Spontaneous growth hormone (GH) secretion is not directly affected by ghrelin in either short normal prepubertal children or children with GH neurosecretory dysfunction

Abstract

Ghrelin, a specific endogenous ligand for the GH secretagogue receptor, stimulates GH secretion in humans when given in pharmacological amounts. Under physiological conditions, however, it is controversial whether ghrelin affects GH secretion and vice versa. No studies have reported on the relationship between daily ghrelin and GH secretion in children. Therefore, plasma ghrelin and GH concentrations over a 24-h period were studied in 10 prepubertal short normal children (five females and five males) to determine the potential relationship between the secretion of these two hormones. Furthermore, five prepubertal patients (two females and three males) with GH neurosecretory dysfunction (GHNSD) were studied in the same way to assess potential alterations in ghrelin secretion in a condition associated with distinct GH changes. No gender difference in ghrelin spontaneous secretion was detected in either short normal children or GHNSD patients, and in both male and female subjects, ghrelin was secreted in a pulsatile and circadian fashion, with a nocturnal surge. Twenty-four-hour secretion and daytime ghrelin secretion of short normal children were similar to those in GHNSD patients, whereas nighttime hormone secretion in the latter group was significantly greater than that in short normal children. The cross-correlation of 24-h ghrelin and GH levels revealed significant positive and negative correlations, which were similar in the two groups examined. The positive one, with GH leading ghrelin, might reflect a somatostatin (SMS)-mediated inhibitory effect on both GH and ghrelin secretion (low SMS levels are followed by Mgh GH and ghrelin levels, and vice versa). The negative correlation, with ghrelin leading GH, might again reflect the positive effect of ghrelin on SMS, as shown in both animal and human studies. In conclusion, the results of the present study indicate that ghrelin secretion in prepubertal children is pulsatile and is not sexually dimorphic. Although the parallelism of ghrelin and GH dynamics hints at the potential relevance of endogenous ghrelin as a promoter of GH release, our data do not support this hypothesis. We suggest that the interactions of ghrelin and GH are the result of SMS action. SMS inhibits GH secretion not only by a direct effect on the pituitary and by inhibiting hypothalamic GHRH, but also through the suppression of ghrelin release.