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Localization and Regulation of a Functional GHRH Receptor in the Rat Renal Medulla

Laboratory of Neuroendocrinology of Aging, Centre Hospitalier de l’Université Montréal Research Center, Notre Dame Hospital and Department of Medicine, University of Montréal, Montréal, Québec, Canada H2L 4M1; and Centre National de la Recherche Scientifique, UMR 5578, Université Claude Bernard Lyon I (G.M.), Villeurbanne 69622, France

Address all correspondence and requests for reprints to: Dr. Pierrette Gaudreau, Laboratory of Neuroendocrinology of Aging, Centre Hospitalier de l’Université de Montréal Research Center, Notre Dame Hospital, Room M-5226, 1560 East Sherbrooke Street, Montréal, Québec, Canada H2L 4M1. E-mail: . pierrette.gaudreau{at}umontreal.ca

To provide information about the kidney GHRH receptor (GHRH-R), we assessed its tissue and cellular localization, defined its pattern of expression in developing and aging rats, and studied the effects of GHRH on the regulation of GHRH-R mRNA levels and receptor internalization. In situ hybridization and ribonuclease protection assay demonstrated that GHRH-R mRNA is restricted to the Henle’s loop (HL). GHRH-R mRNA levels were low in the medulla from 3- and 12-d-old male rats, increased significantly in that from 30- to 70-d-old rats, and decreased in that from 12- and 18-month-old animals. Compared with the GHRH-R mRNA profile obtained in the pituitary, these data support the concept of a tissue-specific regulation of GHRH-R. In HL cell cultures from 70-d-old rats, a 4-h incubation with 1–100 nM rat GHRH-(1–29)NH₂ reduced GHRH-R mRNA levels significantly. As anti-GHRH-R- (392–404) immunoreactivity was demonstrated in HL cells, internalization of [N-5-carboxyfluoresceinyl-D-Ala²,Ala⁸, Ala¹⁵,Lys²²]hGHRH-(1–29)NH₂ in a time- and temperature-dependent manner and inhibition of this process by phenyl arsine oxide indicate that desensitization to GHRH involves both GHRH-R internalization and down-regulation of GHRH-R mRNA levels. Localization of a functional GHRH-R in HL and its regulation during development and aging suggest roles associated with cellular proliferation, differentiation, and/or water/electrolyte transport.

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