Interaction of L-dopa and GHRH on GH secretion in normal men

Endogenous growth hormone (GH)-releasing hormone is required for GH responses to pharmacological stimuli

The roles of hypothalamic growth hormone-releasing hormone (GHRH) and of somatostatin (SRIF) in pharmacologically stimulated growth hormone (GH) secretion in humans are unclear. GH responses could result either from GHRH release or from acute decline in SRIF secretion. To assess directly the role of endogenous GHRH in human GH secretion, we have used a competitive GHRH antagonist, (N-Ac-Tyr1,D-Arg2)GHRH(1-29)NH2 (GHRH-Ant), which we have previously shown is able to block the GH response to GHRH. We first tested whether an acute decline in SRIF, independent of GHRH action, would release GH. Pretreatment with GHRH-Ant abolished the GH response to exogenous GHRH (0.33 microgram/kg i.v.) but did not modify the GH rise after termination of an SRIF infusion. We then investigated the role of endogenous GHRH in the GH responses to pharmacologic stimuli of GH release. The GH responses to arginine (30 g i.v. over 30 min), L-dopa (0.5 g orally), insulin hypoglycemia (0.1 U/Kg i.v.), clonidine (0.25 mg orally), or pyridostigmine (60 mg orally) were measured in healthy young men after pretreatment with either saline of GHRH-Ant 400 microgram/kg i.v. In every case, GH release was significantly suppressed by GHRH-Ant. We conclude that endogenous GHRH is required for the GH response to each of these pharmacologic stimuli. Acute release of hypothalamic GHRH may be a common mechanism by which these compounds mediate GH secretion.

GH secretion status in myotonic dystrophy

Frequent endocrine alterations and abnormal growth hormone (GH) secretion have been reported in myotonic dystrophy (MD). To evaluate GH secretion status in MD, GH response to 100 micrograms of growth hormone releasing hormone (GHRH) with or without pyridostigmine pretreatment and its relation with insulin-induced hypoglycemia was investigated in MD patients and compared with normal controls. The mean peak plasma GH response to GHRH was 27.8 +/- 19.2 micrograms/l normal subjects and 11.4 +/- 8.7 micrograms/l in MD patients. In five of seven patients GH reached a mean peak of 12.6 +/- 4.2 micrograms/l after insulin-induced hypoglycemia, compared with 5 +/- 2.8 micrograms/l after GHRH. Conversely, in two patients GH reached a peak of 16.1 and 32 micrograms/l after GHRH, and only 2.5 and 5.3, respectively, after hypoglycemia. Pretreatment with pyridostigmine in nine patients tested potentiated GHRH-induced GH release with a peak of 17.6 +/- 12.5 micrograms/l, compared with 10.05 +/- 6.7 micrograms/l after GHRH alone; IGF-I levels were normal in all patients.