Reduced left ventricular compliance and mechanical efficiency after prolonged inhibition of NO synthesis in conscious dogs

Expanding the Frontiers of Guardian Antioxidant Selenoproteins in Cardiovascular Pathophysiology

Significance: Physiological levels of reactive oxygen and nitrogen species (ROS/RNS) function as fundamental messengers for many cellular and developmental processes in the cardiovascular system. ROS/RNS involved in cardiac redox-signaling originate from diverse sources, and their levels are tightly controlled by key endogenous antioxidant systems that counteract their accumulation. However, dysregulated redox-stress resulting from inefficient removal of ROS/RNS leads to inflammation, mitochondrial dysfunction, and cell death, contributing to the development and progression of cardiovascular disease (CVD). Recent Advances: Basic and clinical studies demonstrate the critical role of selenium (Se) and selenoproteins (unique proteins that incorporate Se into their active site in the form of the 21st proteinogenic amino acid selenocysteine [Sec]), including glutathione peroxidase and thioredoxin reductase, in cardiovascular redox homeostasis, representing a first-line enzymatic antioxidant defense of the heart. Increasing attention has been paid to emerging selenoproteins in the endoplasmic reticulum (ER) (i.e., a multifunctional intracellular organelle whose disruption triggers cardiac inflammation and oxidative stress, leading to multiple CVD), which are crucially involved in redox balance, antioxidant activity, and calcium and ER homeostasis. Critical Issues: This review focuses on endogenous antioxidant strategies with therapeutic potential, particularly selenoproteins, which are very promising but deserve more detailed and clinical studies. Future Directions: The importance of selective selenoproteins in embryonic development and the consequences of their mutations and inborn errors highlight the need to improve knowledge of their biological function in myocardial redox signaling. This could facilitate the development of personalized approaches for the diagnosis, prevention, and treatment of CVD. Antioxid. Redox Signal. 40, 369-432.

Reduced nitric oxide bioavailability impairs myocardial oxygen balance during exercise in swine with multiple risk factors

In the present study, we tested the hypothesis that multiple risk factors, including diabetes mellitus (DM), dyslipidaemia and chronic kidney disease (CKD) result in a loss of nitric oxide (NO) signalling, thereby contributing to coronary microvascular dysfunction. Risk factors were induced in 12 female swine by intravenous streptozotocin injections (DM), a high fat diet (HFD) and renal artery embolization (CKD). Female healthy swine (n = 13) on normal diet served as controls (Normal). After 5 months, swine were chronically instrumented and studied at rest and during exercise. DM + HFD + CKD swine demonstrated significant hyperglycaemia, dyslipidaemia and impaired kidney function compared to Normal swine. These risk factors were accompanied by coronary microvascular endothelial dysfunction both in vivo and in isolated small arteries, due to a reduced NO bioavailability, associated with perturbations in myocardial oxygen balance at rest and during exercise. NO synthase inhibition caused coronary microvascular constriction in exercising Normal swine, but had no effect in DM + HFD + CKD animals, while inhibition of phosphodiesterase 5 produced similar vasodilator responses in both groups, indicating that loss of NO bioavailability was principally responsible for the observed coronary microvascular dysfunction. This was associated with an increase in myocardial 8-isoprostane levels and a decrease in antioxidant capacity, while antioxidants restored the vasodilation to bradykinin in isolated coronary small arteries, suggesting that oxidative stress was principally responsible for the reduced NO bioavailability. In conclusion, five months of combined exposure to DM + HFD + CKD produces coronary endothelial dysfunction due to impaired NO bioavailability, resulting in impaired myocardial perfusion at rest and during exercise.

Cavin-1 deficiency modifies myocardial and coronary function, stretch responses and ischaemic tolerance: roles of NOS over-activity

Caveolae and associated cavin and caveolins may govern myocardial function, together with responses to mechanical and ischaemic stresses. Abnormalities in these proteins are also implicated in different cardiovascular disorders. However, specific roles of the cavin-1 protein in cardiac and coronary responses to mechanical/metabolic perturbation remain unclear. We characterised cardiovascular impacts of cavin-1 deficiency, comparing myocardial and coronary phenotypes and responses to stretch and ischaemia-reperfusion in hearts from cavin-1 ^+/+ and cavin-1 ^-/- mice. Caveolae and caveolins 1 and 3 were depleted in cavin-1 ^-/- hearts. Cardiac ejection properties in situ were modestly reduced in cavin-1 ^-/- mice. While peak contractile performance in ex vivo myocardium from cavin-1 ^-/- and cavin-1 ^+/+ mice was comparable, intrinsic beating rate, diastolic stiffness and Frank-Starling behaviour (stretch-dependent diastolic and systolic forces) were exaggerated in cavin-1 ^-/- hearts. Increases in stretch-dependent forces were countered by NOS inhibition (100 µM L-NAME), which exposed negative inotropy in cavin-1 ^-/- hearts, and were mimicked by 100 µM nitroprusside. In contrast, chronotropic differences appeared largely NOS-independent. Cavin-1 deletion also induced NOS-dependent coronary dilatation, ≥3-fold prolongation of reactive hyperaemic responses, and exaggerated pressure-dependence of coronary flow. Stretch-dependent efflux of lactate dehydrogenase and cardiac troponin I was increased and induction of brain natriuretic peptide and c-Fos inhibited in cavin-1 ^-/- hearts, while ERK1/2 phospho-activation was preserved. Post-ischaemic dysfunction and damage was also exaggerated in cavin-1 ^-/- hearts. Diverse effects of cavin-1 deletion reveal important roles in both NOS-dependent and -independent control of cardiac and coronary functions, together with governing sarcolemmal fragility and myocardial responses to stretch and ischaemia.

Nitric oxide bioavailability affects cardiovascular regulation dependent on cardiac nerve status

The sympathetic nervous system and nitric oxide (NO) contribute to regulation of vascular tone, blood flow regulation and cardiac function. Intrinsic cardiac neurons are tonically influenced by locally released NO and exogenous NO donors; however, the role of intact central neural connections remains controversial. We investigated the effects of S-nitroso-N-acetylpenicillamine (SNAP) administered into an intracoronary artery near the ventral interventricular ganglionated plexus (VIVGP) to evaluate distribution of myocardial blood flow (MBF) and ventricular function in normal and acute cardiac decentralized dogs. MBF was measured with microspheres during infusion of SNAP (100μM, IC) after systemic administration of 7-nitroindazole (nNOS blocker) followed by N(ω)-nitro-L-arginine methyl ester (LN; non-selective NOS blocker). Cardiac dynamics were not significantly affected by cardiac decentralization; several of these parameters (aortic systolic and diastolic pressures) were significantly increased after systemic administration of LN. Overall SNAP administered to the VIVGP increased blood flow in the anterior LV wall (vs. posterior LV wall) without affecting other cardiodynamic factors. In cardiac decentralized dogs subepicardial blood flow to the anterior LV wall during LN+SNAP was diminished resulting in a significantly higher inner:outer blood flow ratio (index of blood flow uniformity across the LV wall). LV function was not affected by acute cardiac decentralization; however, LV ejection fraction decreased markedly after LN (reduced NO bioavailability). These results validate earlier claims that reduced NO bioavailability imposes an upper limit on myocardial blood flow regulation and its transmural distribution. These effects are exacerbated after disconnection of intrinsic cardiac neurons from intact central neuron connections.

The gut hormone ghrelin partially reverses energy substrate metabolic alterations in the failing heart

BACKGROUND

The gut-derived hormone ghrelin, especially its acylated form, plays a major role in the regulation of systemic metabolism and exerts also relevant cardioprotective effects; hence, it has been proposed for the treatment of heart failure (HF). We tested the hypothesis that ghrelin can directly modulate cardiac energy substrate metabolism.

METHODS AND RESULTS

We used chronically instrumented dogs, 8 with pacing-induced HF and 6 normal controls. Human des-acyl ghrelin [1.2 nmol/kg per hour] was infused intravenously for 15 minutes, followed by washout (rebaseline) and infusion of acyl ghrelin at the same dose. (3)H-oleate and (14)C-glucose were coinfused and arterial and coronary sinus blood sampled to measure cardiac free fatty acid and glucose oxidation and lactate uptake. As expected, cardiac substrate metabolism was profoundly altered in HF because baseline oxidation levels of free fatty acids and glucose were, respectively, >70% lower and >160% higher compared with control. Neither des-acyl ghrelin nor acyl ghrelin significantly affected function and metabolism in normal hearts. However, in HF, des-acyl and acyl ghrelin enhanced myocardial oxygen consumption by 10.2±3.5% and 9.9±3.7%, respectively (P<0.05), and cardiac mechanical efficiency was not significantly altered. This was associated, respectively, with a 41.3±6.7% and 32.5±10.9% increase in free fatty acid oxidation and a 31.3±9.2% and 41.4±8.9% decrease in glucose oxidation (all P<0.05).

CONCLUSIONS

Acute increases in des-acyl or acyl ghrelin do not interfere with cardiac metabolism in normal dogs, whereas they enhance free fatty acid oxidation and reduce glucose oxidation in HF dogs, thus partially correcting metabolic alterations in HF. This novel mechanism might contribute to the cardioprotective effects of ghrelin in HF.

Beneficial effects of acute inhibition of the oxidative pentose phosphate pathway in the failing heart

In vitro studies suggested that glucose metabolism through the oxidative pentose phosphate pathway (oxPPP) can paradoxically feed superoxide-generating enzymes in failing hearts. We therefore tested the hypothesis that acute inhibition of the oxPPP reduces oxidative stress and enhances function and metabolism of the failing heart, in vivo. In 10 chronically instrumented dogs, congestive heart failure (HF) was induced by high-frequency cardiac pacing. Myocardial glucose consumption was enhanced by raising arterial glycemia to levels mimicking postprandial peaks, before and after intravenous administration of the oxPPP inhibitor 6-aminonicotinamide (80 mg/kg). Myocardial energy substrate metabolism was measured with radiolabeled glucose and oleic acid, and cardiac 8-isoprostane output was used as an index of oxidative stress. A group of five chronically instrumented, normal dogs served as control. In HF, raising glycemic levels from ∼ 80 to ∼ 170 mg/dL increased cardiac isoprostane output by approximately twofold, whereas oxPPP inhibition normalized oxidative stress and enhanced cardiac oxygen consumption, glucose oxidation, and stroke work. In normal hearts glucose infusion did not induce significant changes in cardiac oxidative stress. Myocardial tissue concentration of 6P-gluconate, an intermediate metabolite of the oxPPP, was significantly reduced by ∼ 50% in treated versus nontreated failing hearts, supporting the inhibitory effect of 6-aminonicotinamide. Our study indicates an important contribution of the oxPPP activity to cardiac oxidative stress in HF, which is particularly pronounced during common physiological changes such as postprandial glycemic peaks.

Impact of acute changes of left ventricular contractility on the transvalvular impedance: validation study by pressure-volume loop analysis in healthy pigs

BACKGROUND

The real-time and continuous assessment of left ventricular (LV) myocardial contractility through an implanted device is a clinically relevant goal. Transvalvular impedance (TVI) is an impedentiometric signal detected in the right cardiac chambers that changes during stroke volume fluctuations in patients. However, the relationship between TVI signals and LV contractility has not been proven. We investigated whether TVI signals predict changes of LV inotropic state during clinically relevant loading and inotropic conditions in swine normal heart.

METHODS

The assessment of RVTVI signals was performed in anesthetized adult healthy anesthetized pigs (n = 6) instrumented for measurement of aortic and LV pressure, dP/dtmax and LV volumes. Myocardial contractility was assessed with the slope (Ees) of the LV end systolic pressure-volume relationship. Effective arterial elastance (Ea) and stroke work (SW) were determined from the LV pressure-volume loops. Pigs were studied at rest (baseline), after transient mechanical preload reduction and afterload increase, after 10-min of low dose dobutamine infusion (LDDS, 10 ug/kg/min, i.v), and esmolol administration (ESMO, bolus of 500 µg and continuous infusion of 100 µg·kg-1·min-1).

RESULTS

We detected a significant relationship between ESTVI and dP/dtmax during LDDS and ESMO administration. In addition, the fluctuations of ESTVI were significantly related to changes of the Ees during afterload increase, LDDS and ESMO infusion.

CONCLUSIONS

ESTVI signal detected in right cardiac chamber is significantly affected by acute changes in cardiac mechanical activity and is able to predict acute changes of LV inotropic state in normal heart.

Protection against L-NAME-induced reduction in cardiac output persists even after cessation of angiotensin-converting enzyme inhibitor treatment

AIM

We have demonstrated that short-term angiotensin-converting enzyme (ACE) inhibition in adult spontaneously hypertensive rats produces cardiac changes that persist following cessation of treatment that result in a reduced inflammatory, proliferative and fibrotic response to the nitric oxide synthase inhibitor N(ω) -Nitro-l-arginine methyl ester (L-NAME). The present study examines whether prior ACE inhibition with enalapril also protects against L-NAME-induced cardiac dysfunction.

METHODS

Rats were treated with enalapril (Enal + L) or tap water (Con, Con + L) for 2 weeks followed by a 2-week washout period. At this point, Con + L and Enal + L rats were treated with L-NAME for 10 days. Hearts were perfused in the working mode, mean arterial pressure (MAP) was assessed via radiotelemetry, and myocardial injury was evaluated in hematoxylin and eosin-stained sections.

RESULTS

L-NAME increased MAP by a similar magnitude in Con + L and Enal + L. L-NAME-induced statistically significant decreases in flow-mediated functional parameters in Con + L rats including cardiac output, stroke volume and coronary flow. This was prevented by prior enalapril treatment. Prior enalapril did not prevent L-NAME-induced myocardial injury, but may have lessened the degree of it. Regardless of treatment, changes in cardiac function did not correlate with myocardial injury.

CONCLUSION

Despite equivalent impact on MAP and incidence of myocardial infarction, prior enalapril treatment resulted in the preservation of cardiac function following L-NAME. Understanding the mechanisms by which transient ACE inhibition protects against reductions in cardiac function in the absence of ongoing treatment may reveal novel targets for heart failure treatment.

Acute vagal stimulation attenuates cardiac metabolic response to β-adrenergic stress

The effects of vagal stimulation (VS) on cardiac energy substrate metabolism are unknown. We tested the hypothesis that acute VS alters the balance between free fatty acid (FFA) and carbohydrate oxidation and opposes the metabolic effects of β-adrenergic stimulation. A clinical-type selective stimulator of the vagal efferent fibres was connected to the intact right vagus in chronically instrumented dogs. VS was set to reduce heart rate by 30 beats min(-1), and the confounding effects of bradycardia were then eliminated by pacing the heart at 165 beats min(-1). [(3)H]Oleate and [(14)C]glucose were infused to measure FFA and glucose oxidation. The heart was subjected to β-adrenergic stress by infusing dobutamine at 5, 10 and 15 μg kg(-1) min(-1) before and during VS. VS did not significantly affect baseline cardiac performance, haemodynamics or myocardial metabolism. However, at peak dobutamine stress, VS attenuated the increase in left ventricular pressure-diameter area from 235.9 ± 72.8 to 167.3 ± 55.8%, and in cardiac oxygen consumption from 173.9 ± 23.3 to 127.89 ± 6.2% (both P < 0.05), and thus mechanical efficiency was not enhanced. The increase in glucose oxidation fell from 289.3 ± 55.5 to 131.1 ± 20.9% (P < 0.05), while FFA oxidation was not increased by β-adrenergic stress and fell below baseline during VS only at the lowest dose of dobutamine. The functional and in part the metabolic changes were reversed by 0.1 mg kg(-1) atropine i.v. Our data show that acute right VS does not affect baseline cardiac metabolism, but attenuates myocardial oxygen consumption and glucose oxidation in response to adrenergic stress, thus functioning as a cardio-selective antagonist to β-adrenergic activation.

Reassessment of a suggested pharmacological approach to heart failure: L-arginine is only a marginal NO donor in pigs

OBJECTIVES

L-Arginine has been tested in various cardiovascular diseases, mainly to improve endothelial function through NO production. However, as the results have been partly unpredictable, we assessed the hemodynamic, energetic and metabolic effects of L-arginine to clarify any potential benefits in postischemic left ventricular (LV) dysfunction.

METHODS

LV dysfunction was induced by repetitive brief coronary occlusions in 12 anesthetized, open chest pigs. L-Arginine was subsequently infused (bolus 400 mg·kg and continuously for 1 hour, 250 mg·kg·h). Hemodynamic parameters, metabolites of L-arginine and myocardial energetics were assessed sequentially.

RESULTS

L-Arginine infusions caused a substantial rise in plasma L-arginine (3474 ± 358 μmole·L) accompanied by a 2-fold increase in plasma L-citrulline. No significant alterations in vascular resistance or LV contractility were observed from L-arginine. Mean arterial pressure dropped from 78 ± 11 to 72 ± 10 mm Hg (P = 0.019) and 70 ± 8 mm Hg (P = 0.003) after bolus and infusions, respectively. Myocardial oxygen consumption was unaltered, and myocardial creatine content was not increased after 90 minutes of L-arginine infusion.

CONCLUSION

L-Arginine infusion did not influence the energetic cost of myocardial contractility, and only minor hemodynamic changes were observed despite a demonstrable turnover of L-arginine. These findings question the use of L-arginine to promote therapeutic NO formation in the acute setting.

Altered cardiac metabolic phenotype after prolonged inhibition of NO synthesis in chronically instrumented dogs

Acute inhibition of nitric oxide (NO) synthase causes a reversible alteration in myocardial substrate metabolism. We tested the hypothesis that prolonged NO synthase inhibition alters cardiac metabolic phenotype. Seven chronically instrumented dogs were treated with Nω-nitro-l-arginine methyl ester (l-NAME, 35 mg·kg−1·day−1po) for 10 days to inhibit NO synthesis, and seven were used as controls. Cardiac free fatty acid, glucose, and lactate oxidation were measured by infusion of [3H]oleate, [14C]glucose, and [13C]lactate, respectively. After 10 days of l-NAME administration, despite no differences in left ventricular afterload, cardiac O2consumption was significantly increased by 30%, consistent with a marked enhancement in baseline oxidation of glucose (6.9 ± 2.0 vs. 1.7 ± 0.5 μmol·min−1·100 g−1, P < 0.05 vs. control) and lactate (21.6 ± 5.6 vs. 11.8 ± 2.6 μmol·min−1·100 g−1, P < 0.05 vs. control). When left ventricular afterload was increased by ANG II infusion to stimulate myocardial metabolism, glucose oxidation was augmented further in the l-NAME than in the control group, whereas free fatty acid oxidation decreased. Exogenous NO (diethylamine nonoate, 0.01 μmol·kg−1·min−1iv) could not reverse this metabolic alteration. Consistent with the accelerated rate of carbohydrate oxidation, total myocardial pyruvate dehydrogenase activity and protein expression were higher (38 and 34%, respectively) in the l-NAME than in the control group. Also, protein expression of the constitutively active glucose transporter GLUT-1 was significantly elevated (46%) vs. control. We conclude that prolonged NO deficiency causes a profound alteration in cardiac metabolic phenotype, characterized by selective potentiation of carbohydrate oxidation, that cannot be reversed by a short-term infusion of exogenous NO. This phenomenon may constitute an adaptive mechanism to counterbalance cardiac mechanical inefficiency.

What mechanisms underlie diastolic dysfunction in heart failure?

Abnormalities of diastolic function are common to virtually all forms of cardiac failure. However, their underlying mechanisms, precise role in the generation and phenotypic expression of heart failure, and value as specific therapeutic targets remain poorly understood. A growing proportion of heart failure patients, particularly among the elderly, have apparently preserved systolic function, and this is fueling interest for better understanding and treating diastolic abnormalities. Much of the attention in clinical and experimental studies has focused on relaxation and filling abnormalities of the heart, whereas chamber stiffness has been less well studied, particularly in humans. Nonetheless, new insights from basic and clinical research are helping define the regulators of diastolic dysfunction and illuminate novel targets for treatment. This review puts these developments into perspective with the major aim of highlighting current knowledge gaps and controversies.

Nitric oxide's role in the heart: control of beating or breathing?

Beneficial actions of nitric oxide (NO) in failing myocardium have frequently been overshadowed by poorly documented negative inotropic effects mainly derived from in vitro cardiac preparations. NO's beneficial actions include control of myocardial energetics and improvement of left ventricular (LV) diastolic distensibility. In isolated cardiomyocytes, administration of NO increases their diastolic cell length consistent with a rightward shift of the passive length-tension relation. This shift is explained by cGMP-induced phosphorylation of troponin I, which prevents calcium-independent diastolic cross-bridge cycling and concomitant diastolic stiffening of the myocardium. Similar improvements in diastolic stiffness have been observed in isolated guinea pig hearts, in pacing-induced heart failure dogs, and in patients with dilated cardiomyopathy or aortic stenosis and have been shown to result in higher LV preload reserve and stroke work. NO also controls myocardial energetics through its effects on mitochondrial respiration, oxygen consumption, and substrate utilization. The effects of NO on diastolic LV performance appear to be synergistic with its effects on myocardial energetics through prevention of myocardial energy wastage induced by LV contraction against late-systolic reflected arterial pressure waves and through prevention of diastolic LV stiffening, which is essential for the maintenance of adequate subendocardial coronary perfusion. A drop in these concerted actions of NO on diastolic LV distensibility and on myocardial energetics could well be instrumental for the relentless deterioration of failing myocardium.