Ipamorelin and GHRP-6 increase bone mineral content via expanded bone dimensions in adult female rats
Background
Growth hormone (GH) plays a central role in bone remodelling, regulating both formation and resorption throughout adult life. Deficiencies or age-related declines in GH can impair bone mineral content (BMC) and skeletal integrity, raising fracture risk. GH secretagogues (GHSs) represent a pharmacological strategy to amplify endogenous GH release via the GHS-receptor (GHSR) pathway, potentially bypassing the limitations of direct GH replacement. A prior clinical study with MK-677, a small-molecule GHS, showed elevated biochemical markers of both bone formation and bone resorption, suggesting GHSs can engage skeletal remodelling. Whether peptide-class GHSs such as ipamorelin and GHRP-6 replicate this skeletal effect in a controlled preclinical model remained untested at the time of this study.
Study Design
Thirteen-week-old female Sprague-Dawley rats received ipamorelin (IPA) at 0.5 mg/kg/day (n=7), GHRP-6 at 0.5 mg/kg/day (n=8), GH at 3.5 mg/kg/day (n=7), or vehicle (control), all delivered continuously via subcutaneous osmotic minipumps for 12 weeks. Skeletal outcomes were tracked in vivo every 4 weeks using dual X-ray absorptiometry (DXA). After sacrifice, femurs were analysed by peripheral quantitative computed tomography (pQCT) at the mid-diaphysis. Excised femurs and vertebra L6 were further characterised by Archimedes' principle for volumetric bone density and by ash weight determinations to assess mineral concentration.
Results
All active treatments — ipamorelin, GHRP-6, and GH — increased body weight, total tibial BMC, and vertebral BMC measured by DXA in vivo compared with vehicle-treated controls. However, when total BMC was corrected for the gain in body weight, the total BMC:body weight ratio was unaffected across all treatment groups. Tibial area bone mineral density (BMD) was increased, while total and vertebral area BMDs remained unchanged. pQCT analysis of excised femurs showed the cortical BMC rise was attributable to an increased cross-sectional bone area, not to a change in cortical volumetric BMD, which was unchanged.
The in vitro measurements using
pQCT, Archimedes' principle, and ash weight determinations collectively confirm that increases in cortical and total BMC were driven entirely by expanded bone dimensions, with volumetric BMD remaining constant across all treatment arms.
Femur and vertebra L6 volumes were increased by all treatments, but volumetric BMDs assessed by Archimedes' principle were unaffected. Ash weight was elevated by all treatments; however, mineral concentration was unchanged, reinforcing that bone grew larger but not denser.
Key Findings
- IPA and GHRP-6 at 0.5 mg/kg/day both increased total tibial and vertebral BMC by DXA over 12 weeks
- Total BMC corrected for body weight (BMC:body weight ratio) was unaffected by any treatment
- pQCT confirmed cortical BMC rise was due to increased cross-sectional bone area, not volumetric BMD
- Femur and vertebra L6 volumes were increased, but volumetric BMDs by Archimedes' principle were unchanged
- Ash weight was elevated by all treatments; mineral concentration per unit bone remained unchanged
Why It Matters
The practical takeaway for clinicians and researchers is that GHS peptides ipamorelin and GHRP-6 stimulate bone growth in adult female rats, but the mechanism is periosteal/dimensional expansion rather than increased mineralisation density. This distinction matters clinically: larger bone cross-sections improve biomechanical resistance to fracture even at equal volumetric density, so the skeletal benefit is real but operates differently than direct calcium-loading interventions. Translation to humans remains distant — this is a 12-week rat study, and the relationship between GHS-driven bone size changes and fracture risk reduction in osteoporotic or GH-deficient patients requires clinical validation. For protocol designers, the results suggest ipamorelin and GHRP-6 share skeletal activity with exogenous GH at the doses tested, supporting their mechanistic classification as GH-axis activators in bone tissue rather than agents with unique bone-specific pathways.
