Calcitonin suppresses growth hormone (GH) response to growth hormone-releasing hormone (GHRH) in man

Calcitonin (CT) receptors have been found in the hypothalamus, suggesting a neuroendocrine role for this peptide. We have recently shown that, in the rat, central administration of salmon calcitonin (sCT) suppresses basal and GHRH-stimulated GH secretion. To further investigate how sCT alters GH secretion, we studied the effects of sCT (100U MRC, im) or placebo on basal and GHRH (50 micrograms, iv)-stimulated GH secretion in 6 normal men. GHRH was administered 1 h after sCT injection. Basal GH levels were not altered by sCT administration. However, GH response to GHRH was markedly suppressed by sCT (area under the curve - sCT: 574.6 +/- 69.7 vs placebo: 1057.2 +/- 284.8 micrograms. min/L; p less than 0.02). Cortisol levels were higher in sCT-treated subjects compared to controls, from 45 to 105 min after sCT injection (p less than 0.05). However, no correlation was found between GH response to GHRH and cortisol levels. No changes in glucose, calcium and PTH levels were seen. These results demonstrate that sCT inhibits GHRH-induced GH secretion in man by a mechanism apparently independent of changes in peripheral cortisol, glucose, calcium and PTH levels.