Regulation of thyrotropin-releasing hormone receptors is cell type specific: comparison of endogenous pituitary receptors and receptors transfected into non-pituitary cells.

TRH, which does not elevate cyclic AMP, and elevation of cellular cyclic AMP decrease the density (down-regulate) of TRH receptors (TRH-Rs) on pituitary (GH3) cells. In this study we measured the effects of TRH and elevation of cyclic AMP on TRH-Rs expressed in non-pituitary cells transfected with a recently cloned mouse pituitary TRH-R complementary DNA. In stably transfected rat glioma (C6-2) cells and transiently transfected COS-1 cells TRH caused TRH-R down-regulation while elevation of cyclic AMP caused increases in TRH-R density. Hence, the effects of cyclic AMP on TRH-Rs in transfected C6-2 and COS-1 cells are different from those in GH3 cells while the effects of TRH on TRH-R are similar in all three cell types. These data show that regulation of TRH-Rs is cell type specific.