Effects of growth hormone secretagogues on the release of adenohypophyseal hormones in young and old healthy dogs

Abstract

The effects of three growth hormone secretagogues (GHSs), ghrelin, growth hormone-releasing peptide-6 (GHRP-6), and growth hormone-releasing hormone (GHRH), on the release of adenohypophyseal hormones, growth hormone (GH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinising hormone (LH), prolactin (PRL) and on cortisol were investigated in young and old healthy Beagle dogs.Ghrelin proved to be the most potent GHS in young dogs, whereas in old dogs GHRH administration was associated with the highest plasma GH concentrations. The mean plasma GH response after administration of ghrelin was significantly lower in the old dogs compared with the young dogs. The mean plasma GH concentration after GHRH and GHRP-6 administration was lower in the old dogs compared with the young dogs, but this difference did not reach statistical significance. In both age groups, the GHSs were specific for GH release as they did not cause significant elevations in the plasma concentrations of ACTH, cortisol, TSH, LH, and PRL. It is concluded that in young dogs, ghrelin is a more powerful stimulator of GH release than either GHRH or GHRP-6. Ageing is associated with a decrease in GH-releasing capacity of ghrelin, whereas this decline is considerably lower for GHRH or GHRP-6.

Ghrelin, an endogenous growth hormone secretagogue with diverse endocrine and nonendocrine effects

“…31,72,78,79 The effect of ghrelin on PRL secretion is independent of gender and age and likely results from direct stimulation of somatomammotrophs. 72,78,80,81 In dogs 28,29 and rats, 8 synthetic GHSs do not stimulate PRL release. This species-related difference may be explained by differences in the number of somatomammotrophs in various species, with humans having a high proportion of those cells.…”

“…8,21,[27][28][29]31,51,[70][71][72][73] The GH-releasing activity of ghrelin is greater in vivo than in vitro because ghrelin and GHRH act synergistically, consistent with the fact that their actions are at least partially mediated via different mechanisms.…”

“…21,23,26 The synthetic GHSs have potent GH-releasing activity in several species, including humans, mice, rats, swine, goats, cows, and dogs. [27][28][29] In humans, nearly all synthetic GHSs induce the release of more GH than does GHRH. 24 However, the hormone-releasing action of synthetic GHSs is not always specific.…”

Ghrelin-stimulation test in the diagnosis of canine pituitary dwarfism

“…Ghrelin is a potent stimulator of GH release in several species such as humans Peino et al, 2000;Takaya et al, 2000), rats (Kojima et al, 1999) and dogs (Bhatti et al, 2002(Bhatti et al, , 2005. In rats, the GH-releasing potency of ghrelin is similar to that of GHRH (Kojima et al, 1999), whereas in humans, ghrelin releases more GH than GHRH and the synthetic GHS hexarelin .…”

“…In rats, the GH-releasing potency of ghrelin is similar to that of GHRH (Kojima et al, 1999), whereas in humans, ghrelin releases more GH than GHRH and the synthetic GHS hexarelin . In young dogs, ghrelin is a more potent stimulator of GH secretion than GHRH or GHRP-6 (Bhatti et al, 2005). However, the action of ghrelin and the synthetic GHSs is not exclusively confined to GH release.…”