Therapeutic potential of ghrelin in cardiac diseases

Hexarelin targets neuroinflammatory pathways to preserve cardiac morphology and function in a mouse model of myocardial ischemia-reperfusion

Acute myocardial ischemia and reperfusion injury (IRI) underly the detrimental effects of coronary heart disease on the myocardium. Despite the ongoing advances in reperfusion therapies, there remains a lack of effective therapeutic strategies for preventing IRI. Growth hormone secretagogues (GHS) have been demonstrated to improve cardiac function, attenuate inflammation and modulate the autonomic nervous system (ANS) in models of cardiovascular disease. Recently, we demonstrated a reduction in infarct size after administration of hexarelin (HEX), in a murine model of myocardial infarction. In the present study we employed a reperfused ischemic (IR) model, to determine whether HEX would continue to have a cardioprotective influence in a model of higher clinical relevance. Myocardial ischemia was induced by transient ligation of the left descending coronary artery (tLAD) in C57BL/6 J mice followed by HEX (0.3 mg/kg/day; n = 20) or vehicle (VEH) (n = 18) administration for 21 days, first administered immediately prior-to reperfusion. IR-injured and sham mice were subjected to high-field magnetic resonance imaging to assess left ventricular (LV) function, with HEX-treated mice demonstrating a significant improvement in LV function compared with VEH-treated mice. A significant decrease in interstitial collagen, TGF-β1 expression and myofibroblast differentiation was also seen in the HEX-treated mice after 21 days. HEX treatment shifted the ANS balance towards a parasympathetic predominance; combined with a significant decrease in cardiac troponin-I and TNF-α levels, these findings were suggestive of an anti-inflammatory action on the myocardium mediated via HEX. In this model of IR, HEX appeared to rebalance the deregulated ANS and activate vagal anti-inflammatory pathways to prevent adverse remodelling and LV dysfunction. There are limited interventions focusing on IRI that have been successful in improving clinical outcome in acute myocardial infarction (AMI) patients, this study provides compelling evidence towards the translational potential of HEX where all others have largely failed.

Hexarelin treatment preserves myocardial function and reduces cardiac fibrosis in a mouse model of acute myocardial infarction

Ischemic heart disease (IHD) is a leading cause of morbidity and mortality worldwide. Growth hormone secretagogues (GHS) have been shown to improve cardiac function in models of IHD. This study determined whether hexarelin (HEX), a synthetic GHS, preserves cardiac function and morphology in a mouse model of myocardial infarction (MI). MI was induced by ligation of the left descending coronary artery in C57BL/6J mice followed by vehicle (VEH; n = 10) or HEX (0.3 mg/kg/day; n = 11) administration for 21 days. MI-injured and sham mice (treated with VEH; n = 6 or HEX; n = 5) underwent magnetic resonance imaging for measurement of left ventricular (LV) function, mass and infarct size at 24 h and 14 days post-MI. MI-HEX mice displayed a significant improvement (P < 0.05) in LV function compared with MI-VEH mice after 14 days treatment. A significant decrease in LV mass, interstitial collagen and collagen concentration was demonstrated with chronic HEX treatment (for 21 days), accompanied by a decrease in TGF-β1 expression, myofibroblast differentiation and an increase in collagen-degrading MMP-13 expression levels. Furthermore, heart rate variability analysis demonstrated that HEX treatment shifted the balance of autonomic nervous activity toward a parasympathetic predominance and sympathetic downregulation. This was combined with a HEX-dependent decrease in troponin-I, IL-1β and TNF-α levels suggestive of amelioration of cardiomyocyte injury. These results demonstrate that GHS may preserve ventricular function, reduce inflammation and favorably remodel the process of fibrotic healing in a mouse model of MI and hold the potential for translational application to patients suffering from MI.

Effect of ghrelin on autonomic activity in healthy volunteers

Ghrelin is a novel growth hormone (GH)-releasing peptide originally isolated from the stomach. Recently, we have shown that ghrelin suppresses cardiac sympathetic activity and prevents early left ventricular remodeling in rats with myocardial infarction. In the present study, we evaluated the effect of ghrelin on autonomic nerve activity in healthy human subjects. An intravenous bolus of human synthetic ghrelin (10μg/kg) was administered to 10 healthy men (mean age, 33 years). Holter monitoring assessment was performed before and during 2h after the ghrelin therapy. The standard deviation of normal RR intervals (SDNN), square root of the mean of the sum of the squares of differences between adjacent RR intervals (rMSSD), high-frequency power (HF), and low-frequency power (LF) were analyzed. Blood samples were also obtained before and after the therapy. A single administration of ghrelin decreased both heart rate and blood pressure. Interestingly, ghrelin significantly decreased the LF and LF/HF ratio of heart rate variability and increased the SDNN, rMSSD, and HF. Ghrelin also elicited a marked increase in circulating GH, but not insulin-like growth factor-1. These data suggest that ghrelin might suppress cardiac sympathetic nerve activity and stimulate cardiac parasympathetic nerve activity.

Hexarelin treatment in male ghrelin knockout mice after myocardial infarction

Both ghrelin and the synthetic analog hexarelin are reported to possess cardioprotective actions that are mainly exerted through different receptors. However, their effects on acute myocardial infarction have not been compared in vivo. This study aimed to clarify whether hexarelin treatment can compensate for ghrelin deficiency in ghrelin-knockout mice and to compare the effects of hexarelin (400 nmol/kg/d, sc) and equimolar ghrelin treatment after myocardial infarction. Myocardial infarction was produced by left coronary artery ligation in male ghrelin-knockout mice, which then received ghrelin, hexarelin, or vehicle treatment for 2 weeks. The mortality within 2 weeks was significantly lower in the hexarelin group (6.7%) and ghrelin group (14.3%) than in the vehicle group (50%) (P < .05). A comparison of cardiac function 2 weeks after infarction showed that in the ghrelin and hexarelin treatment groups, cardiac output was greater, whereas systolic function, represented by ejection fraction, and diastolic function, represented by dP/dt min (peak rate of pressure decline), were significantly superior compared with the vehicle group (P < .05). Hexarelin treatment was more effective than ghrelin treatment, as indicated by the ejection fraction, dP/dt max (peak rate of pressure rise), and dP/dt min. Telemetry recording and heart rate variability analysis demonstrated that sympathetic nervous activity was clearly suppressed in the hexarelin and ghrelin groups relative to the vehicle group. Our data demonstrated that hexarelin treatment can result in better heart function than ghrelin treatment 2 weeks after myocardial infarction in ghrelin-knockout mice, although both hormones have similar effects on heart rate variability and mortality.

Ghrelin prevents incidence of malignant arrhythmia after acute myocardial infarction through vagal afferent nerves

Ghrelin is a GH-releasing peptide mainly excreted from the stomach. Ghrelin administration has been shown to inhibit cardiac sympathetic nerve activity (CSNA), reduce malignant arrhythmia, and improve prognosis after acute myocardial infarction (MI). We therefore investigated the effects and potential mechanisms of the action of endogenous ghrelin on survival rate and CSNA after MI by using ghrelin-knockout (KO) mice. MI was induced by left coronary artery ligation in 46 KO mice and 41 wild-type mice. On the first day, malignant arrhythmia-induced mortality was observed within 30 min of the ligation and had an incidence of 2.4% in wild-type and 17.4% in KO mice (P < 0.05). We next evaluated CSNA by spectral analysis of heart rate variability. CSNA, represented by the low frequency/high frequency ratio, was higher in KO mice at baseline (2.18 ± 0.43 vs. 0.98 ± 0.09; P < 0.05), and especially after MI (25.5 ± 11.8 vs. 1.4 ± 0.3; P < 0.05), than in wild-type mice. Ghrelin (150 μg/kg, s.c.) 15 min before ligation suppressed the activation of CSNA and reduced mortality in KO mice. Further, this effect of ghrelin was inhibited by methylatropine bromide (1 mg/kg, i.p.) or by perineural treatment of both cervical vagal trunks with capsaicin (a specific afferent neurotoxin). Our data demonstrated that both exogenous and endogenous ghrelin suppressed CSNA, prevented the incidence of malignant arrhythmia, and improved the prognosis after acute MI. These effects are likely to be via the vagal afferent nerves.

One dose of ghrelin prevents the acute and sustained increase in cardiac sympathetic tone after myocardial infarction

Acute myocardial infarction (MI) increases sympathetic nerve activity (SNA) to the heart, which exacerbates chronic cardiac deterioration. The hormone ghrelin, if administered soon after an MI, prevents the increase in cardiac SNA and improves early survival prognosis. Whether these early beneficial effects of ghrelin also impact on cardiac function in chronic heart failure has not yet been addressed and thus was the aim of this study. MI was induced in Sprague Dawley rats by ligating the left coronary artery. One bolus of saline (n = 7) or ghrelin (150 μg/kg, sc, n = 9) was administered within 30 min of MI. Two weeks after the infarct (or sham; n = 7), rats were anesthetized and cardiac function was evaluated using a Millar pressure-volume conductance catheter. Cardiac SNA was measured using whole-nerve electrophysiological techniques. Untreated-MI rats had a high mortality rate (50%), evidence of severe cardiac dysfunction (ejection fraction 28%; P < 0.001), and SNA was significantly elevated (102% increase; P = 0.03). In comparison, rats that received a single dose of ghrelin after the MI tended to have a lower mortality rate (25%; P = NS) and no increase in SNA, and cardiac dysfunction was attenuated (ejection fraction of 43%; P = 0.014). This study implicates ghrelin as a potential clinical treatment for acute MI but also highlights the importance of therapeutic intervention in the early stages after acute MI. Moreover, these results uncover an intricate causal relationship between early and chronic changes in the neural control of cardiac function in heart failure.

Ghrelin and cardiovascular diseases

In 1999, a peptide from the stomach called ghrelin was discovered, which exerts potent growth hormone releasing powers. Subsequent studies revealed that it exerts a potent orexigenic action. In addition, the beneficial effects of ghrelin in cardiovascular diseases have been recently suggested. In humans as well as in animals, administration of ghrelin improves cardiac function and remodeling in chronic heart failure. In an animal model for myocardial infarction, ghrelin treatment early after coronary ligation effectively reduces fatal arrhythmia and, consequently, mortality, suggesting the potential therapeutic role of the peptide in acute myocardial infarction. Although how ghrelin may influence the cardiovascular system is not fully understood, the cardiovascular beneficial effects are mediated possibly through a combination of various actions, such as an increase in growth hormone level, an improvement in energy balance, direct actions to the cardiovascular cells, and regulation of the autonomic nervous activity. Of note, current experimental evidence suggests that ghrelin may act centrally to decrease sympathetic nervous system activity through peripheral afferent nerve. Thus, administration of ghrelin might become a unique new therapy for cardiovascular diseases.