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Growth hormone (GH)-releasing heptapeptide, but not GH-releasing hormone, inhibits thyrotropin-stimulated thyroid hormone secretion and cAMP formation in cultured human thyroid follicles

Kraiem Z., Bowers CY, Sobel E, Laron Z. Growth hormone (GH)-releasing heptapeptide, but not GHreleasing hormone, inhibits thyrotropin-stimulated thyroid hormone secretion and cAMP formation in cultured human thyroid follicles. Eur J Endocrinol 1995;133:117–20. ISSN 0804–4643

Synthetic heptapeptide growth hormone-releasing peptide-1 (GHRP-1) potently stimulates GH release in many species, including humans. We investigated the direct in vitro effect of this peptide, compared to growth hormone-releasing hormone (GHRH), on cultured human thyroid follicles. The results indicate that whereas GHRP-1 (6–600 µg/l) or GHRH (6–600 µg/l) alone had no effect on basal triiodothyronine (T₃) secretion or cAMP formation, the heptapeptide (6–600 µg/l), but not GHRH (6–1200 µg/l), dose-dependently inhibited thyrotropin (TSH)-stimulated T₃ secretion and cAMP formation. Moreover, GHRP-1 also dose-dependently inhibited forskolin-stimulated T₃ secretion. It would seem, therefore, that the GHRP-1-induced inhibitory effect on thyroid function is located downstream of cAMP formation, without necessarily excluding an additional inhibitory action at a pre-cAMP site. These results additionally demonstrate differences in the mode of action of GHRP-1 and GHRH.

Z Kraiem, Endocrine Research Unit, Carmel Medical Center, 7 Michal Street, Haifa 34362, Israel

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