Hexarelin and GHRPs exert direct GH-independent cardiotropic effects via cardiovascular GHRP receptors
Background
Growth hormone deficiency and ischemic heart disease represent overlapping clinical burdens where standard GH-replacement therapy addresses systemic deficiency but not direct myocardial protection. The discovery of specific GHSR-type receptors throughout peripheral tissues — particularly the heart — opened a question: do GH-releasing peptides (GHRPs) act on the cardiovascular system through pathways entirely separate from the hypothalamo-pituitary axis? Prior research established GHRPs as potent GH secretagogues acting via GHSR in the pituitary and hypothalamus, but the presence of high-density GHRP binding sites in ventricular and atrial tissue suggested a direct cardiac mechanism worth investigating. Understanding these extra-neuroendocrine effects is critical for evaluating GHRPs as potential cardioprotective agents beyond their endocrine role.
Study Design
Receptor distribution was mapped using (125)I-Tyr-Ala-hexarelin radioligand binding across human cardiovascular tissues (ventricles, atria, aorta, coronaries, carotid, endocardium, vena cava) and peripheral organs (heart, adrenal, ovary, testis, lung, skeletal muscle). Binding inhibition assays tested unlabeled Tyr-Ala-hexarelin, hexarelin, GHRP-6, GHRP-1, GHRP-2, and MK-677 against the radioligand on cardiac membranes. Anti-apoptotic activity was assessed in H9c2 myocytes (fetal cardiomyocyte-derived cell line) treated with hexarelin. In vivo cardioprotection was evaluated in aged and GH-deficient rats subjected to ischemia-induced myocardial damage. Positive inotropic effects of hexarelin were assessed in normal human subjects and GH-deficient patients.
Results
GHRP receptor binding density in peripheral cardiovascular tissues was significantly higher than that found in the hypothalamo-pituitary system. Within the human cardiovascular system, binding followed a rank order: ventricles > atria > aorta > coronaries > carotid > endocardium > vena cava. Cardiac membrane binding was inhibited by unlabeled Tyr-Ala-hexarelin, hexarelin, GHRP-6, GHRP-1, and GHRP-2, but critically, not by MK-677, the non-peptidyl GHRP analog, indicating a receptor subtype distinct from or differently accessible to small-molecule secretagogues.
Highest GHRP radioligand binding in the entire cardiovascular system was detected in the ventricles, exceeding atria, aorta, and all other vascular structures — and surpassing the density found at the hypothalamo-pituitary axis.
In H9c2 myocytes, specific GHRP binding was confirmed and hexarelin demonstrated anti-apoptotic activity in this cardiomyocyte-derived line. In vivo, hexarelin protected against ischemia-induced myocardial damage in both aged and GH-deficient rats. In human studies, hexarelin produced a positive inotropic effect in both normal subjects and GH-deficient patients, confirming the cardiac actions are GH-independent.
Key Findings
- GHRP receptor density in cardiovascular tissues exceeded that in the hypothalamo-pituitary system
- Highest binding in human heart: ventricles > atria > aorta > coronaries > carotid > endocardium > vena cava
- MK-677 (non-peptidyl analog) did NOT inhibit cardiac membrane GHRP binding, unlike hexarelin and peptidyl GHRPs
- Hexarelin showed anti-apoptotic activity in H9c2 fetal cardiomyocyte-derived cells
- Hexarelin produced positive inotropic effects in both normal subjects and GH-deficient patients
Why It Matters
GHRPs — particularly hexarelin — should be understood as direct cardiac agents, not merely GH secretagogues; clinicians and researchers evaluating these compounds for body composition or GH-axis support must account for concurrent cardiovascular effects. The demonstration of cardioprotection in GH-deficient patients is especially relevant because it separates cardiac benefit from GH replacement entirely, pointing toward standalone cardioprotective indications. For protocol designers, the finding that MK-677 does not compete at cardiac GHRP receptors suggests the oral secretagogue and peptidyl GHRPs occupy pharmacologically distinct niches — stacking them may not replicate cardiac receptor engagement. Translation to clinical cardiology remains early-stage, but the human inotropic data and ischemia protection in animals together provide a meaningful translational signal worth watching in prospective cardiac trials.
