GROWTH-HORMONE (GH)-RELEASING HEPTAPEPTIDE, BUT NOT GH-RELEASING HORMONE, INHIBITS THYROTROPIN-STIMULATED THYROID-HORMONE SECRETION AND CAMP FORMATION IN CULTURED HUMAN THYROID-FOLLICLES

Synthetic heptapeptide growth hormone-releasing peptide-1 (GHRP-1) pot ently stimulates GH release in many species, including humans. We inve stigated the direct in vitro effect of this peptide, compared to growt h hormone-releasing hormone (GHRH), on cultured human thyroid follicle s. The results indicate that whereas GARP-1 (6-600 mu g/l) or GHRH (6- 600 mu g/l) alone had no effect on basal triiodothyronine (T-3) secret ion or cAMP formation, the heptapeptide (6-600 mu g/l), but not GHRH ( 6-1200 mu g/l), dose-dependently inhibited thyrotropin (TSH)-stimulate d T-3 secretion and cAMP formation, Moreover, GHRP-1 also dose-depende ntly inhibited forskolin-stimulated T-3 secretion. It would seem, ther efore, that the GHRP-1-induced inhibitory effect on thyroid function i s located downstream of cAMP formation, without necessarily excluding an additional inhibitory action at a pre-cAMP site. These results addi tionally demonstrate differences in the mode of action of GHRP-1 and G HRH.