Acute administration of hexarelin stimulates GH secretion during day and night in normal men

Hexarelin is a synthetic hexapeptide with potent GH-releasing activity in both animals and men. The aim of this study was to investigate the effect of a bolus injection of hexarelin given in the morning during wakefulness and during nocturnal sleep in a group of normal adult men.

Eight normal men, aged 21-33 years, of normal height and within 10% of ideal body weight were studied. All subjects received in random order saline or hexarelin (2 micrograms/kg) in the morning between 0800 and 0900 h after they had fasted overnight. The same experiments were performed during nocturnal sleep in the same subjects. Saline or hexarelin were injected within 30 minutes after the onset of sleep between 2300 and 2400 h. Sleep was recorded by visual inspection.

In all four test sessions blood samples were taken 30, 15 minutes and immediately before the injection of saline or hexarelin and then every 15 minutes for 2 hours. GH was measured by an immunoradiometric assay. All values are expressed as peak GH levels or as area under the curve (AUC) calculated by trapezoidal integration.

Mean peak GH concentrations after hexarelin during the morning (58.2 +/- 4.7 micrograms/l) (GH micrograms/l l x 2 = mU/l) were not different from those observed during sleep (61.2 +/- 4.3 micrograms/l). The rate of disappearance of GH from plasma was slower during sleep (t1/2 = 64.9 +/- 14.8 min) than during morning hours (t1/2 = 24.9 +/- 1.4 min, P < 0.01). Mean AUC responses to hexarelin during sleep (1466 +/- 145 micrograms.min/l) were significantly higher than during morning hours (903 +/- 94 micrograms.min/l, P < 0.001).

These results show that GH responsiveness to a growth hormone releasing peptide is preserved during the night. This could be exploited for diagnostic and/or therapeutic purposes.