Mechanism of membrane electrical response to thyrotropin-releasing hormone in xenopus oocytes injected with GH₃ pituitary cell messenger ribonucleic acid

TRH evoked a complex electrical membrane response in Xenopus laevis oocytes injected with either total cytosolic or poly(A)⁺-enriched RNA from GH3 pituitary cells but not in uninjected oocytes. A typical response consisted of a transient, rapid depolarizing current followed by a prolonged depolarizing current with superimposed current fluctuations. The reversal potentials of the rapid and the slow components of the response were-23.0 and-22.6 mV, respectively, and were markedly affected by Cl⁻concentration indicating that the TRH response was mainly an increase in Cl⁻conductance. The response to TRH was dose dependent and was inhibited by the TRH antagonist, chlordiazepoxide. TRH caused rapid hydrolysis of labeled phosphatidylinositol 4, 5-bisphosphate and a marked, prolonged increase in⁴⁵Ca²⁺ efflux from injected oocytes. The depolarizing response to TRH was not diminished in oocytes incubated in a Ca²⁺-free medium, but was inhibited by microinjection of EGTA. These data suggest that TRH evokes an electrophysiological response in oocytes injected with RNA from GH₃ cells via activation of the same biochemical pathway that mediates its actions in GH3 cells. This pathway involves hydrolysis of phosphatidylinositol 4, 5-bisphosphate, forming inositol trisphosphate that causes mobilization of cellular Ca²⁺. We suggest that oocytes injected with GH₃ cell RNA, because of their large size and easy access to their intracellular milieu, will be a useful intact cell model in which to define the molecular details of signal transduction by TRH.