Short-term administration of growth hormone (GH) lowers blood pressure by activating eNOS/nitric oxide (NO)-pathway in male hypophysectomized (Hx) rats

Effect of Ghrelin on the Cardiovascular System

Simple Summary

Ghrelin is an octanoylated peptide that was initially isolated from rat and human stomachs in the process of searching for an endogenous ligand to the orphan growth hormone secretagogue receptor (GHS-R), a G-protein-coupled receptor. Exogenous or endogenous ghrelin secreted from the stomach binds to GHS-R on gastric vagal nerve terminals, and the signals are transmitted to the central nervous system via the vagal afferent nerve to facilitate growth hormone (GH) secretion, feeding, sympathetic inhibition, parasympathetic activation, and anabolic effects. Ghrelin also binds directly to the pituitary GHS-R and stimulates GH secretion. Ghrelin has beneficial effects on the cardiovascular system, including cardioprotective effects such as anti-heart failure, anti-arrhythmic, and anti-inflammatory actions, and it enhances vascular activity via GHS-R-dependent stimulation of GH/IGF-1 (insulin-like growth factor-1) and modulation of the autonomic nervous system. The anti-heart failure effects of ghrelin could be useful as a new therapeutic strategy for chronic heart failure.

Abstract

Ghrelin, an n-octanoyl-modified 28-amino-acid-peptide, was first discovered in the human and rat stomach as an endogenous ligand for the growth hormone secretagogue receptor (GHS-R). Ghrelin-GHS-R1a signaling regulates feeding behavior and energy balance, promotes vascular activity and angiogenesis, improves arrhythmia and heart failure, and also protects against cardiovascular disease by suppressing cardiac remodeling after myocardial infarction. Ghrelin’s cardiovascular protective effects are mediated by the suppression of sympathetic activity; activation of parasympathetic activity; alleviation of vascular endothelial dysfunction; and regulation of inflammation, apoptosis, and autophagy. The physiological functions of ghrelin should be clarified to determine its pharmacological potential as a cardiovascular medication.

Functional Changes after Recombinant Human Growth Hormone Replacement in Patients with Chronic Traumatic Brain Injury and Abnormal Growth Hormone Secretion

We explored the effects of recombinant human growth hormone (rhGH) replacement on physical and cognitive functioning in subjects with a moderate-to-severe traumatic brain injury (TBI) with abnormal growth hormone (GH) secretion. Fifteen individuals who sustained a TBI at least 12 months prior to study enrollment were identified as having abnormal GH secretion by glucagon stimulation testing (maximum GH response less than 8 ng/mL). Peak cardiorespiratory capacity, body composition, and muscle force testing were assessed at baseline and one year after rhGH replacement. Additionally, standardized neuropsychological tests that assess memory, processing speed, and cognitive flexibility, as well as self-report inventories related to depression and fatigue, were administered at baseline and 1 year after rhGH replacement. Comparison tests were performed with proper post hoc analyses. All analyses were carried out at α < 0.05. Peak O₂ consumption, peak oxygen pulse (estimate of cardiac stroke volume), and peak ventilation all significantly increased (p < 0.05). Maximal isometric and isokinetic force production were not altered. Skeletal muscle fatigue did not change but the perceptual rating of fatigue was reduced by ∼25% (p = 0.06). Cognitive performance did not change significantly over time, whereas self-reported symptoms related to depression and fatigue significantly improved. The observed changes suggest that rhGH replacement has a positive impact on cardiorespiratory fitness and a positive impact on perceptual fatigue in survivors of TBI with altered GH secretion.

Chronic central ghrelin infusion reduces blood pressure and heart rate despite increasing appetite and promoting weight gain in normotensive and hypertensive rats

Acute studies showed that ghrelin acts on the central nervous system (CNS) to reduce blood pressure (BP), heart rate (HR) and sympathetic activity. However, the long-term CNS cardiovascular actions of ghrelin are still unclear. We tested whether chronic intracerebroventricular (ICV) infusion of ghrelin causes sustained reductions in BP, HR and whether it alters baroreceptor sensitivity (BRS) and autonomic input to the heart. A cannula was placed in the lateral ventricle of male Sprague-Dawley (SD) rats for ICV infusions via osmotic minipump (0.5 μl/h). BP and HR were measured 24-h/day by telemetry. After 5 days of control measurements, ghrelin (0.21 nmol/h) or saline vehicle were infused ICV for 10 days followed by a 5-day post-treatment period. Chronic ICV ghrelin infusion increased food intake (22±3 to 26±1 g/day) leading to ~50 g body weight gain. BP fell slightly during ghrelin infusion while HR decreased by ~26 bpm. In control animals BP and HR increased modestly. ICV Ghrelin infusion caused a 50% reduction in sympathetic tone to the heart but did not alter BRS. We also tested if the depressor responses to ICV ghrelin infusion were enhanced in spontaneously hypertensive rats (SHR) due to their high basal sympathetic tone. However, we observed similar BP and HR responses compared to normotensive rats. These results indicate that ghrelin, acting via direct actions on the CNS, has a sustained effect to lower HR and a modest impact to reduce BP in normotensive and hypertensive animals despite increasing appetite and body weight.

Pre-weaning growth hormone treatment reverses hypertension and endothelial dysfunction in adult male offspring of mothers undernourished during pregnancy

Maternal undernutrition results in elevated blood pressure (BP) and endothelial dysfunction in adult offspring. However, few studies have investigated interventions during early life to ameliorate the programming of hypertension and vascular disorders. We have utilised a model of maternal undernutrition to examine the effects of pre-weaning growth hormone (GH) treatment on BP and vascular function in adulthood. Female Sprague-Dawley rats were fed either a standard control diet (CON) or 50% of CON intake throughout pregnancy (UN). From neonatal day 3 until weaning (day 21), CON and UN pups received either saline (CON-S, UN-S) or GH (2.5 ug/g/day)(CON-GH, UN-GH). All dams were fed ad libitum throughout lactation. Male offspring were fed a standard diet until the end of the study. Systolic blood pressure (SBP) was measured at day 150 by tail cuff plethysmography. At day 160, intact mesenteric vessels mounted on a pressure myograph. Responses to pressure, agonist-induced constriction and endothelium-dependent vasodilators were investigated to determine vascular function. SBP was increased in UN-S groups and normalised in UN-GH groups (CON-S 121±2 mmHg, CON-GH 115±3, UN-S 146±3, UN-GH 127±2). Pressure mediated dilation was reduced in UN-S offspring and normalised in UN-GH groups. Vessels from UN-S offspring demonstrated a reduced constrictor response to phenylephrine and reduced vasodilator response to acetylcholine (ACh). Furthermore, UN-S offspring vessels displayed a reduced vasodilator response in the presence of L-NG-Nitroarginine Methyl Ester (L-NAME), carbenoxolone (CBX), L-NAME and CBX, Tram-34 and Apamin. UN-GH vessels showed little difference in responses when compared to CON and significantly increased vasodilator responses when compared to UN-S offspring. Pre-weaning GH treatment reverses the negative effects of maternal UN on SBP and vasomotor function in adult offspring. These data suggest that developmental cardiovascular programming is potentially reversible by early life GH treatment and that GH can reverse the vascular adaptations resulting from maternal undernutrition.

Hormonal modulation of endothelial NO production

Since the discovery of endothelium-derived relaxing factor and the subsequent identification of nitric oxide (NO) as the primary mediator of endothelium-dependent relaxations, research has focused on chemical and physical stimuli that modulate NO levels. Hormones represent a class of soluble, widely circulating chemical factors that impact production of NO both by rapid effects on the activity of endothelial nitric oxide synthase (eNOS) through phosphorylation of the enzyme and longer term modulation through changes in amount of eNOS protein. Hormones that increase NO production including estrogen, progesterone, insulin, and growth hormone do so through both of these common mechanisms. In contrast, some hormones, including glucocorticoids, progesterone, and prolactin, decrease NO bioavailability. Mechanisms involved include binding to repressor response elements on the eNOS gene, competing for co-regulators common to hormones with positive genomic actions, regulating eNOS co-factors, decreasing substrate for eNOS, and increasing production of oxygen-derived free radicals. Feedback regulation by the hormones themselves as well as the ability of NO to regulate hormonal release provides a second level of complexity that can also contribute to changes in NO levels. These effects on eNOS and changes in NO production may contribute to variability in risk factors, presentation of and treatment for cardiovascular disease associated with aging, pregnancy, stress, and metabolic disorders in men and women.

Growth hormone (GH) treatment does not restore endothelial function in children with GH deficiency

BACKGROUND

In adults with growth hormone (GH) deficiency (GHD), GH treatment restores impaired endothelial function, a precursor of cardiovascular disease. Its effect in children with GHD is unknown.

METHODS

Three months of GH (0.3 mg/kg/wk) were given to nine children with GHD. Endothelial function was measured via reactive hyperemic response. Forearm blood flow (FBF, strain gauge plethysmography) was measured before and after 5 min of upper arm arterial occlusion. Blood pressure and lipid, insulin and glucose levels were measured. Pretreatment endothelial function was compared to 18 previously studied control children.

RESULTS

Percent fall in forearm vascular resistance was greater in controls (81.2 +/- 1.9%) than in children with GHD (69.6 +/- 5.3%, p = 0.021) but was not affected by GH (60.6 +/- 7.5%). GH markedly increased HOMAIR (21 +/- 7 versus 43 +/- 8, p = 0.004). Plasma lipid levels did not significantly differ.

CONCLUSION

These results indicate that endothelial function is impaired in children with GHD. GH therapy does not restore the impairment.

Evidence for a decrease in cardiovascular risk factors following recombinant growth hormone administration in abstinent anabolic-androgenic steroid users

OBJECTIVES

To determine whether six days recombinant human growth hormone (rhGH) in an abstinent anabolic-androgenic steroid (AAS) group had any cardiovascular and biochemical effects compared with a control group.

METHODS

Male subjects (n=48) were randomly divided, using a single blind procedure into two groups: (1) control group (C) n=24, mean+/-SD, age 32+/-11 years; height 1.8+/-0.06m; (2) rhGH using group (0.058IUkg(-1)day(-1)) (GH) n=24, mean+/-SD, age 32+/-9 years; height 1.8+/-0.07m. Physiological responses, anthropometry, arterial pulse wave velocity (APWV), blood pressure (BP), heart rate (HR), peak oxygen uptake (VO(2) peak) and biochemical indices were investigated.

RESULTS

Body mass index, fat-free mass index and VO(2) peak significantly increased while body fat significantly decreased within GH (all P<0.017). Insulin like growth factor-I significantly increased within GH (P<0.017) and compared with C (P<0.05). Serum sodium significantly increased (P<0.017) and serum homocysteine, high sensitivity C-reactive protein, thyroid stimulating hormone and tetra-iodothyronine (T(4)), significantly decreased within GH (all P<0.017). T(4) significantly decreased compared with C (P<0.05). Arterial pulse wave velocity, peak and recovery systolic and diastolic BP, significantly decreased compared with C (P<0.05). Resting HR and rate pressure product (RPP) significantly increased compared with C (P<0.05).

CONCLUSION

The findings of this study suggest that short term use of rhGH may have beneficial effects on endothelial function and specific inflammatory markers of cardiovascular disease in abstinent AAS users, but may have an adverse effect on the cardiovascular system, as evidenced by the increase in resting RPP.

Growth hormone receptor deficiency in mice results in reduced systolic blood pressure and plasma renin, increased aortic eNOS expression, and altered cardiovascular structure and function

To study the role of the growth hormone receptor (GHR) in the development of cardiovascular structure and function, female GHR gene-disrupted or knockout (KO) and wild-type (WT) mice at age 18 wk were used. GHR KO mice had lower plasma renin levels (12 +/- 2 vs. 20 +/- 4 mGU/ml, P < 0.05) and increased aortic endothelial NO synthase (eNOS) expression (146%, P < 0.05) accompanied by a 25% reduction in systolic blood pressure (BP, 110 +/- 4 vs. 147 +/- 3 mmHg, P < 0.001) compared with WT mice. Aldosterone levels were unchanged, whereas the plasma potassium concentration was elevated by 14% (P < 0.05) in GHR KO. Relative left ventricular weight was 14% lower in GHR KO mice (P < 0.05), and cardiac dimensions as analyzed by echocardiography were similarly reduced. Myograph studies revealed a reduced maximum contractile response in the aorta to norepinephrine (NE) and K(+) (P < 0.05), and aorta media thickness was decreased in GHR KO (P < 0.05). However, contractile force was normal in mesenteric arteries, whereas sensitivity to NE was increased (P < 0.05). Maximal acetylcholine-mediated dilatation was similar in WT and GHR KO mice, whereas the aorta of GHR KO mice showed an increased sensitivity to acetylcholine (P < 0.05). In conclusion, loss of GHR leads to low BP and decreased levels of renin in plasma as well as increase in aortic eNOS expression. Furthermore, GHR deficiency causes functional and morphological changes in both heart and vasculature that are beyond the observed alterations in body size. These data suggest an important role for an intact GH/IGF-I axis in the maintenance of a normal cardiovascular system.