Increased Na⁺/Ca²⁺ exchanger expression/activity constitutes a point of inflection in the progression to heart failure of hypertensive rats

Calcium Handling Remodeling Underlies Impaired Sympathetic Stress Response in Ventricular Myocardium from Cacna1c Haploinsufficient Rats

CACNA1C encodes the pore-forming α1C subunit of the L-type Ca²⁺ channel, Cav1.2. Mutations and polymorphisms of the gene are associated with neuropsychiatric and cardiac disease. Haploinsufficient Cacna1c^+/- rats represent a recently developed model with a behavioral phenotype, but its cardiac phenotype is unknown. Here, we unraveled the cardiac phenotype of Cacna1c^+/- rats with a main focus on cellular Ca²⁺ handling mechanisms. Under basal conditions, isolated ventricular Cacna1c^+/- myocytes exhibited unaltered L-type Ca²⁺ current, Ca²⁺ transients (CaTs), sarcoplasmic reticulum (SR) Ca²⁺ load, fractional release, and sarcomere shortenings. However, immunoblotting of left ventricular (LV) tissue revealed reduced expression of Cav1.2, increased expression of SERCA2a and NCX, and augmented phosphorylation of RyR2 (at S2808) in Cacna1c^+/- rats. The β-adrenergic agonist isoprenaline increased amplitude and accelerated decay of CaTs and sarcomere shortenings in both Cacna1c^+/- and WT myocytes. However, the isoprenaline effect on CaT amplitude and fractional shortening (but not CaT decay) was impaired in Cacna1c^+/- myocytes exhibiting both reduced potency and efficacy. Moreover, sarcolemmal Ca²⁺ influx and fractional SR Ca²⁺ release after treatment with isoprenaline were smaller in Cacna1c^+/- than in WT myocytes. In Langendorff-perfused hearts, the isoprenaline-induced increase in RyR2 phosphorylation at S2808 and S2814 was attenuated in Cacna1c^+/- compared to WT hearts. Despite unaltered CaTs and sarcomere shortenings, Cacna1c^+/- myocytes display remodeling of Ca²⁺ handling proteins under basal conditions. Mimicking sympathetic stress with isoprenaline unmasks an impaired ability to stimulate Ca²⁺ influx, SR Ca²⁺ release, and CaTs caused, in part, by reduced phosphorylation reserve of RyR2 in Cacna1c^+/- cardiomyocytes.

CBL aggravates Ang II-induced cardiac hypertrophy via the VHL/HIF-1α pathway

CBL (Casitas B cell lymphoma), an important ubiquitin protein ligase, is involved in protein folding, protein maturation, and proteasome-dependent protein catabolism in different cells. However, its role in cardiac hypertrophy is still unclear. In this study, we found that expression of CBL is increased in an Ang II-induced mouse cardiac hypertrophy animal model and in Ang II-treated H9C2 cells. Interference with CBL expression attenuates the degree of myocardial hypertrophy as well as the expression of hypertrophy-related genes in H9C2 cells. Further research found that CBL aggravates myocardial hypertrophy by activating HIF-1α, which is an aggravating factor for hypertrophy. The effect of CBL on promoting myocardial hypertrophy was reversed by interference with HIF-1α. Mechanistically, we found that CBL directly interacted with and degraded VHL by increasing its ubiquitination level, which is a widely accepted regulatory factor of HIF-1α. Finally, our results showed that CBL was partially dependent on degradation of VHL and that activation of HIF-1α promoted myocardial hypertrophy. Collectively, these findings suggest that strategies based on activation of the CBL/HIF-1α axis might be promising for the treatment of hypertrophic cardiomyopathy.

Simultaneous study of mechanobiology and calcium dynamics on hESC-derived cardiomyocytes clusters

Calcium ions act like ubiquitous second messengers in a wide amount of cellular processes. In cardiac myocytes, Ca²⁺ handling regulates the mechanical contraction necessary to the heart pump function. The field of intracellular and intercellular Ca²⁺ handling, employing in vitro models of cardiomyocytes, has become a cornerstone to understand the role and adaptation of calcium signalling in healthy and diseased hearts. Comprehensive in vitro systems and cell-based biosensors are powerful tools to enrich and speed up cardiac phenotypic and drug response evaluation. We have implemented a combined setup to measure contractility and calcium waves in human embryonic stem cells-derived cardiomyocyte 3D clusters, obtained from embryoid body differentiation. A combination of atomic force microscopy to monitor cardiac contractility, and sensitive fast scientific complementary metal-oxide-semiconductor camera for epifluorescence video recording, provided correlated signals in real time. To speed up the integrated data processing, we tested several post-processing algorithms, to improve the automatic detection of relevant functional parameters. The validation of our proposed method was assessed by caffeine stimulation (10mM) and detection/characterization of the induced cardiac response. We successfully report the first simultaneous recording of cardiac contractility and calcium waves on the described cardiac 3D models. The drug stimulation confirmed the automatic detection capabilities of the used algorithms, measuring expected physiological response, such as elongation of contraction time and Ca²⁺ cytosolic persistence, increased calcium basal fluorescence, and transient peaks. These results contribute to the implementation of novel, integrated, high-information, and reliable experimental systems for cardiac models and drug evaluation.

Cardiomyocyte Functional Etiology in Heart Failure With Preserved Ejection Fraction Is Distinctive-A New Preclinical Model

Background

Among the growing numbers of patients with heart failure, up to one half have heart failure with preserved ejection fraction (HFpEF). The lack of effective treatments for HFpEF is a substantial and escalating unmet clinical need—and the lack of HFpEF‐specific animal models represents a major preclinical barrier in advancing understanding of HFpEF. As established treatments for heart failure with reduced ejection fraction (HFrEF) have proven ineffective for HFpEF, the contention that the intrinsic cardiomyocyte phenotype is distinct in these 2 conditions requires consideration. Our goal was to validate and characterize a new rodent model of HFpEF, undertaking longitudinal investigations to delineate the associated cardiac and cardiomyocyte pathophysiology.

Methods and Results

The selectively inbred Hypertrophic Heart Rat (HHR) strain exhibits adult cardiac enlargement (without hypertension) and premature death (40% mortality at 50 weeks) compared to its control strain, the normal heart rat. Hypertrophy was characterized in vivo by maintained systolic parameters (ejection fraction at 85%–90% control) with marked diastolic dysfunction (increased E/E′). Surprisingly, HHR cardiomyocytes were hypercontractile, exhibiting high Ca²⁺ operational levels and markedly increased L‐type Ca²⁺ channel current. In HHR, prominent regions of reparative fibrosis in the left ventricle free wall adjacent to the interventricular septum were observed.

Conclusions

Thus, the cardiomyocyte remodeling process in the etiology of this HFpEF model contrasts dramatically with the suppressed Ca²⁺ cycling state that typifies heart failure with reduced ejection fraction. These findings may explain clinical observations, that treatments considered appropriate for heart failure with reduced ejection fraction are of little benefit for HFpEF—and suggest a basis for new therapeutic strategies.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

How does pressure overload cause cardiac hypertrophy and dysfunction? High-ouabain affinity cardiac Na+ pumps are crucial

Left ventricular hypertrophy is frequently observed in hypertensive patients and is believed to be due to the pressure overload and cardiomyocyte stretch. Three recent reports on mice with genetically engineered Na⁺ pumps, however, have demonstrated that cardiac ouabain-sensitive α₂-Na⁺ pumps play a key role in the pathogenesis of transaortic constriction-induced hypertrophy. Hypertrophy was delayed/attenuated in mice with mutant, ouabain-resistant α₂-Na⁺ pumps and in mice with cardiac-selective knockout or transgenic overexpression of α₂-Na⁺ pumps. The latter, seemingly paradoxical, findings can be explained by comparing the numbers of available (ouabain-free) high-affinity (α₂) ouabain-binding sites in wild-type, knockout, and transgenic hearts. Conversely, hypertrophy was accelerated in α₂-ouabain-resistant (R) mice in which the normally ouabain-resistant α₁-Na⁺ pumps were mutated to an ouabain-sensitive (S) form (α₁^S/Sα₂^R/R or "SWAP" vs. wild-type or α₁^R/R α₂^S/S mice). Furthermore, transaortic constriction-induced hypertrophy in SWAP mice was prevented/reversed by immunoneutralizing circulating endogenous ouabain (EO). These findings show that EO and its receptor, ouabain-sensitive α₂, are critical factors in pressure overload-induced cardiac hypertrophy. This complements reports linking elevated plasma EO to hypertension, cardiac hypertrophy, and failure in humans and elucidates the underappreciated role of the EO-Na⁺ pump pathway in cardiovascular disease.

Sex-Specific Effects of Adrenergic-Induced Left Ventricular Remodeling in Spontaneously Hypertensive Rats

OBJECTIVE

The aim of this work was to determine whether adrenergic-induced left ventricular (LV) dilation and eccentric remodeling in pressure-overload hypertrophy is sex specific.

METHODS AND RESULTS

Chronic β-adrenoreceptor activation was produced in male and female spontaneously hypertensive rats (SHRs) by means of daily administration of isoproterenol (ISO; 0.04 mg/kg daily) from 9 to 15 months of age. LV chamber dimensions were determined in vivo by means of echocardiography and ex vivo in isolated perfused heart preparations. The acute hemodynamic response to ISO, the degree of myocardial necrosis and apoptosis, and collagen distribution were also assessed. Female SHRs demonstrated inotropic and chronotropic responses to ISO similarly to male SHRs. Compared with control subjects (saline solution vehicle), following chronic ISO administration, LV end-diastolic diameter (mm) was increased in male (ISO 7.8 ± 0.3 vs control 6.6 ± 0.2; P < .001) but not in female (ISO 6.3 ± 0.2 vs control 6.2 ± 0.2; P = .23) SHRs. Similarly, compared with control, ISO administration increased the volume intercept of the LV end-diastolic pressure-volume relation (mL) in male (ISO 0.31 ± 0.02 vs control 0.22 ± 0.01; P < .0001) but not in female (ISO 0.17 ± 0.01 vs control 0.17 ± 0.01; P = 1.00) SHRs. Relative wall thickness was also decreased in male SHRs receiving ISO but not in female SHRs receiving ISO. Chronic ISO administration increased the percentage of area covered by interstitial collagen in male but not in female SHRs. Finally, chronic adrenergic stimulation failed to influence LV chamber or myocardial systolic function in either male or female SHRs.

CONCLUSIONS

Male SHRs are more susceptible to adrenergic-induced LV dilation and eccentric LV remodeling than female SHRs. These effects are associated with increased collagen deposition. In pressure-overload hypertrophy, LV dilation and eccentric LV remodeling occur before LV dysfunction in male rats.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Low-Level Vagus Nerve Stimulation Reverses Cardiac Dysfunction and Subcellular Calcium Handling in Rats With Post-Myocardial Infarction Heart Failure

Vagus nerve stimulation (VNS), targeting the imbalanced autonomic nervous system, is a promising therapeutic approach for chronic heart failure (HF). Moreover, calcium cycling is an important part of cardiac excitation-contraction coupling (ECC), which also participates in the antiarrhythmic effects of VNS. We hypothesized that low-level VNS (LL-VNS) could improve cardiac function by regulation of intracellular calcium handling properties. The experimental HF model was established by ligation of the left anterior descending coronary artery (LAD). Thirty-two male Sprague-Dawley rats were divided into 3 groups as follows; control group (sham operated without coronary ligation, n = 10), HF-VNS group (HF rats with VNS, n = 12), and HF-SS group (HF rats with sham nerve stimulation, n = 10). After 8 weeks of treatment, LL-VNS significantly improved left ventricular ejection fraction (LVEF) and attenuated myocardial interstitial fibrosis in the HF-VNS group compared with the HF-SS group. Elevated plasma norepinephrine and dopamine, but not epinephrine, were partially reduced by LL-VNS. Additionally, LL-VNS restored the protein and mRNA levels of sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a), Na(+)-Ca(2+) exchanger 1 (NCX1), and phospholamban (PLB) whereas the expression of ryanodine receptor 2 (RyR2) as well as mRNA level was unaffected. Thus, our study results suggest that the improvement of cardiac performance by LL-VNS is accompanied by the reversal of dysfunctional calcium handling properties including SERCA2a, NCX1, and PLB which may be a potential molecular mechanism of VNS for HF.