GH response to hypoglycemia and clonidine in the GH-releasing hormone resistance syndrome.

GH secretion by the pituitary is the result of the balance between the stimulatory effect of GHRH and the inhibitory effect of SS. Patients with mutations in GHRH receptor (GHRH-R) gene (GHRH-R) offer a unique model to study the mechanism of action of different GH secretion stimuli. In the past, we have demonstrated a small but significant GH response to a GH secretagogue (GHRP-2) in a homogenous cohort of patients with severe GH deficiency (GHD) due to a homozygous null mutation in GHRH-R (IVS1+1G-->A). Now, we sought to determine if we could detect a GH response to hypoglycemia (ITT: insulin tolerance test) or clonidine (CL) in these patients. Nine young GHD subjects underwent both ITT and CL tests, and 2 additional subjects underwent only CL test. There was a small but significant GH increase during ITT, but not during CL test. These results indicate that a minimal albeit significant GH response to ITT can occur despite complete lack of GHRH-R function.