Mitochondrial nitric oxide metabolism during rat heart adaptation to high altitude: effect of sildenafil, L-NAME, and L-arginine treatments

Temporal evolution of cardiac mitochondrial dysfunction in a type 1 diabetes model. Mitochondrial complex I impairment, and H2O2 and NO productions as early subcellular events

The aim of this work was to study the early events that occur in heart mitochondria and to analyse the temporal evolution of cardiac mitochondrial dysfunction in a type 1 diabetes model. Male Wistar rats were injected with Streptozotocin (STZ, single dose, 60 mg × kg^-1, i.p.) and hyperglycemic state was confirmed 72 h later. The animals were sacrificed 10 or 14 days after STZ-injection. Heart mitochondrial state 3 O₂ consumption sustained by malate-glutamate (21%) or by succinate (16%), and complexes I-III (27%), II-III (24%) and IV (22%) activities were lower in STZ group, when animals were sacrificed at day 14, i.e. ~11 days of hyperglycemia. In contrast, after 10 days of STZ-injection (~7 days of hyperglycemia), only the state 3 O₂ consumption sustained by malate-glutamate (23%) and its corresponding respiratory control (30%) were lower in diabetic rats, in accordance with complex I-III activity reduction (17%). Therefore, this time (~7 days of hyperglycemia) has been considered as an "early stage" of cardiac mitochondrial dysfunction. At this point, mitochondrial production rates of H₂O₂ (117%), NO (30%) and ONOO^- (~225%), and mtNOS expression (29%) were higher; and mitochondrial SOD activity (15%) and [GSH + GSSG] (28%) were lower in diabetic rats. Linear correlations between the modified mitochondrial parameters and glycemias were observed. PGC-1α expression was similar between groups, suggesting that mitochondrial biogenesis was not triggered in this initial phase of mitochondrial dysfunction. Consequently, complex I, H₂O₂ and NO could be considered early subcellular signals of cardiac mitochondrial dysfunction, with NO and H₂O₂ being located upstream de novo synthesis of mitochondria.

Functional interactions between complex I and complex II with nNOS in regulating cardiac mitochondrial activity in sham and hypertensive rat hearts

Nitric oxide (NO) affects mitochondrial activity through its interactions with complexes. Here, we investigated regulations of complex I (C-I) and complex II (C-II) by neuronal NO synthase (nNOS) in the presence of fatty acid supplementation and the impact on left ventricular (LV) mitochondrial activity from sham and angiotensin II (Ang-II)-induced hypertensive (HTN) rats. Our results showed that nNOS protein was expressed in sham and HTN LV mitochondrial enriched fraction. In sham, oxygen consumption rate (OCR) and intracellular ATP were increased by palmitic acid (PA) or palmitoyl-carnitine (PC). nNOS inhibitor, S-methyl-l-thiocitrulline (SMTC), did not affect OCR or cellular ATP increment by PA or PC. However, SMTC increased OCR with PA + malonate (a C-II inhibitor), but not with PA + rotenone (a C-I inhibitor), indicating that nNOS attenuates C-I with fatty acid supplementation. Indeed, SMTC increased C-I activity but not that of C-II. Conversely, nNOS-derived NO was increased by rotenone + PA in LV myocytes. In HTN, PC increased the activity of C-I but reduced that of C-II, consequently OCR was reduced. SMTC increased both C-I and C-II activities with PC, resulted in OCR enhancement in the mitochondria. Notably, SMTC increased OCR only with rotenone, suggesting that nNOS modulates C-II-mediated OCR in HTN. nNOS-derived NO was partially reduced by malonate + PA. Taken together, nNOS attenuates C-I-mediated mitochondrial OCR in the presence of fatty acid in sham and C-I modulates nNOS activity. In HTN, nNOS attenuates C-I and C-II activities whereas interactions between nNOS and C-II maintain mitochondrial activity.

Chronic aerobic exercise associated to low-dose L-NAME improves contractility without changing calcium handling in rat cardiomyocytes

Nitric oxide (NO) inhibition by high-dose NG-nitro-L-arginine methyl ester (L-NAME) is associated with several detrimental effects on the cardiovascular system. However, low-dose L-NAME increases NO synthesis, which in turn induces physiological cardiovascular benefits, probably by activating a protective negative feedback mechanism. Aerobic exercise, likewise, improves several cardiovascular functions in healthy hearts, but its effects are not known when chronically associated with low-dose L-NAME. Thus, we tested whether the association between low-dose L-NAME administration and chronic aerobic exercise promotes beneficial effects to the cardiovascular system, evaluating the cardiac remodeling process. Male Wistar rats were randomly assigned to control (C), L-NAME (L), chronic aerobic exercise (Ex), and chronic aerobic exercise associated to L-NAME (ExL). Aerobic training was performed with progressive intensity for 12 weeks; L-NAME (1.5 mg·kg^-1·day^-1) was administered by orogastric gavage. Low-dose L-NAME alone did not change systolic blood pressure (SBP), but ExL significantly increased SBP at week 8 with normalization after 12 weeks. Furthermore, ExL promoted the elevation of left ventricle (LV) end-diastolic pressure without the presence of cardiac hypertrophy and fibrosis. Time to 50% shortening and relaxation were reduced in ExL, suggesting a cardiomyocyte contractile improvement. In addition, the time to 50% Ca²⁺ peak was increased without alterations in Ca²⁺ amplitude and time to 50% Ca²⁺ decay. In conclusion, the association of chronic aerobic exercise and low-dose L-NAME prevented cardiac pathological remodeling and induced cardiomyocyte contractile function improvement; however, it did not alter myocyte affinity and sensitivity to intracellular Ca²⁺ handling.

Biochemical and Morphological Alterations in Hearts of Copper-Deficient Bovines

Copper deficiency is an important disease of cattle that produces several clinical signs and lesions, due to alterations in copper-dependent enzymes. One of the organs affected by this deficiency is the heart (falling disease), but nevertheless, these cardiac lesions have not been extensively studied in bovines. The aim of this work was to propose a possible pathogenic mechanism for cardiac lesions in cattle affected by copper deficiency. Because of the possible existence of oxidative distress caused by low levels of copper-zinc-superoxide dismutase and cytochrome oxidase, ultrastructural and histological lesions have been evaluated in the heart of bovines in which a Cu deficiency had been induced using high molybdenum and sulfur levels in the diet. Our results indicated that copper deficiency produces significant damage in myocardium with high levels of lipid oxidation and a significant reduction in copper-zinc-superoxide dismutase activity leading to an oxidative distress situation. However, cytochrome oxidase activity was not significantly reduced. Histological observation revealed a significant increase in the amount of connective tissue, enlarged basement membranes of myocytes, and numerous Anichkov cells, in the hearts of deficient animals. Ultrastructural observation showed a significant enhancement in the mitochondrial volume density, with presence of lesions such as swelling and cristae disruption. We conclude that copper deficiency in bovines causes morphological lesions in the heart due to an oxidative damage produced by copper-dependent enzyme alterations.

Participation of opioid receptors in the cytoprotective effect of chronic normobaric hypoxia

We studied the role of the delta, micro, and kappa opioid receptor (OR) subtypes in the cardioprotective effect of chronic continuous normobaric hypoxia (CNH) in the model of acuteanoxia-reoxygenation of isolated cardiomyocytes. Adaptation of rats to CNH was performed by their exposure to atmosphere containing 12% of O(2) for 21 days. Anoxia-reoxygenation of cardiomyocytes isolated from normoxiccontrol rats caused the death of 51 % of cells and lactate dehydrogenase (LDH) release. Adaptation of rats to CNH resulted in the anoxia/reoxygenation-induced cardiomyocyte death of only 38 %, and reduced the LDH release by 25 %. Pre-incubation of the cells with either the non-selective OR (opioid receptor) blocker naloxone (300 nM/l), the delta OR antagonist TIPP(psi) (30 nM/l), the selective delta(2) OR antagonist naltriben (1 nM/l) or the micro OR antagonist CTAP (100 nM/l) for 25 minutes before anoxia abolished the reduction of cell death and LDH release afforded by CNH. The antagonist of delta(1) OR BNTX (1 nM/l) or the kappa OR antagonist nor-binaltorphimine (3 nM/l) did not influence the cytoprotective effects of CNH. Taken together, the cytoprotective effect of CNH is associated with the activation of the delta(2) and micro OR localized on cardiomyocytes.

The Effect of Hypoxia on Cardiovascular Disease: Friend or Foe?

Savla, Jainy J., Benjamin D. Levine, and Hesham A. Sadek. The effect of hypoxia on cardiovascular disease: Friend or foe? High Alt Med Biol. 19:124-130, 2018.-Over 140 million people reside at altitudes exceeding 2500 m across the world, resulting in exposure to atmospheric (hypobaric) hypoxia. Whether this chronic exposure is beneficial or detrimental to the cardiovascular system, however, is uncertain. On one hand, multiple studies have suggested a protective effect of living at moderate and high altitudes for cardiovascular risk factors and cardiovascular disease (CVD) events. Conversely, residence at high altitude comes at the tradeoff of developing diseases such as chronic mountain sickness and high-altitude pulmonary hypertension and worsens outcomes for diseases such as chronic obstructive pulmonary disease. Interestingly, recently published data show a potential role for severe hypoxia as a unique and unexpected therapy after myocardial infarction. In this review, we will discuss the current literature evaluating the effects of altitude exposure and the accompanying hypoxia on health and the potential therapeutic applications of hypoxia on CVD.

Selective replacement of mitochondrial DNA increases the cardioprotective effect of chronic continuous hypoxia in spontaneously hypertensive rats

Mitochondria play an essential role in improved cardiac ischaemic tolerance conferred by adaptation to chronic hypoxia. In the present study, we analysed the effects of continuous normobaric hypoxia (CNH) on mitochondrial functions, including the sensitivity of the mitochondrial permeability transition pore (MPTP) to opening, and infarct size (IS) in hearts of spontaneously hypertensive rats (SHR) and the conplastic SHR-mt^BN strain, characterized by the selective replacement of the mitochondrial genome of SHR with that of the more ischaemia-resistant brown Norway (BN) strain. Rats were adapted to CNH (10% O₂, 3 weeks) or kept at room air as normoxic controls. In the left ventricular mitochondria, respiration and cytochrome c oxidase (COX) activity were measured using an Oxygraph-2k and the sensitivity of MPTP opening was assessed spectrophotometrically as Ca²⁺-induced swelling. Myocardial infarction was analysed in anaesthetized open-chest rats subjected to 20 min of coronary artery occlusion and 3 h of reperfusion. The IS reached 68±3.0% and 65±5% of the area at risk in normoxic SHR and SHR-mt^BN strains, respectively. CNH significantly decreased myocardial infarction to 46±3% in SHR. In hypoxic SHR-mt^BN strain, IS reached 33±2% and was significantly smaller compared with hypoxic SHR. Mitochondria isolated from hypoxic hearts of both strains had increased detergent-stimulated COX activity and were less sensitive to MPTP opening. The maximum swelling rate was significantly lower in hypoxic SHR-mt^BN strain compared with hypoxic SHR, and positively correlated with myocardial infarction in all experimental groups. In conclusion, the mitochondrial genome of SHR modulates the IS-limiting effect of adaptation to CNH by affecting mitochondrial energetics and MPTP sensitivity to opening.

Diabetes impairs heart mitochondrial function without changes in resting cardiac performance

Diabetes is a chronic disease associated to a cardiac contractile dysfunction that is not attributable to underlying coronary artery disease or hypertension, and could be consequence of a progressive deterioration of mitochondrial function. We hypothesized that impaired mitochondrial function precedes Diabetic Cardiomyopathy. Thus, the aim of this work was to study the cardiac performance and heart mitochondrial function of diabetic rats, using an experimental model of type I Diabetes. Rats were sacrificed after 28days of Streptozotocin injection (STZ, 60mgkg^-1, ip.). Heart O₂ consumption was declined, mainly due to the impairment of mitochondrial O₂ uptake. The mitochondrial dysfunction observed in diabetic animals included the reduction of state 3 respiration (22%), the decline of ADP/O ratio (∼15%) and the decrease of the respiratory complexes activities (22-26%). An enhancement in mitochondrial H₂O₂ (127%) and NO (23%) production rates and in tyrosine nitration (58%) were observed in heart of diabetic rats, with a decrease in Mn-SOD activity (∼50%). Moreover, a decrease in contractile response (38%), inotropic (37%) and lusitropic (58%) reserves were observed in diabetic rats only after a β-adrenergic stimulus. Therefore, in conditions of sustained hyperglycemia, heart mitochondrial O₂ consumption and oxidative phosphorylation efficiency are decreased, and H₂O₂ and NO productions are increased, leading to a cardiac compromise against a work overload. This mitochondrial impairment was detected in the absence of heart hypertrophy and of resting cardiac performance changes, suggesting that mitochondrial dysfunction could precede the onset of diabetic cardiac failure, being H₂O₂, NO and ATP the molecules probably involved in mitochondrion-cytosol signalling.

Mitochondrial nitric oxide production supported by reverse electron transfer

Heart phosphorylating electron transfer particles (ETPH) produced NO at 1.2 ± 0.1 nmol NO. min(-1) mg protein(-1) by the mtNOS catalyzed reaction. These particles showed a NAD(+) reductase activity of 64 ± 3 nmol min(-1) mg protein(-1) sustained by reverse electron transfer (RET) at expenses of ATP and succinate. The same particles, without NADPH and in conditions of RET produced 0.97 ± 0.07 nmol NO. min(-1) mg protein(-1). Rotenone inhibited NO production supported by RET measured in ETPH and in coupled mitochondria, but did not reduce the activity of recombinant nNOS, indicating that the inhibitory effect of rotenone on NO production is due to an electron flow inhibition and not to a direct action on mtNOS structure. NO production sustained by RET corresponds to 20% of the total amount of NO released from heart coupled mitochondria. A mitochondrial fraction enriched in complex I produced 1.7 ± 0.2 nmol NO. min(-1) mg protein(-1) and reacted with anti-75 kDa complex I subunit and anti-nNOS antibodies, suggesting that complex I and mtNOS are located contiguously. These data show that mitochondrial NO production can be supported by RET, and suggest that mtNOS is next to complex I, reaffirming the idea of a functional association between these proteins.

Synergistic impact of endurance training and intermittent hypobaric hypoxia on cardiac function and mitochondrial energetic and signaling

BACKGROUND

Intermittent hypobaric-hypoxia (IHH) and endurance-training (ET) are cardioprotective strategies against stress-stimuli. Mitochondrial modulation appears to be an important step of the process. This study aimed to analyze whether a combination of these approaches provides additive or synergistic effects improving heart-mitochondrial and cardiac-function.

METHODS

Two-sets of rats were divided into normoxic-sedentary (NS), normoxic-exercised (NE, 1 h/day/5 weeks treadmill-running), hypoxic-sedentary (HS, 6000 m, 5h/day/5 weeks) and hypoxic-exercised (HE) to study overall cardiac and mitochondrial function. In vitro cardiac mitochondrial oxygen consumption and transmembrane potential were evaluated. OXPHOS subunits and ANT protein content were semi-quantified by Western blotting. HIF-1α, VEGF, VEGF-R1 VEGF-R2, BNP, SERCA2a and PLB expressions were measured by qRT-PCR and cardiac function was characterized by echocardiography and hemodynamic parameters.

RESULTS

Respiratory control ratio (RCR) increased in NE, HS and HE vs. NS. Susceptibility to anoxia/reoxygenation-induced dysfunction decreased in NE, HS and HE vs. NS. HS decreased mitochondrial complex-I and -II subunits; however HE completely reverted the decreased content in complex-II subunits. ANT increased in HE. HE presented normalized ventricular-arterial coupling (Ea) and BNP myocardial levels and significantly improved myocardial performance as evaluated by increased cardiac output and normalization of the Tei index vs.

HS CONCLUSION

Data demonstrates that IHH and ET confer cardiac mitochondria with a more resistant phenotype although without visible addictive effects at least under basal conditions. It is suggested that the combination of both strategies, although not additive, results into improved cardiac function.

Strategic localization of heart mitochondrial NOS: a review of the evidence

Heart mitochondria play a central role in cell energy provision and in signaling. Nitric oxide (NO) is a free radical with primary regulatory functions in the heart and involved in a broad array of key processes in cardiac metabolism. Specific NO synthase (NOS) isoforms are confined to distinct locations in cardiomyocytes. The present article reviews the chemical reactions through which NO interacts with biomolecules and exerts some of its crucial roles. Specifically, the article discusses the reactions of NO with mitochondrial targets and the subcellular localization of NOS within the myocardium and analyzes the available data about heart mitochondrial NOS activity and identity. The article also describes the regulation of heart mtNOS by the distinctive mitochondrial environment by showing the effects of Ca(2+), O(2), l-arginine, mitochondrial transmembrane potential, and the metabolic states on heart mitochondrial NO production. The article depicts the effects of NO on heart function and highlights the relevance of NO production within mitochondria. Finally, the evidence on the functional implications of heart mitochondrial NOS is delineated with emphasis on chronic hypoxia and ischemia-reperfusion studies.

Usefulness of combining intermittent hypoxia and physical exercise in the treatment of obesity

Obesity is an important public health problem worldwide and is a major risk factor for a number of chronic diseases such as type II diabetes, adverse cardiovascular events and metabolic syndrome-related features. Different treatments have been applied to tackle body fat accumulation and its associated clinical manifestations. Often, relevant weight loss is achieved during the first 6 months under different dietary treatments. From this point, a plateau is reached, and a gradual recovery of the lost weight may occur. Therefore, new research approaches are being investigated to assure weight maintenance. Pioneering investigations have reported that oxygen variations in organic systems may produce changes in body composition. Possible applications of intermittent hypoxia to promote health and in various pathophysiological states have been reported. The hypoxic stimulus in addition to diet and exercise can be an interesting approach to lose weight, by inducing higher basal noradrenalin levels and other metabolic changes whose mechanisms are still unclear. Indeed, hypoxic situations increase the diameter of arterioles, produce peripheral vasodilatation and decrease arterial blood pressure. Furthermore, hypoxic training increases the activity of glycolytic enzymes, enhancing the number of mitochondria and glucose transporter GLUT-4 levels as well as improving insulin sensitivity. Moreover, hypoxia increases blood serotonin and decreases leptin levels while appetite is suppressed. These observations allow consideration of the hypothesis that intermittent hypoxia induces fat loss and may ameliorate cardiovascular health, which might be of interest for the treatment of obesity. This new strategy may be useful and practical for clinical applications in obese patients.

Morphological and functional alterations in the aorta of the chronically hypoxic fetal rat

In human pregnancy, reduced placental perfusion has been associated with fetal aortic thickening. However, the relative contributions of fetal undernutrition versus fetal underoxygenation to triggering alterations in fetal cardiovascular development remain uncertain. Here, we isolate the effects of chronic fetal hypoxia on fetal cardiovascular development in a specific rodent model of chronic fetal hypoxia independent of changes in nutrition during pregnancy. Pregnant rats were housed under normoxic (21% O(2)) or hypoxic (13% O(2)) conditions from day 6 to day 20 of gestation. At day 20, pups and placentas were weighed. Fetal thoraces were fixed for quantitative histological analysis of the aorta. In a separate group, fetal aortic reactivity was assessed via in vitro wire myography. The experiments controlled for sex and within-litter variation. Placental weight was increased and fetal weight maintained in hypoxic pregnancy. Hypoxic pregnancy led to a 176% increment in wall thickness and a 170% increment in the wall-to-lumen area ratio of the fetal aorta. Fetal aortic vascular reactivity was markedly impaired, showing reduced constrictor and relaxant responsiveness in hypoxic pregnancy. Chronic developmental hypoxia independent of changes in nutrition has profound effects on the morphology and function of the fetal aorta in a mammalian species.

Complex I syndrome in myocardial stunning and the effect of adenosine

Isolated rabbit hearts were exposed to ischemia (I; 15 min) and reperfusion (R; 5-30 min) in a model of stunned myocardium. I/R decreased left-ventricle O(2) consumption (46%) and malate-glutamate-supported mitochondrial state 3 respiration (32%). Activity of complex I was 28% lower after I/R. The pattern observed for the decline in complex I activity was also observed for the reduction in mitochondrial nitric oxide synthase (mtNOS) biochemical (28%) and functional (50%) activities, in accordance with the reported physical and functional interactions between complex I and mtNOS. Malate-glutamate-supported state 4 H(2)O(2) production was increased by 78% after I/R. Rabbit heart Mn-SOD concentration in the mitochondrial matrix (7.4±0.7 μM) was not modified by I/R. Mitochondrial phospholipid oxidation products were increased by 42%, whereas protein oxidation was only slightly increased. I/R produced a marked (70%) enhancement in tyrosine nitration of the mitochondrial proteins. Adenosine attenuated postischemic ventricular dysfunction and protected the heart from the declines in O(2) consumption and in complex I and mtNOS activities and from the enhancement of mitochondrial phospholipid oxidation. Rabbit myocardial stunning is associated with a condition of dysfunctional mitochondria named "complex I syndrome." The beneficial effect of adenosine could be attributed to a better regulation of intracellular cardiomyocyte Ca(2+) concentration.

L-NAME treatment enhances exercise-induced content of myocardial heat shock protein 72 (Hsp72) in rats

BACKGROUND/AIM

Nitric oxide (NO) modulates the expression of the chaperone Hsp72 in the heart, and exercise stimulates both NO production and myocardial Hsp72 expression. The main purpose of the study was to investigate whether NO interferes with an exercise-induced myocardial Hsp72 expression.

METHODS

Male Wistar rats (70-100 days) were divided into control (C, n=12), L-NAME-treated (L, n=12), exercise (E, n=13) and exercise plus L-NAME-treated (EL, n=20) groups. L-NAME was given in drinking water (700 mg·L(-1)) and the exercise was performed on a treadmill (15-25 m·min(-1), 40-60 min.day(-1)) for seven days. Left ventricle (LV) protein Hsp content, NOS and phosphorylated-NOS (p-NOS) isoforms were measured using Western blotting. The activity of NOS was assayed in LV homogenates by the conversion of [(3)H]L-arginine to [(3)H]L-citrulline.

RESULTS

Hsp72 content was increased significantly (223%; p < 0.05) in the E group compared to the C group, but exercise alone did not alter the NOS content, p-NOS isoforms or NOS activity. Contrary to our expectation, L-NAME enhanced (p < 0.05) the exercise-induced Hsp72 content (EL vs. C, L and E groups = 1019%, 548% and 457%, respectively). Although the EL group had increased stimulatory p-eNOS(Ser1177) (over 200%) and decreased inhibitory p-nNOS(Ser852) (ñ50%) compared to both the E and L groups (p < 0.05), NOS activity was similar in all groups.

CONCLUSIONS

Our results suggest that exercise-induced cardiac Hsp72 expression does not depend on NO. Conversely, the in vivo L-NAME treatment enhances exercise-induced Hsp72 production. This effect may be due to an increase in cardiac stress.

Right ventricular dysfunction and failure in chronic pressure overload

Right ventricular (RV) dysfunction is the main cause of death in pulmonary arterial hypertension (PAH). Our understanding of the pathophysiology of RV dysfunction is limited but improving. Methods to better diagnose RV dysfunction earlier and treatments specifically designed to minimize or reverse the remodeling process are likely to improve outcomes. We review the current understanding of RV dysfunction in chronic pressure overload and introduce some novel insights based on recent investigations into pathophysiology, diagnosis, and treatment.