Specificity of TRH receptor coupling to G‐proteins for regulation of ERG K+ channels in GH3 rat anterior pituitary cells

The identity of the G-protein coupling thyrotropin-releasing hormone (TRH) receptors to rat ether-a-go-go related gene (r-ERG) K+ channel modulation was studied in situ using perforated-patch clamped adenohypophysial GH3 cells and dominant-negative variants (Gα-QL/DN) of G-protein α subunits. Expression of dominant-negative Gαq/11 that minimizes the TRH-induced Ca2+ signal had no effect on r-ERG current inhibition elicited by the hormone. In contrast, the introduction of dominant-negative variants of Gα13 and the small G-protein Rho caused a significant loss of the inhibitory effect of TRH on r-ERG. A strong reduction of this TRH effect was also obtained in cells expressing either dominant-negative Gαs or transducin α subunits, an agent known to sequester free G-protein βγ dimers. As a further indication of specificity of the dominant-negative effects, only the dominant-negative variants of Gα13 and Rho (but not Gαs-QL/DN or Gαt) were able to reduce the TRH-induced shifts of human ERG (HERG) activation voltage dependence in HEK293 cells permanently expressing HERG channels and TRH receptors. Our results demonstrate that whereas the TRH receptor uses a Gq/11 protein for transducing the Ca2+ signal during the initial response to TRH, this G-protein is not involved in the TRH-induced inhibition of endogenous r-ERG currents in pituitary cells. They also identify Gs (or a Gs-like protein) and G13 as important contributors to the hormonal effect in these cells and suggest that βγ dimers released from these proteins may participate in modulation of ERG currents triggered by TRH.