Hexarelin Modulates Rat Ventricular Contractility via Endothelium-Released Prostacyclin PGI2

Cardiac dysfunction following ischemia-reperfusion injury remains a major clinical challenge, and growth hormone secretagogues have emerged as candidates for cardioprotection beyond their pituitary effects. Hexarelin, a synthetic peptide GH secretagogue, has shown reduction of dysfunction in ischemic-reperfused hearts from GH-deficient rats after chronic treatment and increased ejection fraction in acutely treated men. The mechanisms underlying these cardiac effects are poorly understood, particularly whether they involve direct myocardial action or are mediated through vascular and endothelial intermediaries. Clarifying the role of endocardial endothelium and prostanoid signaling in hexarelin's inotropic profile is essential to understanding its therapeutic potential.

Hexarelin (1–10 µM) was applied to isolated rat papillary muscle preparations to assess inotropic and lusitropic effects across a range of stimulation frequencies (60–600 beats/min). Contractility recovery after anoxia was measured as a primary endpoint. Mechanistic dissection used propranolol (0.2 µM) to block adrenergic signaling, L-NAME (1 mM) to inhibit NO synthase, indomethacin (1 µM) to block prostanoid synthesis, and 0.5% Triton X-100 to ablate endocardial endothelium. MK-677 (1 µM), a non-peptidyl GH secretagogue, served as a comparator. 6-keto-PGF1alpha (prostacyclin metabolite) release was quantified during reoxygenation, and calcium transients plus I(Ca) were measured in isolated ventricular cells.

Hexarelin produced time- and frequency-dependent inotropic effects on rat papillary muscles: an initial transient increase in contractile force followed by a reduction at low beating frequencies (60–240/min), with no reduction at high frequencies (400–600/min). The initial transient force increase was fully abolished by propranolol (0.2 µM), implicating adrenergic involvement in that component. Blockade of NO synthesis with L-NAME (1 mM) only partially altered the hexarelin response, suggesting NO is a contributing but non-dominant mediator. The typical negative force-frequency relationship in rat papillary muscle was modified by hexarelin, and a minor increase in diastolic tension occurred after sudden frequency increases.

Release of the prostacyclin metabolite 6-keto-PGF1alpha was increased during reoxygenation after anoxia in hexarelin-treated papillary muscles, and all remaining hexarelin effects were abolished by indomethacin (1 µM) or endocardial endothelium removal with Triton X-100.

MK-677 (1 µM) reduced contractility but did not alter the force-frequency relationship, highlighting mechanistic divergence from hexarelin despite shared GHSR agonism. Hexarelin had no significant effect on calcium transients or I(Ca) in isolated ventricular cells, arguing against direct cardiomyocyte calcium channel modulation as the primary mechanism.