Unraveling Growth Hormone Releasing Substances and Their Receptor Mechanisms

For decades, Growth Hormone (GH) deficiency has been a significant clinical challenge, leading to conditions like growth retardation in children and metabolic dysfunction in adults. While Growth Hormone Releasing Hormone (GHRH) was known to stimulate GH, its therapeutic utility was limited by its short half-life and peptide nature. There was a critical need for orally active, non-peptidic compounds that could effectively stimulate GH release. This review addresses the comprehensive understanding of diverse growth hormone secretagogue (GHS) types and their distinct receptor mechanisms.

The review confirmed that synthetic Growth Hormone Secretagogues (GHS) exert their effects primarily through a specific receptor, later cloned as the Growth Hormone Secretagogue Receptor type 1a (GHSR-1a). This receptor was shown to bind synthetic GHS with high affinity, often exhibiting IC50 values in the low nanomolar range (e.g., 0.5-5 nM for potent compounds like MK-0677). Critically, this receptor was distinct from the GHRH receptor, indicating a novel and independent pathway for GH regulation. The most pivotal finding highlighted was the identification of ghrelin in 1999 as the endogenous ligand for the GHSR-1a receptor, demonstrating a 10-fold greater potency than some early synthetic GHS in stimulating GH release in certain in vitro models and confirming the physiological relevance of this pathway. The review detailed how GHS compounds, unlike GHRH, stimulate GH release by increasing intracellular calcium levels and activating specific signaling cascades, leading to a robust, pulsatile GH secretion that could be sustained over time.