Behaviour of a somatotroph population under a growth hormone releasing peptide treatment

In this investigation, we studied the effects of Momany peptide (GHRP-5), on somatotroph secretory activity. Acute and chronic administration of GHRP-5 provokes a significant release of growth hormone that can be closely correlated with ultrastructural changes in somatotroph populations. After 3,5 and 7 days of GHRP-5 treatment, two somatotroph cell subpopulations coexist. One of them has an enhanced secretory activity and the other presents a quiescent appearance. Therefore, pituitary growth hormone content was not affected in the first seven days of GHRP-5 treatment. After 14 days, there was a significant depletion of growth hormone pituitary content coincident with the highest levels of serum growth hormone. These results concur with the surge of a new hyperactive somatotroph subtype characterised by numerous immature secretory granules that are discharged bypassing the maturation step. Acute and chronic treatments caused no changes in somatotroph cell density, the area immunostained for growth hormone and the levels of total mRNA for transcription factor pit-1. The results of pituitary cell cultures incubated with specific blockers for different signalling pathways demonstrated an involvement of the phospholipase C-inositol phosphate system in GHRP-5 stimulated somatotroph secretion. GHRP-5 treatment enhanced significantly the release of growth hormone, thereby eliciting ultrastructural modifications in somatotrophs that can be correlated with an increased secretory activity devoid of cell density changes.