Differential alterations in cardiac adrenergic signaling in chronic hypoxia or norepinephrine infusion

Cardiovascular physiology and pathophysiology at high altitude

Oxygen is vital for cellular metabolism; therefore, the hypoxic conditions encountered at high altitude affect all physiological functions. Acute hypoxia activates the adrenergic system and induces tachycardia, whereas hypoxic pulmonary vasoconstriction increases pulmonary artery pressure. After a few days of exposure to low oxygen concentrations, the autonomic nervous system adapts and tachycardia decreases, thereby protecting the myocardium against high energy consumption. Permanent exposure to high altitude induces erythropoiesis, which if excessive can be deleterious and lead to chronic mountain sickness, often associated with pulmonary hypertension and heart failure. Genetic factors might account for the variable prevalence of chronic mountain sickness, depending on the population and geographical region. Cardiovascular adaptations to hypoxia provide a remarkable model of the regulation of oxygen availability at the cellular and systemic levels. Rapid exposure to high altitude can have adverse effects in patients with cardiovascular diseases. However, intermittent, moderate hypoxia might be useful in the management of some cardiovascular disorders, such as coronary heart disease and heart failure. The aim of this Review is to help physicians to understand the cardiovascular responses to hypoxia and to outline some recommendations that they can give to patients with cardiovascular disease who wish to travel to high-altitude destinations.

Effects of Normobaric Hypoxia and Adrenergic Blockade over 72 h on Cardiac Function in Rats

In rats, acute normobaric hypoxia depressed left ventricular (LV) inotropic function. After 24 h of hypoxic exposure, a slight recovery of LV function occurred. We speculated that prolonged hypoxia (72 h) would induce acclimatization and, hence, recovery of LV function. Moreover, we investigated biomarkers of nitrosative stress and apoptosis as possible causes of hypoxic LV depression. To elucidate the role of hypoxic sympathetic activation, we studied whether adrenergic blockade would further deteriorate the general state of the animals and their cardiac function. Ninety-four rats were exposed over 72 h either to normal room air (N) or to normobaric hypoxia (H). The rodents received infusion (0.1 mL/h) with 0.9% NaCl or with different adrenergic blockers. Despite clear signs of acclimatization to hypoxia, the LV depression continued persistently after 72 h of hypoxia. Immunohistochemical analyses revealed significant increases in markers of nitrosative stress, adenosine triphosphate deficiency and apoptosis in the myocardium, which could provide a possible explanation for the absence of LV function recovery. Adrenergic blockade had a slightly deteriorative effect on the hypoxic LV function compared to the hypoxic group with maintained sympathetic efficacy. These findings show that hypoxic sympathetic activation compensates, at least partially, for the compromised function in hypoxic conditions, therefore emphasizing its importance for hypoxia adaptation.

Modeling the oxygen transport to the myocardium at maximal exercise at high altitude

Exposure to high altitude induces a decrease in oxygen pressure and saturation in the arterial blood, which is aggravated by exercise. Heart rate (HR) at maximal exercise decreases when altitude increases in prolonged exposure to hypoxia. We developed a simple model of myocardial oxygenation in order to demonstrate that the observed blunting of maximal HR at high altitude is necessary for the maintenance of a normal myocardial oxygenation. Using data from the available scientific literature, we estimated the myocardial venous oxygen pressure and saturation at maximal exercise in two conditions: (1) with actual values of maximal HR (decreasing with altitude); (2) with sea-level values of maximal heart rate, whatever the altitude (no change in HR). We demonstrated that, in the absence of autoregulation of maximal HR, myocardial tissue oxygenation would be incompatible with life above 6200 m-7600 m, depending on the hypothesis concerning a possible increase in coronary reserve (increase in coronary blood flow at exercise). The decrease in maximal HR at high altitude could be explained by several biological mechanisms involving the autonomic nervous system and its receptors on myocytes. These experimental and clinical observations support the hypothesis that there exists an integrated system at the cellular level, which protects the myocardium from a hazardous disequilibrium between O₂  supply and O₂ consumption at high altitude.

The effect of adaptation to hypoxia on cardiac tolerance to ischemia/reperfusion

The acute myocardial infarction (AMI) and sudden cardiac death (SCD), both associated with acute cardiac ischemia, are one of the leading causes of adult death in economically developed countries. The development of new approaches for the treatment and prevention of AMI and SCD remains the highest priority for medicine. A study on the cardiovascular effects of chronic hypoxia (CH) may contribute to the development of these methods. Chronic hypoxia exerts both positive and adverse effects. The positive effects are the infarct-reducing, vasoprotective, and antiarrhythmic effects, which can lead to the improvement of cardiac contractility in reperfusion. The adverse effects are pulmonary hypertension and right ventricular hypertrophy. This review presents a comprehensive overview of how CH enhances cardiac tolerance to ischemia/reperfusion. It is an in-depth analysis of the published data on the underlying mechanisms, which can lead to future development of the cardioprotective effect of CH. A better understanding of the CH-activated protective signaling pathways may contribute to new therapeutic approaches in an increase of cardiac tolerance to ischemia/reperfusion.

Plasticity of non-shivering thermogenesis and brown adipose tissue in high-altitude deer mice

High altitude environments challenge small mammals with persistent low ambient temperatures that require high rates of aerobic heat production in face of low O2 availability. An important component of thermogenic capacity in rodents is non-shivering thermogenesis (NST) mediated by uncoupled mitochondrial respiration in brown adipose tissue (BAT). NST is plastic, and capacity for heat production increases with cold acclimation. However, in lowland native rodents, hypoxia inhibits NST in BAT. We hypothesize that highland deer mice (Peromyscus maniculatus) overcome the hypoxic inhibition of NST through changes in BAT mitochondrial function. We tested this hypothesis using lab born and raised highland and lowland deer mice, and a lowland congeneric (Peromyscus leucopus), acclimated to either warm normoxia (25°C, 760 mmHg) or cold hypoxia (5°C, 430 mmHg). We determined the effects of acclimation and ancestry on whole-animal rates of NST, the mass of interscapular BAT (iBAT), and uncoupling protein (UCP)-1 protein expression. To identify changes in mitochondrial function, we conducted high-resolution respirometry on isolated iBAT mitochondria using substrates and inhibitors targeted to UCP-1. We found that rates of NST increased with cold hypoxia acclimation but only in highland deer mice. There was no effect of cold hypoxia acclimation on iBAT mass in any group, but highland deer mice showed increases in UCP-1 expression and UCP-1-stimulated mitochondrial respiration in response to these stressors. Our results suggest that highland deer mice have evolved to increase the capacity for NST in response to chronic cold hypoxia, driven in part by changes in iBAT mitochondrial function.

β-Adrenergic signaling, monoamine oxidase A and antioxidant defence in the myocardium of SHR and SHR-mtBN conplastic rat strains: the effect of chronic hypoxia

The β-adrenergic signaling pathways and antioxidant defence mechanisms play important roles in maintaining proper heart function. Here, we examined the effect of chronic normobaric hypoxia (CNH, 10% O2, 3 weeks) on myocardial β-adrenergic signaling and selected components of the antioxidant system in spontaneously hypertensive rats (SHR) and in a conplastic SHR-mtBN strain characterized by the selective replacement of the mitochondrial genome of SHR with that of the more ischemia-resistant Brown Norway strain. Our investigations revealed some intriguing differences between the two strains at the level of β-adrenergic receptors (β-ARs), activity of adenylyl cyclase (AC) and monoamine oxidase A (MAO-A), as well as distinct changes after CNH exposure. The β2-AR/β1-AR ratio was significantly higher in SHR-mtBN than in SHR, apparently due to increased expression of β2-ARs. Adaptation to hypoxia elevated β2-ARs in SHR and decreased the total number of β-ARs in SHR-mtBN. In parallel, the ability of isoprenaline to stimulate AC activity was found to be higher in SHR-mtBN than that in SHR. Interestingly, the activity of MAO-A was notably lower in SHR-mtBN than in SHR, and it was markedly elevated in both strains after exposure to hypoxia. In addition to that, CNH markedly enhanced the expression of catalase and aldehyde dehydrogenase-2 in both strains, and decreased the expression of Cu/Zn superoxide dismutase in SHR. Adaptation to CNH intensified oxidative stress to a similar extent in both strains and elevated the IL-10/TNF-α ratio in SHR-mtBN only. These data indicate that alterations in the mitochondrial genome can result in peculiar changes in myocardial β-adrenergic signaling, MAO-A activity and antioxidant defence and may, thus, affect the adaptive responses to hypoxia.

Role of the β3-adrenergic receptor subtype in catecholamine-induced myocardial remodeling

β₃-Adrenoceptors (AR) stimulate cardiac Na⁺/K⁺ pump in healthy hearts. β₃-ARs are upregulated by persistent sympathetic hyperactivity; however, their effect on Na⁺/K⁺ ATPase activity and ventricular function in this condition is still unknown. Here, we investigate preventive effects of additional β₃-AR activation (BRL) on Na⁺/K⁺ ATPase activity and in vivo hemodynamics in a model of noradrenaline-induced hypertrophy. Rats received NA or NA plus simultaneously administered BRL in vivo infusion for 14 days; their cardiac function was investigated by left ventricular pressure-volume analysis. Moreover, fibrosis and apoptosis were also assessed histologically. NA induced an hypertrophic pattern, as detected by morphological, histological, and biochemical markers. Additional BRL exposure reversed the hypertrophic pattern and restored Na⁺/K⁺ ATPase activity. NA treatment increased systolic function and depressed diastolic function (slowed relaxation). Additional BRL treatment reversed most NA-induced hemodynamic changes. NA decreased Na⁺/K⁺ pump α2 subunit expression selectively, a change also reversed by additional BRL treatment. Increasing β₃-AR stimulation may prevent the consequences of chronic NA exposure on Na⁺/K⁺ pump and in vivo hemodynamics. β₃-AR agonism may thus represent a new therapeutic strategy for pharmacological modulation of hypertrophy under conditions of chronically enhanced sympathetic activity.

Effect of disease states on α1 -adrenoceptor binding and signal transduction parameters in isolated perfused heart: quantification by pharmacokinetic-pharmacodynamic modelling

OBJECTIVES

To employ a pharmacokinetic-pharmacodynamic modelling approach for analysing the effect of experimental endotoxemia and mild hypoxia on α1 -adrenoceptor (α1 AR) binding and signal transduction.

METHODS

In Langendorff-perfused rat hearts, phenylephrine was continuously infused, and [(3) H]-prazosin was injected as single dose (infused over 1 min). Simultaneous analysis of the time courses of prazosin outflow concentration and inotropic response (left ventricular developed pressure) using an agonist-antagonist interaction model and nonlinear regression allowed to estimate receptor affinity, as well as the parameters of the operational model of agonism.

KEY FINDINGS

Both endotoxemia and hypoxia, significantly reduced the maximum response achievable in the system to 67% and 49% of the control group mean, respectively. In addition, endotoxemia decreased the efficiency of stimulus-response coupling and increased the steepness of the stimulus-response curve. In both disease models, no change in receptor affinity and density were found.

CONCLUSIONS

The results revealed the causes of reduced α1 AR-mediated inotropic responsiveness in endotoxemia and hypoxia. In contrast with traditional dose-response studies, it was possible to quantify separately the underlying changes in α1 AR binding and signal transduction.

Novel proteins associated with human dilated cardiomyopathy: selective reduction in α(1A)-adrenergic receptors and increased desensitization proteins

Abstract Therapeutics to treat human heart failure (HF) and the identification of proteins associated with HF are still limited. We analyzed α(1)-adrenergic receptor (AR) subtypes in human HF and performed proteomic analysis on more uniform samples to identify novel proteins associated with human HF. Six failing hearts with end-stage dilated cardiomyopathy (DCM) and four non-failing heart controls were subjected to proteomic analysis. Out of 48 identified proteins, 26 proteins were redundant between samples. Ten of these 26 proteins were previously reported to be associated with HF. Of the newly identified proteins, we found several muscle proteins and mitochondrial/electron transport proteins, while novel were functionally similar to previous reports. However, we also found novel proteins involved in functional classes such as β-oxidation and G-protein coupled receptor signaling and desensitization not previously associated with HF. We also performed radioligand-binding studies on the heart samples and not only confirmed a large loss of β(1)-ARs in end-stage DCM, but also found a selective decrease in the α(1A)-AR subtype not previously reported. We have identified new proteins and functional categories associated with end-stage DCM. We also report that similar to the previously characterized loss of β(1)-AR in HF, there is also a concomitant loss of α(1A)-ARs, which are considered cardioprotective proteins.

Prolyl hydroxylase 2: a novel regulator of β2 -adrenoceptor internalization

Adrenergic receptor (AR)-mediated signalling is modulated by oxygen levels. Prolyl hydroxylases (PHDs) are crucial for intracellular oxygen sensing and organism survival. However, it remains to be clarified whether or how PHDs are involved in the regulation of β₂-adrenoceptor (β₂-AR) signalling. Here we show that PHD2 can modulate the rate of β₂-AR internalization through interactions with β-arrestin 2. PHD2 hydroxylates β-arrestin 2 at the proline (Pro)¹⁷⁶, Pro¹⁷⁹ and Pro¹⁸¹ sites, which retards the recruitment of β-arrestin 2 to the plasma membrane and inhibits subsequent co-internalization with β₂-AR into the cytosol. β₂-AR internalization is critical to control the temporal and spatial aspects of β₂-AR signalling. Identifying novel regulators of β₂-AR internalization will enable us to develop new strategies to manipulate receptor signalling and provide potential targets for drug development in the prevention and treatment of diseases associated with β₂-AR signalling dysregulation.

Subsequent stress increases gene expression of catecholamine synthetic enzymes in cardiac ventricles of chronic-stressed rats

Since previous experience of stressful situation profoundly affects response to a subsequent novel stressor, we examined changes in gene expression and protein levels of catecholamine biosynthetic enzymes in cardiac ventricles after exposure of chronic psychosocially isolated adult Wistar male rats to short-term immobilization stress. Chronic social isolation did not affect gene expression of tyrosine hydroxylase (TH) in either right or left ventricle. Subsequent immobilization of these animals produced an elevation of TH mRNA level in right and left ventricles. The levels of dopamine-β-hydroxylase (DBH) mRNA were detectable only after immobilization both in right and left ventricles of control and chronically isolated rats. Chronic isolation stress increased phenylethanolamine N-methyltransferase (PNMT) mRNA levels in the right ventricle. Immobilization led to an elevated PNMT mRNA level in right and left ventricles of both control and chronically stressed animals. Protein levels of TH, DBH, and PNMT in right and left ventricles of socially isolated rats were increased after subsequent immobilization. Taking into consideration the role of cardiac catecholamines in physiological and pathophysiological processes, it could be hypothesized that increased catecholamine synthesis in the ventricles after novel immobilization stress could point to the susceptibility of the heart to subsequent stress.

Chronic intermittent hypobaric hypoxia decreases β-adrenoceptor activity in right ventricular papillary muscle

Chronic intermittent hypobaric hypoxia (CIHH) has an effective cardiac protection against ischemia-reperfusion injury. However, the underlying mechanisms are not fully known. It has been shown that blockade of β-adrenergic receptor exerts anti-arrhythmic action and improves cardiac remodeling in ischemic myocardium. Thus we determined the influence of CIHH on β-adrenergic receptor activity in right ventricular papillary muscle of rats. We found that the action potential duration in right ventricular papillary muscle was significantly longer in CIHH rats than in control rats. Activation of β-adrenergic receptor with dl-isoproterenol dose-dependently increased action potential duration and the contractility in right ventricular papillary muscle. In CIHH rats, the prolonged effect of dl-isoproterenol on action potential duration and the positive inotropic effect were significantly decreased compared with that in control rats. Furthermore, radioligand-binding experiments revealed that the density and affinity of β-adrenergic receptor in right ventricular myocardium was significantly lower in CIHH rats. In addition, Western blot analysis revealed that the membrane-bound G protein Gsα expression level in cardiac myocardium was significantly lower in CIHH rats than that in control rats. Collectively, these data suggest that CIHH suppresses β-adrenergic receptor action in right ventricular papillary muscle through decreasing receptor density and affinity, as well as membrane-bound Gsα. This mechanism may be involved in the cardiac protective effect of CIHH.

The autonomic nervous system at high altitude

The effects of hypobaric hypoxia in visitors depend not only on the actual elevation but also on the rate of ascent. Sympathetic activity increases and there are increases in blood pressure and heart rate. Pulmonary vasoconstriction leads to pulmonary hypertension, particularly during exercise. The sympathetic excitation results from hypoxia, partly through chemoreceptor reflexes and partly through altered baroreceptor function. High pulmonary arterial pressures may also cause reflex systemic vasoconstriction. Most permanent high altitude dwellers show excellent adaptation although there are differences between populations in the extent of the ventilatory drive and the erythropoiesis. Some altitude dwellers, particularly Andeans, may develop chronic mountain sickness, the most prominent characteristic of which being excessive polycythaemia. Excessive hypoxia due to peripheral chemoreceptor dysfunction has been suggested as a cause. The hyperviscous blood leads to pulmonary hypertension, symptoms of cerebral hypoperfusion, and eventually right heart failure and death.

Oxidative stress and oxidant signaling in obstructive sleep apnea and associated cardiovascular diseases

Obstructive sleep apnea (OSA) has emerged as a major public health problem and increasing evidence indicates that untreated OSA can lead to the development of various cardiovascular disorders. One important mechanism by which OSA may promote cardiovascular diseases is intermittent hypoxia, in which patients are subjected to repeated episodes of brief oxygen desaturation in the blood, followed by reoxygenation. Such cycles of hypoxia/reoxygenation may result in the generation of reactive oxygen species. Some studies have demonstrated the presence of oxidative stress in OSA patients as well as in animals subjected to intermittent hypoxia. Further, modulations of nitric oxide and biothiol status might also play important roles in the pathogenesis of OSA-associated diseases. Reactive oxygen species and redox events are also involved in the regulation of signal transduction for oxygen-sensing mechanisms. This review summarizes currently available information on the evidence for and against the occurrence of oxidative stress in OSA and the role of reactive oxygen species in cardiovascular changes associated with OSA.

Hypoxia-induced adaptational shift in MHC-beta isoform expression in rat ventricles

We investigated whether the shift of cardiac myosin heavy chain (MHC) isoform observed during exposure to hypoxia is secondary to hypertrophy, or whether it is directly related to the hypoxic stress. Twelve male Wistar-Kyoto rats, 14 weeks old, were randomly assigned to two groups: sea-level control group (CO) and hypoxia group (HX). The CO group was housed 4 weeks at 1,011 hPa, and the HX group was housed for 4 weeks at 701 hPa. The expression of MHC-beta was significantly increased (600%) in the HX group as compared to the CO group in the right ventricle (p < 0.01). An increased ventricular mass induced by hypoxic exposure was associated with an increased expression of MHC-beta in the right ventricle (p < 0.05). In the left ventricle, the MHC-b expression was significantly increased (295%) in the HX group as compared to the CO group without ventricular hypertrophy (p < 0.01). No differences were observed in the adenylyl cyclase activity or in the phosphodiesterase activities in both ventricles between the CO and HX groups (p > 0.05). Oxidative enzymatic activities (citrate synthase and three-hydroxyacyl-CoA dehydrogenase) were unchanged in both ventricles following 4 weeks of hypoxia (p > 0.05). These findings suggest that, besides cardiac hypertrophy, the hypoxia-induced adaptational change to the MHC-b isoform may be mediated through a specific mechanism related to the stress of hypoxia.

Cardiovascular and autonomic nervous functions during acclimatization to hypoxia in conscious rats

The time courses of changes in cardiovascular and autonomic nervous functions during acclimatization to hypoxia were studied in conscious Sprague-Dawley rats. The animals were kept under a 12:12-h light-dark cycle and exposed to hypoxia (1 atm, 10% O2). Implanted telemetry transmitters were used to record blood pressure (BP). Changes in heart rate (HR) and BP were monitored over a 21-day period, and variations before and during hypoxia were analyzed using the wavelet transform method. The HR, high-frequency power of HR variability (HR-HF) and low-frequency power of BP variability (BP-LF) were all significantly increased after 1 h of hypoxia, whereas the LF/HF ratio of HR variability did not change. After this initial increase, both HR and the BP-LF were found to decrease. On the first day of hypoxia, HR and BP-LF values were significantly lower than those of the control rats, whereas the HR-HF was higher. Subsequently, these values altered so that they were similar to the control after 14 days of hypoxia. In addition, the amplitude of diurnal variation in HR was reduced during hypoxia. These results suggest that a sequence of dynamic interactions between sympathetic and parasympathetic nervous activities might have important roles in the regulation of cardiovascular function during acclimatization to hypoxia.

Impaired Gene Expression of .BETA.1-Adrenergic Receptor, but Not Stimulatory G-Protein Gs.ALPHA., in Rat Ventricular Myocardium Treated with Isoproterenol

We investigated the gene expression of beta(1)-adrenergic receptor (beta(1)AR) and stimulatory G-protein Gsalpha, important signal transduction elements for regulating heart rate and contractility, in ventricle after chronic treatment with isoproterenol (ISO) in rat. Rats were treated with ISO (4 mg/kg, intraperitoneal) twice a day for 4 d. Ventricle weight of the heart and ventricle weight/body weight ratio were increased by 23% and 25% compared with control, respectively. Positive inotropic responses to ISO in left atrial muscle preparations isolated from ISO-treated rats were markedly decreased. Northern blot hybridization showed that the mRNA transcript of beta(1)AR was significantly decreased in ventricle of ISO-treated rats, whereas Gsalpha mRNA level was unchanged. Present results demonstrate that the gene expression of myocardial beta(1)AR, but not Gsalpha, was decreased in rat myocardium of ISO-induced cardiac hypertrophy, and suggesting that decrease in the gene expression of beta(1)AR may be one of the mechanisms responsible for the diminished cardiac function.

Altered myocardial Gs protein and adenylyl cyclase signaling in rats exposed to chronic hypoxia and normoxic recovery

The present work has analyzed the consequences of chronic intermittent high-altitude hypoxia for functioning of the G protein-mediated adenylyl cyclase (AC) signaling system in the right (RV) and left ventricular (LV) myocardium in rats. Adaptation to hypoxia did not appreciably affect the number of beta-adrenoceptors and the content of predominantly membrane-bound alpha-subunit (G(s)alpha) of the stimulatory G protein, but it raised the amount of cytosolic G(s)alpha in RV. The levels of myocardial inhibitory Galpha protein were not altered. Activity of AC stimulated by GTP, fluoride, forskolin, or isoprotertenol was reduced by approximately 50% in RV from chronically hypoxic rats, and a weaker depression was also found in LV. In addition, hypoxia significantly diminished a functional activity of membrane-bound G(s)alpha in both RV and LV. The RV baseline contractile function was markedly increased in chronically hypoxic animals, and its sensitivity to beta-adrenergic stimulation was decreased. Animals recovering from hypoxia for 5 wk still exhibited markedly elevated levels of cytosolic G(s)alpha and significantly lower activity of AC in RV than did age-matched controls, but contractile responsiveness to beta-agonists was normal.

Adaptations to iron deficiency: cardiac functional responsiveness to norepinephrine, arterial remodeling, and the effect of beta-blockade on cardiac hypertrophy

BACKGROUND

Iron deficiency (ID) results in ventricular hypertrophy, believed to involve sympathetic stimulation. We hypothesized that with ID 1) intravenous norepinephrine would alter heart rate (HR) and contractility, 2) abdominal aorta would be larger and more distensible, and 3) the beta-blocker propanolol would reduce hypertrophy.

METHODS

1) 30 CD rats were fed an ID or replete diet for 1 week or 1 month. Norepinephrine was infused via jugular vein; pressure was monitored at carotid artery. Saline infusions were used as a control. The pressure trace was analyzed for HR, contractility, systolic and diastolic pressures. 2) Abdominal aorta catheters inflated the aorta, while digital microscopic images were recorded at stepwise pressures to measure arterial diameter and distensibility. 3) An additional 10 rats (5 ID, 5 control) were given a daily injection of propanolol or saline. After 1 month, the hearts were excised and weighed.

RESULTS

Enhanced contractility, but not HR, was associated with ID hypertrophic hearts. Systolic and diastolic blood pressures were consistent with an increase in arterial diameter associated with ID. Aortic diameter at 100 mmHg and distensibility were increased with ID. Propanolol was associated with an increase in heart to body mass ratio.

CONCLUSIONS

ID cardiac hypertrophy results in an increased inotropic, but not chronotropic response to the sympathetic neurotransmitter, norepinephrine. Increased aortic diameter is consistent with a flow-dependent vascular remodeling; increased distensibility may reflect decreased vascular collagen content. The failure of propanolol to prevent hypertrophy suggests that ID hypertrophy is not mediated via beta-adrenergic neurotransmission.