Tissue-Specific Peptide Effects
How peptides exert different effects in various tissues based on receptor distribution.
Tissue Distribution of Key Receptors
A peptide acts only where its receptor is. The receptor binding overview explains the binding itself.
GH/GHS Receptors
| Tissue | GHR | GHS-R1a | Primary Effect |
|---|---|---|---|
| Liver | High | Low | IGF-1 production |
| Muscle | Present | Low | Protein synthesis |
| Fat | Present | Low | Lipolysis |
| Pituitary | Present | Highest | GH release |
| Hypothalamus | Present | High (animal data) | Appetite regulation |
A 2002 human tissue study found GHS-R1a mainly in the pituitary. See the ghrelin receptor and growth hormone receptor articles.
IGF-1 Receptors
| Tissue | IGF-1R Density | Primary Effect |
|---|---|---|
| Muscle | Very high | Hypertrophy |
| Bone | High | Growth/density |
| Cartilage | High | Repair |
Melanocortin and GLP-1 Receptors
A 2025 review maps melanocortin subtypes: MC1R to skin, MC2R to the adrenal cortex, MC3R and MC4R to the brain, MC5R to exocrine glands. A 2014 primate and human tissue study found GLP-1 receptors mainly in pancreatic beta cells, not the liver.
Peptide-Specific Tissue Effects
GH Secretagogues
| Tissue | Effect | 2-year MK-677 trial |
|---|---|---|
| Pituitary | GH release | Shown |
| Liver | IGF-1 production | Shown |
| Muscle | Lean mass | Rose; strength didn't |
| Fat | Lipolysis | No visceral fat change |
| Hypothalamus | Appetite | Increased |
BPC-157
| Tissue | Effect | Proposed mechanism |
|---|---|---|
| GI tract | Healing | Primary action site |
| Tendons | Repair | Growth factor effects |
| Muscle | Regeneration | Angiogenesis, VEGF pathway |
These are animal findings; a 2025 systematic review found no clinical safety data.
TB-500
| Tissue | Effect | Proposed mechanism |
|---|---|---|
| All tissues | Cell migration | Actin sequestering |
| Heart | Repair | Angiogenesis |
| Tendons | Healing | Collagen |
Most data are on full-length thymosin beta-4; as of October 2026, FDA reports no human exposure data for the TB-500 fragment.
Targeted Administration
Site-Specific Injection
Injecting near an injury is a user practice with no human evidence behind it; BPC-157 animal studies also used systemic routes.
Systemic vs. Local Effects
| Peptide | Administration | Distribution |
|---|---|---|
| GH secretagogues | Subcutaneous | Systemic |
| BPC-157 | Near site or systemic | Not compared in humans |
| TB-500 | Any site | "Systemic" claim untested |
| IGF-1 LR3 | Intramuscular or SubQ | No human data |
Optimization Based on Goals
| Goal | Users choose | Evidence |
|---|---|---|
| Muscle growth | GH secretagogues, IGF-1 LR3 | No strength gain shown |
| Tendon healing | BPC-157, TB-500 | Animal data |
| Fat loss | Fragment 176-191 | Related AOD-9604 failed |
| General recovery | Stacks | One uncontrolled series |
AOD-9604, a modified hGH fragment, missed its weight-loss endpoint in an oral trial of 536 adults with obesity.
What We Still Don't Know
No human study has tested whether injection site changes tissue exposure.
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References
- Peptides and their receptors: Treasures for pharmacological interventions (Peptides, 2019)
- The tissue distribution of the mRNA of ghrelin and subtypes of its receptor, GHS-R, in humans (Journal of Clinical Endocrinology & Metabolism, 2002)
- The growth hormone receptor: mechanism of receptor activation, cell signaling, and physiological aspects (Frontiers in Endocrinology, 2018)
- Melanocortin receptors: their functions and regulation by physiological agonists and antagonists (Cellular and Molecular Life Sciences, 2001)
- Polymorphism of melanocortin receptor genes: association with inflammatory traits and diseases (Diseases, 2025)
- GLP-1 receptor localization in monkey and human tissue (Endocrinology, 2014)
- Effects of an oral ghrelin mimetic (MK-677) on body composition and clinical outcomes in healthy older adults (Annals of Internal Medicine, 2008)
- Emerging use of BPC-157 in orthopaedic sports medicine: a systematic review (HSS Journal, 2025)
- Regeneration or risk? A narrative review of BPC-157 for musculoskeletal healing (Current Reviews in Musculoskeletal Medicine, 2025)
- Certain bulk drug substances for use in compounding that may present significant safety risks (FDA, accessed October 2026)
- FDA briefing document on AOD-9604, Pharmacy Compounding Advisory Committee (FDA, December 2024)