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 (PIP2). 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 PIP2 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 PIP2. A Kir2.1 (IRK1) mutant, Kir2.1(R218Q), despite having a weaker binding to PIP2 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 PIP2 theory of channel inhibition. Co-immunoprecipitation reveals that Gαq binds with Kir3.2, but not with Kir2.2 or Kir2.1. These functional results and co-immunoprecipitation data suggest that Gq activation rapidly inhibits Kir3 (but not Kir2), possibly by direct binding of Gαq to the channel.
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