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Characterization of Growth Hormone-Releasing Hormone Receptors in Pituitary Adenomas from Patients with Acromegaly^*

Division of Metabolism and Endocrinology, Department of Internal Medicine, Saga Medical School (S.I., T.K., T.S.) Saga 840-01, Japan
The Third Department of Internal Medicine, Faculty of Medicine, Kyushu University (S.N., H.N., K.K., H.I.) Fukuoka 812, Japan
The Clayton Foundation Laboratories for Peptide Biology, The Salk Institute (J.R., W. V.) La Jolla, California 92037

Address all correspondence and requests for reprints to: Dr. S. Ikuyama, M.D., Department of Internal Medicine, Saga Medical School, Nabeshima, Saga 840-01, Japan.

GHRH receptors in pituitary adenoma cell membranes from five patients with acromegaly were characterized using [¹²⁵I] [His¹,Nle²⁷]GHRH-(1–32)NH₂ ([¹²⁵I]GHRHa) as a ligand. Specific binding of [¹²⁵I]GHRHa to adenoma cell membranes was maximal within 20 min at 24 C, remained stable for 60 min, and was reversible in the presence of 500 nmol/L human GHRH-(1–44)NH₂ (hGHRH). The specific binding increased linearly with 10–160 µg cell membrane protein. This binding was inhibited by 10^–11–10^–6 mol/L hGHRH in a dosedependent manner, with an ID₅₀ of 0.20 nmol/L, but not by 10^–7 mol/L vasoactive intestinal peptide, glucagon, somatostatin-14, somatostatin-28, TRH, LHRH, and CRH. The specific binding of [¹²⁵I] GHRHa to the membranes was saturable, and Scatchard analysis of the data revealed an apparent single class of high affinity GHRH receptors in five adenomas from acromegalic patients; the mean dissociation constant was 0.30 ± 0.07 (±SE) nmol/L, and the mean maximal binding capacity was 26.7 ± 7.0 (±SE) fmol/mg protein. In three nonfunctioning pituitary adenomas, GHRH receptors were not detected.

The plasma GH response to hGHRH (100 µg) injection was studied in four acromegalic patients before surgery. Plasma GH levels increased variably in response to hGHRH injection in all four patients. However, there was no correlation between the characteristics of the tumor GHRH receptors and plasma GH responsiveness in these patients.

We conclude that pituitary GH-secreting adenomas have specific GHRH receptors. Exogenously administered GHRH presumably acts via these receptors, but the variations in plasma GH responsiveness to hGHRH in these patients cannot be directly related to the variations in binding characteristics of the GHRH receptors on the GH-secreting adenoma cells.

^* This work was supported in part by a Grant-in-Aid for Special Project Research of Selected Intractable Neurological Disorders, the Ministry of Education, Science, and Culture, Japan, and a research grant from the Division of Intractable Diseases, the Ministry of Health and Welfare, Japan.

Received June 29, 1987.

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