Effects of intranasal calcitonin administration on pituitary GH response to hGHRH (1-29)NH2 in normal adult subjects.

Studies in man demonstrated that intramuscular salmon (s) calcitonin (CT) administration blunted pituitary GH response to hypothalamic stimuli such as arginine infusion and insulin-induced hypoglycaemia. However, the mechanisms underlying this inhibiting action of CT are still unclear. The aim of our study was to investigate the effects of intranasal (i.n.) and i.m. sCT administration on GH secretion elicited by direct pituitary stimulation in man with human GH-releasing hormone (GHRH) (1-29)NH2. Seven healthy non-obese volunteers (five men, two women; mean age +/- SDM 25 +/- 2) underwent a bolus intravenous injection of GHRH, 100 micrograms, alone or associated with sCT, administered either i.n., 300 IU, or i.m., 100 IU. Our data demonstrate a significant decrease in GH secretion after GHRH when either i.n. or i.m. sCT is administered. GH peak (mean +/- SDM); GHRH alone 22.9 +/- 2.5 vs GHRH plus i.n. sCT, 8.9 +/- 1.5 micrograms/l, P less than 0.001; or vs GHRH plus i.m. sCT 12.3 +/- 2.5 micrograms/l, P less than 0.001. Area under the curve of GH secretion: GHRH alone 1211 +/- 196 vs GHRH plus i.n. sCT 551 +/- 116 micrograms 120 min/l. P less than 0.001; or vs GHRH plus i.m. sCT 700 +/- 167 micrograms 120 min/l, P less than 0.001. We conclude that sCT is able to inhibit GHRH-stimulated GH secretion in man.