Figure 4.

Ghrelin and pituitary GH secretion. (A) Ghrelin directly stimulates pituitary GH secretion (blue arrow), but indirectly it also contributes to this secretion, since ghrelin inhibits IGF-I (red arrow) and, therefore, the inhibitory effect of this peptide on GH; furthermore, the inhibition of IGF-I impedes the stimulatory effect of this hormone on the hypothalamic release of SS (blue arrow), which allows the release of GHRH to the portal circulation (blue arrow) with its consequent positive effect on the synthesis and secretion of GH. Furthermore, it appears that ghrelin co-localizes with GHRH in the hypothalamic arcuate nucleus, inducing its release, directly or inhibiting the release of SS from the periventricular nucleus (red arrow), thus inhibiting the inhibitory effect that SS exerts on the release of GHRH and on the synthesis and secretion of pituitary GH. (B) ghrelin and its receptor are expressed in the pituitary, where they could play an auto/paracrine role in the regulation of GH release. In fact, GHRH infusion (1) increases pituitary ghrelin mRNA levels that could induce GH stimulation (2) and release (3).