Ipamorelin

Ipamorelin is a synthetic pentapeptide (Aib-His-D-2-Nal-D-Phe-Lys-NH₂) studied as a selective growth hormone secretagogue (GHS) and agonist of the ghrelin / GHS-R1a receptor. In its original characterization it released growth hormone from rat pituitary cells in vitro and in vivo with potency comparable to GHRP-6, yet — unlike earlier GHRPs — did not significantly elevate ACTH or cortisol even at doses far above those needed for GH release. Researchers described it as the first GHRP-receptor agonist with GH-release selectivity similar to GHRH, making it a useful reference tool for studying receptor selectivity. [1]

Research models position ipamorelin as an agonist at the GHS-R1a (ghrelin) receptor on pituitary somatotroph cells. The proposed signaling cascade involves phospholipase C and the second messengers IP₃ and DAG, leading to intracellular calcium release and exocytosis of GH-containing vesicles. In one in vitro study, chronic exposure in young female rats was associated with increased somatotroph secretory-granule density and intracellular GH content, suggesting a dynamic effect on the somatotroph population. [2]

Because ipamorelin is derived from the GHRP series and targets the ghrelin receptor, it is used to probe how growth-hormone-releasing peptides interact with endogenous ghrelin signaling. In rats, GHRP-induced GH release was attenuated by roughly 60–70% after resection of the gastrointestinal tract — where ghrelin is synthesized — supporting the hypothesis that gastric ghrelin may mediate part of the GH-releasing effect of these peptides. [3]

In an adult rat model, ipamorelin administered alongside a glucocorticoid (methylprednisolone) counteracted the glucocorticoid-induced decline in bone formation and muscle strength: the periosteal bone-formation rate increased roughly four-fold versus glucocorticoid treatment alone. The authors attributed the effect to GHS-driven modulation of the GH/IGF-1 axis. [4]

As a ghrelin mimetic, ipamorelin has been examined for effects on gut motility and feeding. In a rodent model of post-operative ileus, repeated dosing accelerated colonic transit and increased fecal output, food intake, and body-weight gain. [5] More recent work in ferrets reported that ipamorelin (and the related agonist anamorelin) reduced cisplatin-induced weight loss, consistent with appetite- and motility-related activity at the ghrelin receptor. [6]