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Mechanism of Membrane Electrical Response to Thyrotropin-Releasing Hormone in Xenopus Oocytes Injected with GH₃ Pituitary Cell Messenger Ribonucleic Acid

Division of Endocrinology and Metabolism, Department of Medicine, Cornell University Medical College and The New York Hospital New York, New York 10021

Address requests for reprints to: Marvin C. Gershengorn, M.D., Cornell University Medical College, 1300 York Avenue, New York, NY 10021.

Abstract

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 in ⁴⁵Ca²⁺ 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.

FOOTNOTES

This research project was supported by Grants GA-PS-8524 from the Rockefeller Foundation, 5-RO1-DK 33468-05 and BRSG S07 RR05396 from NIH (to M.C.G.), and by United States-Israel Binational Foundation grant (to Y.O. and M.C.G.).

^* Permanent address: Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, Israel.

Received for publication August 27, 1987. Accepted for publication October 7, 1987.

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