Recruiting of somatotroph cells after combined somatostatin, GHRH and growth hormone (GH) secretagogue stimulation in a study of pituitary GH reserve in prepuberal female rats

Diagnostic confirmation of growth hormone (GH) deficiency in children and adults is based on stimulation tests designed to assess the pituitary reserve by measuring the amount of GH released into the bloodstream; however, the results obtained by this means cannot provide any direct indication of the amount of GH actually produced by pituitary somatotroph cells. The present paper sought to test the hypothesis that release of GH following administration of specific stimuli does not accurately reflect the somatotroph cell response, and that the amount of GH released into the bloodstream may often be greater or smaller than the amount synthesized. GH release and changes in the proportion of somatotroph cells were charted in prepuberal female Wistar rats, following administration of several different GH stimuli: GHRH (1 microg/kg), GHRP-6 (1 microg/kg), GHRELIN (1 microg/kg) and combined GHRH-based treatments, with or without SRIH pretreatment (1 microg/kg) 90 minutes earlier. Peak serum GH values were recorded 15 minutes after administration of GHRH+GHRELIN and GHRH+GHRP-6; maximum stimulation in terms of an increased proportion of somatotroph cells occurred 15 minutes after combined adminstration of GHRH + GHRELIN. SRIH pretreatment (- 90 min) inhibited GH release, with a subsequent "escape" and lack of response to stimulation which lasted at least 30 minutes except following administration of GHRH. However, combined administration of GHRH+GHRELIN maintained stimulation of the somatotroph cell population. In conclusion, the results suggest that the enhanced GH release prompted by stimulation tests used to diagnose GH deficiency in prepuberal female rats does not fully reflect somatroph cell dynamics, and that not all the GH produced and stored by somatotroph cells is released into the bloodstream.