Depressed contractile reserve and impaired calcium handling of cardiac myocytes from chronically unloaded hearts are ameliorated with the administration of physiological treatment dose of T3 in rats

Function and regulation of phosphatase 1 in healthy and diseased heart

Reversible phosphorylation of ion channels and calcium-handling proteins provides precise post-translational regulation of cardiac excitation and contractility. Serine/threonine phosphatases govern dephosphorylation of the majority of cardiac proteins. Accordingly, dysfunction of this regulation contributes to the development and progression of heart failure and atrial fibrillation. On the molecular level, these changes include alterations in the expression level and phosphorylation status of Ca²⁺ handling and excitation-contraction coupling proteins provoked by dysregulation of phosphatases. The serine/threonine protein phosphatase PP1 is one a major player in the regulation of cardiac excitation-contraction coupling. PP1 essentially impacts on cardiac physiology and pathophysiology via interactions with the cardiac ion channels Cav1.2, NKA, NCX and KCNQ1, sarcoplasmic reticulum-bound Ca²⁺ handling proteins such as RyR2, SERCA and PLB as well as the contractile proteins MLC2, TnI and MyBP-C. PP1 itself but also PP1-regulatory proteins like inhibitor-1, inhibitor-2 and heat-shock protein 20 are dysregulated in cardiac disease. Therefore, they represent interesting targets to gain more insights in heart pathophysiology and to identify new treatment strategies for patients with heart failure or atrial fibrillation. We describe the genetic and holoenzymatic structure of PP1 and review its role in the heart and cardiac disease. Finally, we highlight the importance of the PP1 regulatory proteins for disease manifestation, provide an overview of genetic models to study the role of PP1 for the development of heart failure and atrial fibrillation and discuss possibilities of pharmacological interventions.

Cardioprotection and Thyroid Hormones in the Clinical Setting of Heart Failure

Ischemic heart disease is the main cause of morbidity and mortality worldwide and is becoming more widespread with population aging. Cardioprotection is a dynamic process characterized by mechanisms related to myocardial damage and activation of protective factors. Targeting these processes could be attractive as a new therapeutic strategy in the evolution of post-ischemic heart failure (HF). In this context, the role of thyroid hormone (TH)-mediated cardioprotection is supported by a number of findings regarding the modulation of neuroendocrine systems, inflammatory and oxidative stress status, pro-survival intracellular pathways, and epigenetic factors, its effects on cardiac angiogenesis, structure, and function and on the preservation of mitochondrial function and morphology, and its beneficial effects on cell growth and redifferentiation. Moreover, the numerous effects of TH on the heart involve genomic mechanisms, which include cardiac differentiation during the perinatal period and non-genomic action, directed toward the maintenance of cardiovascular homeostasis. This evidence suggests that there is an opportunity to treat HF patients with TH. This review is mainly focused on the clinical evidence of the role of the thyroid system in the complex setting of HF.

Depressed Myocardial Contractility: Can It Be Rescued?

Current dogma suggests patients with advanced systolic heart failure have an irreversible depression in myocardial contractility. Recent experience with improved ventricular function during continuous flow ventricular assist devices used as destination therapy would suggest otherwise. Herein, cellular and molecular signaling involved in reversing depressed myocardial contractility would be addressed. This includes cardiomyocyte thyroid hormone signaling responsible for the reexpression of fetal gene program that preserves cell efficiency (work and energy consumed) and the rescue of an endogenous population of atrophic myocytes bordering on microdomains of fibrosis to improve contractile mass.

Rat Heterotopic Heart Transplantation Model to Investigate Unloading-Induced Myocardial Remodeling

Unloading of the failing left ventricle in order to achieve myocardial reverse remodeling and improvement of contractile function has been developed as a strategy with the increasing frequency of implantation of left ventricular assist devices in clinical practice. But, reverse remodeling remains an elusive target, with high variability and exact mechanisms still largely unclear. The small animal model of heterotopic heart transplantation (hHTX) in rodents has been widely implemented to study the effects of complete and partial unloading on cardiac failing and non-failing tissue to better understand the structural and molecular changes that underlie myocardial recovery. We herein review the current knowledge on the effects of volume unloading the left ventricle via different methods of hHTX in rats, differentiating between changes that contribute to functional recovery and adverse effects observed in unloaded myocardium. We focus on methodological aspects of heterotopic transplantation, which increase the correlation between the animal model and the setting of the failing unloaded human heart. Last, but not least, we describe the late use of sophisticated techniques to acquire data, such as small animal MRI and catheterization, as well as ways to assess unloaded hearts under "reloaded" conditions. While giving regard to certain limitations, heterotopic rat heart transplantation certainly represents the crucial model to mimic unloading-induced changes in the heart and as such the intricacies and challenges deserve highest consideration. Careful translational research will further improve our knowledge of the reverse remodeling process and how to potentiate its effect in order to achieve recovery of contractile function in more patients.

Non-thyroidal illness syndrome in patients with cardiovascular diseases: A systematic review and meta-analysis

BACKGROUND

Non-thyroidal illness syndrome (NTIS) is characterized by decreased serum triiodothyronine level without increased thyroid-stimulating hormone level during critical illness. The summary data on the prevalence of NTIS in cardiovascular patients are lacking, and its prognostic role in cardiovascular patients is also unclear.

METHODS

We performed a systematic review and meta-analysis to comprehensively determine the prevalence and the prognostic role of NTIS in cardiovascular patients. The prevalence of NTIS was pooled using random-effect meta-analysis and the hazard ratios (HRs) for all-cause mortality, cardiac mortality and major adverse cardiovascular events (MACE) were also pooled.

RESULTS

Forty-one studies were finally included. The pooled prevalence of NTIS in cardiovascular patients was 21.7% (95% CI 18.4%-25.3%). Subgroup by the types of cardiovascular diseases showed the prevalence of NTIS was highest in patients with heart failure (24.5%), followed by acute myocardial infarction (18.9%) and acute coronary syndrome (17.1%). Meta-analysis of studies using strict diagnostic criteria of NITS showed that the pooled prevalence of NTIS in cardiovascular patients was 17.6% (95% CI 14.5%-21.2%). NTIS was independently associated with increased risks of all-cause mortality (HR=2.52, 95% CI 1.87-3.40, P<0.001) and cardiac mortality (HR=2.06, 95% CI 1.58-2.69, P<0.001) in cardiovascular patients. NTIS was also an independent predictor of MACE in cardiovascular patients (HR=1.73, 95% CI 1.32-2.26, P<0.001).

CONCLUSION

NTIS is very common in patients with cardiovascular diseases. NTIS is an independent prognostic factor in cardiovascular patients and is associated with increased risks of all-cause mortality, cardiac mortality and MACE.

Caloric restriction induces energy-sparing alterations in skeletal muscle contraction, fiber composition and local thyroid hormone metabolism that persist during catch-up fat upon refeeding

Weight regain after caloric restriction results in accelerated fat storage in adipose tissue. This catch-up fat phenomenon is postulated to result partly from suppressed skeletal muscle thermogenesis, but the underlying mechanisms are elusive. We investigated whether the reduced rate of skeletal muscle contraction-relaxation cycle that occurs after caloric restriction persists during weight recovery and could contribute to catch-up fat. Using a rat model of semistarvation-refeeding, in which fat recovery is driven by suppressed thermogenesis, we show that contraction and relaxation of leg muscles are slower after both semistarvation and refeeding. These effects are associated with (i) higher expression of muscle deiodinase type 3 (DIO3), which inactivates tri-iodothyronine (T₃), and lower expression of T₃-activating enzyme, deiodinase type 2 (DIO2), (ii) slower net formation of T₃ from its T₄ precursor in muscles, and (iii) accumulation of slow fibers at the expense of fast fibers. These semistarvation-induced changes persisted during recovery and correlated with impaired expression of transcription factors involved in slow-twitch muscle development. We conclude that diminished muscle thermogenesis following caloric restriction results from reduced muscle T₃ levels, alteration in muscle-specific transcription factors, and fast-to-slow fiber shift causing slower contractility. These energy-sparing effects persist during weight recovery and contribute to catch-up fat.

Enhanced Ca²+ influx through cardiac L-type Ca²+ channels maintains the systolic Ca²+ transient in early cardiac atrophy induced by mechanical unloading

Cardiac atrophy as a consequence of mechanical unloading develops following exposure to microgravity or prolonged bed rest. It also plays a central role in the reverse remodelling induced by left ventricular unloading in patients with heart failure. Surprisingly, the intracellular Ca²⁺ transients which are pivotal to electromechanical coupling and to cardiac plasticity were repeatedly found to remain unaffected in early cardiac atrophy. To elucidate the mechanisms underlying the preservation of the Ca²⁺ transients, we investigated Ca²⁺ cycling in cardiomyocytes from mechanically unloaded (heterotopic abdominal heart transplantation) and control (orthotopic) hearts in syngeneic Lewis rats. Following 2 weeks of unloading, sarcoplasmic reticulum (SR) Ca²⁺ content was reduced by ~55 %. Atrophic cardiac myocytes also showed a much lower frequency of spontaneous diastolic Ca²⁺ sparks and a diminished systolic Ca²⁺ release, even though the expression of ryanodine receptors was increased by ~30 %. In contrast, current clamp recordings revealed prolonged action potentials in endocardial as well as epicardial myocytes which were associated with a two to fourfold higher sarcolemmal Ca²⁺ influx under action potential clamp. In addition, Cav1.2 subunits which form the pore of L-type Ca²⁺ channels (LTCC) were upregulated in atrophic myocardium. These data suggest that in early cardiac atrophy induced by mechanical unloading, an augmented sarcolemmal Ca²⁺ influx through LTCC fully compensates for a reduced systolic SR Ca²⁺ release to preserve the Ca²⁺ transient. This interplay involves an electrophysiological remodelling as well as changes in the expression of cardiac ion channels.

Experimental hyperthyroidism decreases gene expression and serum levels of adipokines in obesity

Aims. To analyze the influence of hyperthyroidism on the gene expression and serum concentration of leptin, resistin, and adiponectin in obese animals. Main Methods. Male Wistar rats were randomly divided into two groups: control (C)—fed with commercial chow ad libitum—and obese (OB)—fed with a hypercaloric diet. After group characterization, the OB rats continued receiving a hypercaloric diet and were randomized into two groups: obese animals (OB) and obese with 25 μg triiodothyronine (T₃)/100 BW (OT). The T₃ dose was administered every day for the last 2 weeks of the study. After 30 weeks the animals were euthanized. Samples of blood and adipose tissue were collected for biochemical and hormonal analyses as well as gene expression of leptin, resistin, and adiponectin. Results. T₃ treatment was effective, increasing fT₃ levels and decreasing fT₄ and TSH serum concentration. Administration of T₃ promotes weight loss, decreases all fat deposits, and diminishes serum levels of leptin, resistin, and adiponectin by reducing their gene expression. Conclusions. Our results suggest that T₃ modulate serum and gene expression levels of leptin, resistin, and adiponectin in experimental model of obesity, providing new insights regarding the relationship between T₃ and adipokines in obesity.

Thyroid hormone and chronically unloaded hearts

The heart is subjected to chronic mechanical unloading during prolonged spaceflight and microgravity. The heart in patients with end-stage heart failure is also unloaded in prolonged duration after left ventricular assist devices (LVAD) are implanted. Heterotopic heart transplantation in rats is an established model of chronic cardiac unloading, and has been used to investigate the effects of chronic cardiac unloading on the heart. Observations that have been found using this experimental model are as follow. Chronic cardiac unloading induces time-dependent depressions of Ca2+ handling and myocyte contractility, which are associated with the shift of myosin heavy chain (MHC) isozymes and altered expressions of Ca2+ cycling-related proteins. Treatment with the physiological treatment dose of thyroid hormone restores the expression levels of Ca2+ cycling-related proteins, Ca2+ handling, and contractile function of cardiac myocytes in chronically unloaded hearts. Although future studies are required to determine precise mechanisms of the beneficial effects of thyroid hormone on chronically unloaded hearts, these observations may have clinical implications in the future for chronic cardiac unloading in the space industry as well as in the treatment of patients with end-stage heart failure supported by LVAD.