ACUTE ADMINISTRATION OF HEXARELIN STIMULATES GH SECRETION DURING DAY AND NIGHT IN NORMAL MEN

OBJECTIVE Hexarelin is a synthetic hexapeptide with potent GH-releasin g activity in both animals and men. The aim of this study was to inves tigate the effect of a bolus injection of hexarelin given in the morni ng during wakefulness and during nocturnal sleep in a group of normal adult men. DESIGN AND SUBJECTS Eight normal men, aged 21-33 years, of normal height and within 10% of ideal body weight were studied. All su bjects received in random order saline or hexarelin (2 mu g/kg) in the morning between 0800 and 0900h after they had fasted overnight. The s ame experiments were performed during nocturnal sleep in the same subj ects. Saline or hexarelin were injected within 30 minutes after the on set of sleep between 2300 and 2400h. Sleep was recorded by visual insp ection. MEASUREMENTS In all four test sessions blood samples were take n 30, 15 minutes and immediately before the injection of saline or hex arelin and then every 15 minutes for 2 hours. GH was measured by an im munoradiometric assay. All values are expressed as peak GH levels or a s area under the curve (AUG) calculated by trapezoidal integration. RE SULTS Mean peak GH concentrations after hexarelin during the morning ( 58.2+/-4.7 mu g/l) (GH mu g/1x2=mU/l) were not different from those ob served during sleep (61.2+/-43 mu g/l). The rate of disappearance of G H from plasma was slower during sleep (t(1/2) = 64.9+/-14.8 min) than during morning hours (t(1/2),, = 24.9+/-1.4 min, P < 0.01). Mean AUC r esponses to hexarelin during sleep (1466+/-145 mu g.min/l) were signif icantly higher than during morning hours (903+/-94 mu g.min/l, P < 0.0 01). CONCLUSIONS These results show that GH responsiveness to a growth hormone releasing peptide is preserved during the night. This could b e exploited for diagnostic and/or therapeutic purposes.