GH responsiveness to repeated GHRH or hexarelin administration in normal adults.
Sartorio. A A; Conti. A A; Ferrero. S S; Spada. A A; Faglia. G G
Key Findings
- Hexarelin produces a significantly larger GH release than GHRH after a single dose.
- A second hexarelin dose still raises GH, though the response is reduced (tachyphylaxis), and the reduction is less severe than with repeated GHRH.
- When hexarelin follows an initial GHRH dose, it sustains GH release, unlike a second GHRH dose which fails to do so.
Practical Outcomes
- For those experimenting with GH‑boosting protocols, hexarelin appears to be a more potent and longer‑lasting option than GHRH. To avoid rapid loss of effect, space repeat doses of hexarelin at least several hours apart (e.g., >2 h) and consider limiting the number of consecutive doses. Combining an initial GHRH with a later hexarelin dose can maintain GH elevation better than using GHRH alone.
Summary
In healthy adults, a single IV dose of the peptide hexarelin triggers a much larger growth‑hormone (GH) spike than the same dose of the hormone‑releasing factor GHRH. When you give hexarelin a second time after two hours, the GH response drops, but it still stays higher than a second GHRH dose. So hexarelin works better and loses its effect more slowly than GHRH.
Abstract
GH responses, calculated as the net incremental area under the curve (GH nAUC/h), to two consecutive 1 microgram/kg/bw iv GHRH boluses (administered at 0 and 120 min, test a), to one 1 microgram/kg/bw iv GHRH bolus followed by a 1 microgram/kg/bw iv hexarelin bolus (administered at 0 and 120 min respectively, test b) and to two consecutive 1 microgram/kg/bw iv hexarelin boluses (administered at 0 and 120 min, test c) were evaluated in 6 normal adults. The first GHRH injection caused a clear rise in serum GH levels in all subjects (mean GH nAUC/h, test a: 832.1 +/- 59.4 ng/ml/h, range: 723.7-1074.0 ng/ml/h; test b: 859.2 +/- 122.9 ng/ml/h, range 618.0-1422.7 ng/ml/h). Hexarelin administration elicited a marked GH release (test c: 1424 +/- 208.2 ng/ml/h, range: 810.0-2154.0 ng/ml/h), which was significantly higher than those observed after GHRH (vs test a: p < 0.02, vs test b: p < 0.05). After the first GHRH bolus, the second GHRH injection (test a) was unable to sustain GH elevated levels (mean GH nAUC: 74.5 +/- 26.5 ng/ml/h, range: 9.7-182.2 ng/ml/h), while hexarelin administration (test b) caused a clear GH rise in all subjects (GH nAUC: 1049.7 +/- 105.2 ng/ml/h, range: 786.0-1356.0 ng/ml/h). Repeated hexarelin administration (test c) was associated with a significant (p < 0.02) reduction of GH responses to the second bolus (286.6 +/- 43.1 ng/ml/h), which were however significantly higher than those observed after the second GHRH bolus (p < 0.05). In conclusion, these results demonstrate that repeated hexarelin administration causes a reduction of GH responsiveness, which is less marked than that induced by repeated GHRH administration, probably due to the more potent GH-releasing effect of hexarelin. Moreover, when administered after GHRH, hexarelin has a sustained GH-releasing effect in contrast to the poor efficacy of GHRH, thus suggesting that the acute effect of hexarelin is probably not mediated through hypothalamic GHRH pathways.
Study Information
pubmed
1995
10.1007/bf03349794