (n.s.: P > 0.05, *P < 0.05)