A, diagrams depict channel mutants: Kir3.4 (S143T) parent, N-terminal truncation, and chimeras. We produced the Kir3.4(S143T) using a cDNA template for cRNA coding a Kir3.4(S143T) having a truncated (1–57) N terminus. We used Kir3 chimeras: Kir3.4(S143T)-(1–338)/Kir3.1-(333–501) and Kir3.4(S143T)-(1–249)/Kir3.1-(244–501). B, oocytes were injected with 1 ng of MOR and 1 ng of HETAR, and either 1 ng of Kir3.4(S143T), 1 ng of Kir3.4(S143T)-(Δ1–57), 1 ng of Kir3.4(S143T)-(1–338)/Kir3.1-(333–501), or 1 ng of Kir3.4(S143T)-(1–249)/Kir3.1-(244–501). The percent inhibition of the second mu opioid response after Et-1 pretreatment is shown. The bar graph summarizes the effects of Et-1 on the DAMGO activation of MOR compared with untreated controls from the same batch. Data are means ± S.E. from four to seven oocytes and two to three independent experiments.