The interaction between the inhibitory effect on growth hormone secretion of a 75 g oral glucose load and the stimulatory effect of human pancreatic growth hormone releasing factor 1-44 (hpGRF 1-44, 10 micrograms i.v.) has been studied in six normal subjects. hpGRF 1-44 alone induced a rise in growth hormone concentrations (maximum mean +/- SEM, 16.5 +/- 1.7 mU/l 15 min after injection) while growth hormone levels were suppressed by oral glucose alone (less than 1.5 mU/l from 45 to 135 min after glucose ingestion). When hpGRF 1-44 was injected 60 min after oral glucose, the growth hormone response was attenuated (maximum, 6.7 +/- 1.4 mU/l at 15 min, P less than 0.05). Increments of blood glucose within the physiological range diminish the growth hormone response to hpGRF 1-44 in normal man.
[Show abstract][Hide abstract] ABSTRACT: Continuous infusion of GHRH, as well as a bolus injection of GHRH, specifically stimulates the release of GH by the anterior pituitary. However, repetitive bolus injections of GHRH result in diminished responses of GH, while a constant infusion of GHRH cannot maintain high serum GH levels. To investigate whether different GH-releasing stimuli are able further to challenge the somatotroph being exposed to continuous infusion of GHRH, we determined the GH response to insulin-induced hypoglycaemia and bicycle exercise. Six normal volunteers received a bolus of 50 micrograms GHRH or vehicle followed by a continuous infusions of GHRH 1-29 amide (1 microgram/kg/h) or vehicle for 2.5 h. GHRH bolus and infusion resulted in elevated GH levels, but GH levels fell to values not significantly different from baseline levels after 150 min, GH plasma levels rose again, however, in response to insulin hypoglycaemia and bicycle exercise after both GHRH or vehicle infusion. Thus, the somatotroph's responsiveness to GHRH remains intact to respond to stress stimuli after continuous GHRH infusion.
[Show abstract][Hide abstract] ABSTRACT: The effect of pretreatment with biosynthetic methionyl human GH (hGH) on the GH response to GHRH has been studied in normal subjects. Eight volunteers were given either 4 IU hGH or placebo s.c. 12-hourly for 72 h before a GHRH test, or a single s.c. dose of 4 IU hGH 12 h before a GHRH test. Somatomedin-C (Sm-C) levels at the time of the GHRH tests were significantly elevated after treatment with hGH compared to placebo, and the GH response to GHRH was significantly attenuated. A further six subjects were given 2 IU hGH or placebo i.v., and i.v. GHRH 3 h later; there was no rise in Sm-C for the 5 h of the study after either treatment; nevertheless, the response to GHRH was completely abolished by pretreatment with hGH. These results demonstrate that GH can regulate its own secretion independently of changes in Sm-C levels, through a mechanism other than the inhibition of GHRH release. The attenuated response to GHRH in the presence of elevated Sm-C levels may be related to Sm-C, or be a more direct effect of the recently elevated GH levels.
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