Signal transduction pathway for the substance P‐induced inhibition of rat Kir3 (GIRK) channel

Certain transmitters inhibit Kir3 (GIRK) channels, resulting in neuronal excitation. We analysed signalling mechanisms for substance P (SP)‐induced Kir3 inhibition in relation to the role of phosphatidylinositol 4,5‐bisphosphate (PIP 2 ). SP rapidly – with a half‐time of ∼10 s with intracellular GTPγS and ∼14 s with intracellular GTP – inhibits a robustly activated Kir3.1/Kir3.2 current. A mutant Kir3 channel, Kir3.1(M223L)/Kir3.2(I234L), which has a stronger binding to PIP 2 than does the wild type Kir3.1/Kir3.2, is inhibited by SP as rapidly as the wild type Kir3.1/Kir3.2. This result contradicts the idea that Kir3 inhibition originates from the depletion of PIP 2 . A Kir2.1 (IRK1) mutant, Kir2.1(R218Q), despite having a weaker binding to PIP 2 than wild type Kir3.1/Kir3.2, shows a SP‐induced inhibition slower than the wild type Kir3.1/Kir3.2 channel, again conflicting with the PIP 2 theory of channel inhibition. Co‐immunoprecipitation reveals that Gα q binds with Kir3.2, but not with Kir2.2 or Kir2.1. These functional results and co‐immunoprecipitation data suggest that G q activation rapidly inhibits Kir3 (but not Kir2), possibly by direct binding of Gα q to the channel.