Mechanical unloading versus neurohumoral stimulation on myocardial structure and endocrine function In vivo

Does Myocardial Atrophy Represent Anti-Arrhythmic Phenotype?

This review focuses on cardiac atrophy resulting from mechanical or metabolic unloading due to various conditions, describing some mechanisms and discussing possible strategies or interventions to prevent, attenuate or reverse myocardial atrophy. An improved awareness of these conditions and an increased focus on the identification of mechanisms and therapeutic targets may facilitate the development of the effective treatment or reversion for cardiac atrophy. It appears that a decrement in the left ventricular mass itself may be the central component in cardiac deconditioning, which avoids the occurrence of life-threatening arrhythmias. The depressed myocardial contractility of atrophied myocardium along with the upregulation of electrical coupling protein, connexin43, the maintenance of its topology, and enhanced PKCƐ signalling may be involved in the anti-arrhythmic phenotype. Meanwhile, persistent myocardial atrophy accompanied by oxidative stress and inflammation, as well as extracellular matrix fibrosis, may lead to severe cardiac dysfunction, and heart failure. Data in the literature suggest that the prevention of heart failure via the attenuation or reversion of myocardial atrophy is possible, although this requires further research.

Dynamic and static biomechanical traits of cardiac fibrosis

Cardiac fibrosis is a common pathology in cardiovascular diseases which are reported as the leading cause of death globally. In recent decades, accumulating evidence has shown that the biomechanical traits of fibrosis play important roles in cardiac fibrosis initiation, progression and treatment. In this review, we summarize the four main distinct biomechanical traits (i.e., stretch, fluid shear stress, ECM microarchitecture, and ECM stiffness) and categorize them into two different types (i.e., static and dynamic), mainly consulting the unique characteristic of the heart. Moreover, we also provide a comprehensive overview of the effect of different biomechanical traits on cardiac fibrosis, their transduction mechanisms, and in-vitro engineered models targeting biomechanical traits that will aid the identification and prediction of mechano-based therapeutic targets to ameliorate cardiac fibrosis.

Angiotensin (1-12) in Humans With Normal Blood Pressure and Primary Hypertension

The importance of canonical versus noncanonical mechanisms for the generation of angiotensins remains a major challenge that, in part, is heavily swayed by the relative efficacy of therapies designed to inhibit renin, ACE (angiotensin-converting enzyme), or the Ang II (Angiotensin II) receptor. Ang (1-12) (angiotensin [1-12]) is an Ang II forming substrate serving as a source for Ang II-mediated tissue actions. This study identifies for the first time the presence of Ang (1-12) in the blood of 52 normal (22 women) and 19 (13 women) patients with hypertension not receiving antihypertensive medication at the time of the study. Normal subjects of comparable ages and body habitus had similar circulating plasma Ang (1-12) concentrations (women: 2.02±0.62 [SD] ng/mL; men 2.05±0.55 [SD] ng/mL, P>0.05). The higher values of plasma Ang (1-12) concentrations in hypertensive men (2.51±0.49 ng/mL, n=6) and women (2.33±0.63 [SD] ng/mL, n=13) were statistically significant (P<0.02) and correlated with elevated plasma renin activity, systolic and pulse pressure, and plasma concentrations of NT-proBNP (N-terminal prohormone BNP). The increased plasma Ang (1-12) in patients with hypertension was not mirrored by similar changes in plasma angiotensinogen and Ang II concentrations. The first identification of an age-independent presence of Ang (1-12) in the blood of normotensive subjects and patients with hypertension, irrespective of sex, implicates this non-renin dependent substrate as a source for Ang II production in the blood and its potential contribution to the hypertensive process.

Current animal models for the study of congestion in heart failure: an overview

Congestion (i.e., backward failure) is an important culprit mechanism driving disease progression in heart failure. Nevertheless, congestion remains often underappreciated and clinicians underestimate the importance of congestion on the pathophysiology of decompensation in heart failure. In patients, it is however difficult to study how isolated congestion contributes to organ dysfunction, since heart failure and chronic kidney disease very often coexist in the so-called cardiorenal syndrome. Here, we review the existing relevant and suitable backward heart failure animal models to induce congestion, induced in the left- (i.e., myocardial infarction, rapid ventricular pacing) or right-sided heart (i.e., aorta-caval shunt, mitral valve regurgitation, and monocrotaline), and more specific animal models of congestion, induced by saline infusion or inferior vena cava constriction. Next, we examine critically how representative they are for the clinical situation. After all, a relevant animal model of isolated congestion offers the unique possibility of studying the effects of congestion in heart failure and the cardiorenal syndrome, separately from forward failure (i.e., impaired cardiac output). In this respect, new treatment options can be discovered.

Selective abdominal venous congestion to investigate cardiorenal interactions in a rat model

Abdominal congestion may play an important role in the cardiorenal syndrome and has been demonstrated to drive disease progression. An animal model for abdominal congestion, without other culprit mechanisms that are often present in patients such as low cardiac output or chronic kidney disease, might be interesting to allow a better study of the pathophysiology of the cardiorenal syndrome. The objective of this study was to develop a clinically relevant and valid rat model with abdominal venous congestion and without pre-existing heart and/or kidney dysfunction. To do so, a permanent surgical constriction (20 Gauge) of the thoracic inferior vena cava (IVC) was applied in male Sprague Dawley rats (IVCc, n = 7), which were compared to sham-operated rats (SHAM, n = 6). Twelve weeks after surgery, abdominal venous pressure (mean: 13.8 vs 4.9 mmHg, p < 0.01), plasma creatinine (p < 0.05), plasma cystatin c (p < 0.01), urinary albumin (p < 0.05), glomerular surface area (p < 0.01) and width of Bowman's space (p < 0.05) of the IVCc group were significantly increased compared to the SHAM group for a comparable absolute body weight between groups (559 vs 530g, respectively, p = 0.73). Conventional cardiac echocardiographic and hemodynamic parameters did not differ significantly between both groups, indicating that cardiac function was not compromised by the surgery. In conclusion, we demonstrate that constriction of the thoracic IVC in adult rats is feasible and significantly increases the abdominal venous pressure to a clinically relevant level, thereby inducing abdominal venous congestion.

Combination of Hydralazine and Isosorbide-Dinitrate in the Treatment of Patients with Heart Failure with Reduced Ejection Fraction

The use of direct acting vasodilators (the combination of hydralazine and isosorbide dinitrate -Hy+ISDN-) in heart failure with reduced ejection fraction (HFrEF) is supported by evidence, but rarely used.However, treatment with Hy+ISDN is guideline-recommended for HFrEF patients who cannot receive either angiotensin-converting enzyme inhibitors or angiotensin receptor blockers due to intolerance or contraindication, and in self-identified African-American HFrEF patients who are symptomatic despite optimal neurohumoral therapy.The Hy+ISDN combination has arterial and venous vasodilating properties. It can decrease preload and afterload, decrease left ventricular end-diastolic diameter and the volume of mitral regurgitation, reduce left atrial and left ventricular wall tension, decrease pulmonary artery pressure and pulmonary arterial wedge pressure, increase stroke volume, and improve left ventricular ejection fraction, as well as induce left ventricular reverse remodelling. Furthermore, Hy+ISDN combination has antioxidant property, it affects endothelial dysfunction beneficially and improves NO bioavailability. Because of these benefits, this combination can improve the signs and symptoms of heart failure, exercise capacity and quality of life, and, most importantly, reduce morbidity and mortality in well-defined subgroups of HFrEF patients.Accordingly, this therapeutic option can in many cases play an essential role in the treatment of HFrEF.

Cardiorenal fibrosis and dysfunction in aging: Imbalance in mediators and regulators of collagen

Cardiorenal fibrosis is a biological process that increases with age and contributes to dysfunction of the heart and kidney. While numerous circulating and tissue hormones, cytokines and enzymes have been identified in the development of cardiorenal fibrosis, several reports have suggested that the anti-fibrotic natriuretic peptide system (NPS), pro-fibrotic renin-angiotensin-aldosterone system (RAAS), transforming growth factor-beta 1 (TGF-β1), matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinases (TIMPs) are fundamental regulators and mediators of this process. However, the simultaneous assessment of these components in the development of age-mediated cardiorenal fibrotic remodeling is not completely understood. Thus, we assessed cardiorenal structure and function, the circulating NPS and RAAS and the cardiorenal tissue gene expression of collagen (Col) I, Col III, TGF-β1, MMP-9 and TIMP-1 in 2 and 20 month old Fischer rats. Our studies determined that aging was characterized by an increase in cardiorenal fibrosis that was accompanied with cardiorenal dysfunction. These alterations were associated with lower circulating atrial and C-type natriuretic peptides and higher angiotensin II and aldosterone levels in the aged rats. Moreover, we observed a decrease in Col I and III and an increase in TIMP- mRNA expressions in the aged heart and kidney, while TGF-β1 expression increased and MMP-9 decreased only in the aged kidney. We conclude that the age-mediated alterations in these fibrotic regulator and mediator profiles favors collagen accumulation due to an imbalance between the NPS and RAAS as well as a decline in the degradative pathway, thus suggesting a therapeutic opportunity to target these components.

Small dedifferentiated cardiomyocytes bordering on microdomains of fibrosis: evidence for reverse remodeling with assisted recovery

With the perspective of functional myocardial regeneration, we investigated small cardiomyocytes bordering on microdomains of fibrosis, where they are dedifferentiated re-expressing fetal genes, and determined: (1) whether they are atrophied segments of the myofiber syncytium, (2) their redox state, (3) their anatomic relationship to activated myofibroblasts (myoFb), given their putative regulatory role in myocyte dedifferentiation and redifferentiation, (4) the relevance of proteolytic ligases of the ubiquitin-proteasome system as a mechanistic link to their size, and (5) whether they could be rescued from their dedifferentiated phenotype. Chronic aldosterone/salt treatment (ALDOST) was invoked, where hypertensive heart disease with attendant myocardial fibrosis creates the fibrillar collagen substrate for myocyte sequestration, with propensity for disuse atrophy, activated myoFb, and oxidative stress. To address phenotype rescue, 4 weeks of ALDOST was terminated followed by 4 weeks of neurohormonal withdrawal combined with a regimen of exogenous antioxidants, ZnSO4, and nebivolol (assisted recovery). Compared with controls, at 4 weeks of ALDOST, we found small myocytes to be: (1) sequestered by collagen fibrils emanating from microdomains of fibrosis and representing atrophic segments of the myofiber syncytia, (2) dedifferentiated re-expressing fetal genes (β-myosin heavy chain and atrial natriuretic peptide), (3) proximal to activated myoFb expressing α-smooth muscle actin microfilaments and angiotensin-converting enzyme, (4) expressing reactive oxygen species and nitric oxide with increased tissue 8-isoprostane, coupled to ventricular diastolic and systolic dysfunction, and (5) associated with upregulated redox-sensitive proteolytic ligases MuRF1 and atrogin-1. In a separate study, we did not find evidence of myocyte replication (BrdU labeling) or expression of stem cell antigen (c-Kit) at weeks 1-4 ALDOST. Assisted recovery caused complete disappearance of myoFb from sites of fibrosis with redifferentiation of these myocytes, loss of oxidative stress, and ubiquitin-proteasome system activation, with restoration of nitric oxide and improved ventricular function. Thus, small dedifferentiated myocytes bordering on microdomains of fibrosis can re-differentiate and represent a potential source of autologous cells for functional myocardial regeneration.

Cardiomyocyte Ca2+ handling and structure is regulated by degree and duration of mechanical load variation

Cardiac transverse (t)-tubules are altered during disease and may be regulated by stretch-sensitive molecules. The relationship between variations in the degree and duration of load and t-tubule structure remains unknown, as well as its implications for local Ca²⁺-induced Ca²⁺ release (CICR). Rat hearts were studied after 4 or 8 weeks of moderate mechanical unloading [using heterotopic abdominal heart–lung transplantation (HAHLT)] and 6 or 10 weeks of pressure overloading using thoracic aortic constriction. CICR, cell and t-tubule structure were assessed using confocal-microscopy, patch-clamping and scanning ion conductance microscopy. Moderate unloading was compared with severe unloading [using heart-only transplantation (HAHT)]. Mechanical unloading reduced cardiomyocyte volume in a time-dependent manner. Ca²⁺ release synchronicity was reduced at 8 weeks moderate unloading only. Ca²⁺ sparks increased in frequency and duration at 8 weeks of moderate unloading, which also induced t-tubule disorganization. Overloading increased cardiomyocyte volume and disrupted t-tubule morphology at 10 weeks but not 6 weeks. Moderate mechanical unloading for 4 weeks had milder effects compared with severe mechanical unloading (37% reduction in cell volume at 4 weeks compared to 56% reduction after severe mechanical unloading) and did not cause depression and delay of the Ca²⁺ transient, increased Ca²⁺ spark frequency or impaired t-tubule and cell surface structure. These data suggest that variations in chronic mechanical load influence local CICR and t-tubule structure in a time- and degree-dependent manner, and that physiological states of increased and reduced cell size, without pathological changes are possible.

Simulation of dilated heart failure with continuous flow circulatory support

Lumped parameter models have been employed for decades to simulate important hemodynamic couplings between a left ventricular assist device (LVAD) and the native circulation. However, these studies seldom consider the pathological descending limb of the Frank-Starling response of the overloaded ventricle. This study introduces a dilated heart failure model featuring a unimodal end systolic pressure-volume relationship (ESPVR) to address this critical shortcoming. The resulting hemodynamic response to mechanical circulatory support are illustrated through numerical simulations of a rotodynamic, continuous flow ventricular assist device (cfVAD) coupled to systemic and pulmonary circulations with baroreflex control. The model further incorporated septal interaction to capture the influence of left ventricular (LV) unloading on right ventricular function. Four heart failure conditions were simulated (LV and bi-ventricular failure with/without pulmonary hypertension) in addition to normal baseline. Several metrics of LV function, including cardiac output and stroke work, exhibited a unimodal response whereby initial unloading improved function, and further unloading depleted preload reserve thereby reducing ventricular output. The concept of extremal loading was introduced to reflect the loading condition in which the intrinsic LV stroke work is maximized. Simulation of bi-ventricular failure with pulmonary hypertension revealed inadequacy of LV support alone. These simulations motivate the implementation of an extremum tracking feedback controller to potentially optimize ventricular recovery.

M-atrial natriuretic peptide and nitroglycerin in a canine model of experimental acute hypertensive heart failure: differential actions of 2 cGMP activating therapeutics

Background

Systemic hypertension is a common characteristic in acute heart failure (HF). This increasingly recognized phenotype is commonly associated with renal dysfunction and there is an unmet need for renal enhancing therapies. In a canine model of HF and acute vasoconstrictive hypertension we characterized and compared the cardiorenal actions of M‐atrial natriuretic peptide (M‐ANP), a novel particulate guanylyl cyclase (pGC) activator, and nitroglycerin, a soluble guanylyl cyclase (sGC) activator.

Methods and Results

HF was induced by rapid RV pacing (180 beats per minute) for 10 days. On day 11, hypertension was induced by continuous angiotensin II infusion. We characterized the cardiorenal and humoral actions prior to, during, and following intravenous M‐ANP (n=7), nitroglycerin (n=7), and vehicle (n=7) infusion. Mean arterial pressure (MAP) was reduced by M‐ANP (139±4 to 118±3 mm Hg, P<0.05) and nitroglycerin (137±3 to 116±4 mm Hg, P<0.05); similar findings were recorded for pulmonary wedge pressure (PCWP) with M‐ANP (12±2 to 6±2 mm Hg, P<0.05) and nitroglycerin (12±1 to 6±1 mm Hg, P<0.05). M‐ANP enhanced renal function with significant increases (P<0.05) in glomerular filtration rate (38±4 to 53±5 mL/min), renal blood flow (132±18 to 236±23 mL/min), and natriuresis (11±4 to 689±37 mEq/min) and also inhibited aldosterone activation (32±3 to 23±2 ng/dL, P<0.05), whereas nitroglycerin had no significant (P>0.05) effects on these renal parameters or aldosterone activation.

Conclusions

Our results advance the differential cardiorenal actions of pGC (M‐ANP) and sGC (nitroglycerin) mediated cGMP activation. These distinct renal and aldosterone modulating actions make M‐ANP an attractive therapeutic for HF with concomitant hypertension, where renal protection is a key therapeutic goal.

BNP molecular forms and processing by the cardiac serine protease corin

The cardiac hormone, B-type natriuretic peptide (BNP), is one of human natriuretic peptides which possesses cardiorenal protective actions and is used as a therapeutic and a biomarker for heart failure (HF). Its prohormone, proBNP1_108, is processed by the proNPs convertases, corin or furin, to inactive NT-proBNP1_76 and active BNP1-32. Paradoxically, circulating NT-proBNP and BNP are elevated in HF leading to the use of BNP as a sensitive and predictive marker of HF. This paradox may be explained by the "nonspecific" nature of conventional assays and/or a relative deficiency state of "active BNP" as characterized by an increase in inactive proBNP_108 and a decrease in active BNP1-32. Therefore, understanding the regulation of proBNP1_108 processing and the role of the convertase corin may be important in understanding the physiology of HF. Corin is expressed in heart and kidney and may play an important role in regulating blood pressure and remodeling of the heart. The processing of proBNP1_108 by corin may be controlled by O-linked glycosylation of proBNP1-108. A potential impairment of proBNP1lo8 processing in HF may be linked to dysregulation of the convertase corin, which may offer therapeutic opportunities to control proBNPlo0s processing and its activation in HF.

Electrophysiologic remodeling of the left ventricle in pressure overload-induced right ventricular failure

OBJECTIVES

The purpose of this study was to analyze the electrophysiologic remodeling of the atrophic left ventricle (LV) in right ventricular (RV) failure (RVF) after RV pressure overload.

BACKGROUND

The LV in pressure-induced RVF develops dysfunction, reduction in mass, and altered gene expression, due to atrophic remodeling. LV atrophy is associated with electrophysiologic remodeling.

METHODS

We conducted epicardial mapping in Langendorff-perfused hearts, patch-clamp studies, gene expression studies, and protein level studies of the LV in rats with pressure-induced RVF (monocrotaline [MCT] injection, n = 25; controls with saline injection, n = 18). We also performed epicardial mapping of the LV in patients with RVF after chronic thromboembolic pulmonary hypertension (CTEPH) (RVF, n = 10; no RVF, n = 16).

RESULTS

The LV of rats with MCT-induced RVF exhibited electrophysiologic remodeling: longer action potentials (APs) at 90% repolarization and effective refractory periods (ERPs) (60 ± 1 ms vs. 44 ± 1 ms; p < 0.001), and slower longitudinal conduction velocity (62 ± 2 cm/s vs. 70 ± 1 cm/s; p = 0.003). AP/ERP prolongation agreed with reduced Kcnip2 expression, which encodes the repolarizing potassium channel subunit KChIP2 (0.07 ± 0.01 vs. 0.11 ± 0.02; p < 0.05). Conduction slowing was not explained by impaired impulse formation, as AP maximum upstroke velocity, whole-cell sodium current magnitude/properties, and mRNA levels of Scn5a were unaltered. Instead, impulse transmission in RVF was hampered by reduction in cell length (111.6 ± 0.7 μm vs. 122.0 ± 0.4 μm; p = 0.02) and width (21.9 ± 0.2 μm vs. 25.3 ± 0.3 μm; p = 0.002), and impaired cell-to-cell impulse transmission (24% reduction in Connexin-43 levels). The LV of patients with CTEPH with RVF also exhibited ERP prolongation (306 ± 8 ms vs. 268 ± 5 ms; p = 0.001) and conduction slowing (53 ± 3 cm/s vs. 64 ± 3 cm/s; p = 0.005).

CONCLUSIONS

Pressure-induced RVF is associated with electrophysiologic remodeling of the atrophic LV.

Reverse remodeling and recovery from cachexia in rats with aldosteronism

The congestive heart failure (CHF) syndrome with soft tissue wasting, or cachexia, has its pathophysiologic origins rooted in neurohormonal activation. Mechanical cardiocirculatory assistance reveals the potential for reverse remodeling and recovery from CHF, which has been attributed to device-based hemodynamic unloading whereas the influence of hormonal withdrawal remains uncertain. This study addresses the signaling pathways induced by chronic aldosteronism in normal heart and skeletal muscle at organ, cellular/subcellular, and molecular levels, together with their potential for recovery (Recov) after its withdrawal. Eight-week-old male Sprague-Dawley rats were examined at 4 wk of aldosterone/salt treatment (ALDOST) and following 4-wk Recov. Compared with untreated, age-/sex-/strain-matched controls, ALDOST was accompanied by 1) a failure to gain weight, reduced muscle mass with atrophy, and a heterogeneity in cardiomyocyte size across the ventricles, including hypertrophy and atrophy at sites of microscopic scarring; 2) increased cardiomyocyte and mitochondrial free Ca(2+), coupled to oxidative stress with increased H(2)O(2) production and 8-isoprostane content, and increased opening potential of the mitochondrial permeability transition pore; 3) differentially expressed genes reflecting proinflammatory myocardial and catabolic muscle phenotypes; and 4) reversal to or toward recovery of these responses with 4-wk Recov. Aldosteronism in rats is accompanied by cachexia and leads to an adverse remodeling of the heart and skeletal muscle at organ, cellular/subcellular, and molecular levels. However, evidence presented herein implicates that these tissues retain their inherent potential for recovery after complete hormone withdrawal.

Clinical, molecular, and genomic changes in response to a left ventricular assist device

The use of left ventricular assist devices in treating patients with end-stage heart failure has increased significantly in recent years, both as a bridge to transplantation and as destination therapy in those who are ineligible for cardiac transplantation. This increase is based largely on the results of several recently completed clinical trials with the new second-generation continuous-flow devices that showed significant improvements in survival, functional capacity, and quality of life. Additional information on the use of the first- and second-generation left ventricular assist devices has come from a recently released report spanning the years 2006 to 2009, from the Interagency Registry for Mechanically Assisted Circulatory Support, a National Heart, Lung, and Blood Institute-sponsored collaboration between the U.S. Food and Drug Administration, the Centers for Medicare and Medicaid Services, and the scientific community. The authors review the latest clinical trials and data from the registry, with tight integration of the landmark molecular, cellular, and genomic research that accompanies the reverse remodeling of the human heart in response to a left ventricular assist device and functional recovery that has been reported in a subset of these patients.

Right ventricular failure following chronic pressure overload is associated with reduction in left ventricular mass: evidence for atrophic remodeling

OBJECTIVES

We sought to study whether patients with right ventricular failure (RVF) secondary to chronic thromboembolic pulmonary hypertension (CTEPH) have reduced left ventricular (LV) mass, and whether LV mass reduction is caused by atrophy.

BACKGROUND

The LV in patients with CTEPH is underfilled (unloaded). LV unloading may cause atrophic remodeling that is associated with diastolic and systolic dysfunction.

METHODS

We studied LV mass using cardiac magnetic resonance imaging (MRI) in 36 consecutive CTEPH patients (before/after pulmonary endarterectomy [PEA]) and 11 healthy volunteers selected to match age and sex of patients. We studied whether LV atrophy is present in monocrotaline (MCT)-injected rats with RVF or controls by measuring myocyte dimensions and performing in situ hybridization.

RESULTS

At baseline, CTEPH patients with RVF had significantly lower LV free wall mass indexes than patients without RVF (35 ± 6 g/m(2) vs. 44 ± 7 g/m(2), p = 0.007) or volunteers (42 ± 6 g/m(2), p = 0.006). After PEA, LV free wall mass index increased (from 38 ± 6 g/m(2) to 44 ± 9 g/m(2), p = 0.001), as right ventricular (RV) ejection fraction improved (from 31 ± 8% to 56 ± 12%, p < 0.001). Compared with controls, rats with RVF had reduced LV free wall mass and smaller LV free wall myocytes. Expression of atrial natriuretic peptide was higher, whereas that of α-myosin heavy chain and sarcoplasmic reticulum calcium ATPase-2 were lower in RVF than in controls, both in RV and LV.

CONCLUSIONS

RVF in patients with CTEPH is associated with reversible reduction in LV free wall mass. In a rat model of RVF, myocyte shrinkage due to atrophic remodeling contributed to reduction in LV free wall mass.

A novel atrial natriuretic peptide based therapeutic in experimental angiotensin II mediated acute hypertension

M-atrial natriuretic peptide (ANP; M-ANP) is a novel next generation 40 amino acid peptide based on ANP, which is highly resistant to enzymatic degradation and has greater and more sustained beneficial actions compared with ANP. The current study was designed to advance our understanding of the therapeutic potential of M-ANP in a canine model of acute angiotensin II-induced hypertension with elevated cardiac filling pressures and aldosterone activation. We compare M-ANP with vehicle and equimolar human B-type natriuretic peptide, which possesses the most potent in vivo actions of the native natriuretic peptides. M-ANP significantly lowered mean arterial pressure and systemic vascular resistance. Importantly, despite a reduction in blood pressure, renal function was enhanced with significant increases in renal blood flow, glomerular filtration rate, diuresis, and natriuresis after M-ANP infusion. Although angiotensin II induced an acute increase in pulmonary capillary wedge pressure, M-ANP significantly lowered pulmonary capillary wedge pressure, pulmonary artery pressure, and right atrial pressure. Further, M-ANP significantly suppressed angiotensin II-induced activation of aldosterone. These cardiovascular and renal enhancing actions of M-ANP were accompanied by significant increases in plasma and urinary cGMP, the second messenger molecule of the natriuretic peptide system. When compared with human B-type natriuretic peptide, M-ANP had comparable cardiovascular actions but resulted in a greater natriuretic effect. These results suggest that M-ANP, which is more potent than ANP in normal canines, has potent blood pressure lowering and renal enhancing properties and may, therefore, serve as an ANP based therapeutic for acute hypertension.

A human atrial natriuretic peptide gene mutation reveals a novel peptide with enhanced blood pressure-lowering, renal-enhancing, and aldosterone-suppressing actions

OBJECTIVES

We sought to determine the physiologic actions and potential therapeutic applications of mutant atrial natriuretic peptide (mANP).

BACKGROUND

The cardiac hormone atrial natriuretic peptide (ANP) is a 28-amino acid (AA) peptide that consists of a 17-AA ring structure together with a 6-AA N-terminus and a 5-AA C-terminus. In a targeted scan for sequence variants within the human ANP gene, a mutation was identified that results in a 40-AA peptide consisting of native ANP((1-28)) and a C-terminal extension of 12 AA. We have termed this peptide mutant ANP.

METHODS

In vitro 3',5'-cyclic guanosine monophosphate (cGMP) activation in response to mANP was studied in cultured human cardiac fibroblasts known to express natriuretic peptide receptor A. The cardiorenal and neurohumoral properties of mANP compared with ANP were assessed in vivo in normal dogs.

RESULTS

We observed an incremental in vitro cGMP dose response with increasing concentrations of mANP. In vivo with high-dose mANP (33 pmol/kg/min), we observed significantly greater plasma cGMP activation, diuretic, natriuretic, glomerular filtration rate enhancing, renin-angiotensin-aldosterone system inhibiting, cardiac unloading, and blood pressure lowering properties when compared with native ANP. Low-dose mANP (2 pmol/kg/min) has natriuretic and diuretic properties without altering systemic hemodynamics compared with no natriuretic or diuretic response with low-dose native ANP.

CONCLUSIONS

These studies establish that mANP activates cGMP in vitro and exerts greater and more sustained natriuretic, diuretic, glomerular filtration rate, and renal blood flow enhancing actions than native ANP in vivo.

Diastolic suction is impaired by bed rest: MRI tagging studies of diastolic untwisting

Bed rest deconditioning leads to physiological cardiac atrophy, which may compromise left ventricular (LV) filling during orthostatic stress by reducing diastolic untwisting and suction. To test this hypothesis, myocardial-tagged magnetic resonance imaging (MRI) was performed, and maximal untwisting rates of the endocardium, midwall, and epicardium were calculated by Harmonic Phase Analysis (HARP) before and after -6 degrees head-down tilt bed rest for 18 days with (n = 14) and without exercise training (n = 10). LV mass and LV end-diastolic volume were measured using cine MRI. Exercise subjects cycled on a supine ergometer for 30 min, three times per day at 75% maximal heart rate (HR). After sedentary bed rest, there was a significant reduction in maximal untwisting rates of the midwall (-46.8 +/- 14.3 to -35.4 +/- 12.4 degrees /s; P = 0.04) where untwisting is most reliably measured, and to a lesser degree of certainty in the endocardium (-50.3 +/- 13.8 to -40.1 +/- 18.5 degrees /s; P = 0.09); the epicardium was unchanged. In contrast, when exercise was performed in bed, untwisting rates were enhanced at the endocardium (-48.4 +/- 20.8 to -72.3 +/- 22.3 degrees /ms; P = 0.05) and midwall (-39.2 +/- 12.2 to -59.0 +/- 19.6 degrees /s; P = 0.03). The differential response was significant between groups at the endocardium (interaction P = 0.02) and the midwall (interaction P = 0.004). LV mass decreased in the sedentary group (156.4 +/- 30.3 to 149.5 +/- 27.9 g; P = 0.07), but it increased slightly in the exercise-trained subjects (156.4 +/- 34.3 to 162.3 +/- 40.5 g; P = 0.16); (interaction P = 0.03). We conclude that diastolic untwisting is impaired following sedentary bed rest. However, exercise training in bed can prevent the physiological cardiac remodeling associated with bed rest and preserve or even enhance diastolic suction.

BNP-induced activation of cGMP in human cardiac fibroblasts: interactions with fibronectin and natriuretic peptide receptors

Cardiac remodeling involves the accumulation of extracellular matrix (ECM) proteins including fibronectin (FN). FN contains RGD motifs that bind integrins at DDX sequences allowing signaling from the ECM to the nucleus. We noted that the natriuretic peptide receptor A (NPR-A) sequence contains both RGD and DDX sequences. The goal of the current investigation was to determine potential interactions between FN and NPR-A on BNP induction of cGMP in cultured human cardiac fibroblasts (CFs). Further, we sought to determine whether a Mayo designed NPR-A specific RGD peptide could modify this interaction. Here we reconfirm the presence of all three natriuretic peptide receptors (NPR) in CFs. CFs plated on FN demonstrated a pronounced increase in cGMP production to BNP compared to non-coated plates. This production was also enhanced by the NPR-A specific RGD peptide, which further augmented FN associated cGMP production. Addition of HS-142-1, a NPR-A/B antagonist, abrogated the responses of BNP to both FN and the NPR-A specific RGD peptide. Finally, we defined a possible role for the NPR-C through non-cGMP mechanisms in mediating the anti-proliferative actions of BNP in CFs where the NPR-C antagonist cANF 4-28 but not HS-142-1 blocked BNP-mediated inhibition of proliferation of CFs. We conclude that NPR-A interacts with components of the ECM such as FN to enhance BNP activation of cGMP and that a small NPR-A specific RGD peptide augments this action of BNP with possible therapeutic implications. Lastly, the NPR-C may also have a role in mediating anti-proliferative actions of BNP in CFs.

Myocardial dysfunction and neurohumoral activation without remodeling in left ventricle of monocrotaline-induced pulmonary hypertensive rats

In monocrotaline (MCT)-induced pulmonary hypertension (PH), only the right ventricle (RV) endures overload, but both ventricles are exposed to enhanced neuroendocrine stimulation. To assess whether in long-standing PH the left ventricular (LV) myocardium molecular/contractile phenotype can be disturbed, we evaluated myocardial function, histology, and gene expression of autocrine/paracrine systems in rats with severe PH 6 wk after subcutaneous injection of 60 mg/kg MCT. The overloaded RV underwent myocardial hypertrophy ( P < 0.001) and fibrosis ( P = 0.014) as well as increased expression of angiotensin-converting enzyme (ACE) (8-fold; P < 0.001), endothelin-1 (ET-1) (6-fold; P < 0.001), and type B natriuretic peptide (BNP) (15-fold; P < 0.001). Despite the similar upregulation of ET-1 (8-fold; P < 0.001) and overexpression of ACE (4-fold; P < 0.001) without BNP elevation, the nonoverloaded LV myocardium was neither hypertrophic nor fibrotic. LV indexes of contractility ( P < 0.001) and relaxation ( P = 0.03) were abnormal, however, and LV muscle strips from MCT-treated compared with sham rats presented negative ( P = 0.003) force-frequency relationships (FFR). Despite higher ET-1 production, BQ-123 (ETA antagonist) did not alter LV MCT-treated muscle strip contractility distinctly ( P = 0.005) from the negative inotropic effect exerted on shams. Chronic daily therapy with 250 mg/kg bosentan (dual endothelin receptor antagonist) after MCT injection not only attenuated RV hypertrophy and local neuroendocrine activation but also completely reverted FFR of LV muscle strips to positive values. In conclusion, the LV myocardium is altered in advanced MCT-induced PH, undergoing neuroendocrine activation and contractile dysfunction in the absence of hypertrophy or fibrosis. Neuroendocrine mediators, particularly ET-1, may participate in this functional deterioration.

Lower body negative pressure vs. lower body positive pressure to prevent cardiac atrophy after bed rest and spaceflight. What caused the controversy?

Cardiac muscle adapts well to changes in loading conditions. For example, left ventricular (LV) hypertrophy may be induced physiologically (via exercise training) or pathologically (via hypertension or valvular heart disease). If hypertension is treated, LV hypertrophy regresses, suggesting a sensitivity to LV work. However, whether physical inactivity in nonathletic populations causes adaptive changes in LV mass or even frank atrophy is not clear. We exposed previously sedentary men to 6 ( n = 5) and 12 ( n = 3) wk of horizontal bed rest. LV and right ventricular (RV) mass and end-diastolic volume were measured using cine magnetic resonance imaging (MRI) at 2, 6, and 12 wk of bed rest; five healthy men were also studied before and after at least 6 wk of routine daily activities as controls. In addition, four astronauts were exposed to the complete elimination of hydrostatic gradients during a spaceflight of 10 days. During bed rest, LV mass decreased by 8.0 ± 2.2% ( P = 0.005) after 6 wk with an additional atrophy of 7.6 ± 2.3% in the subjects who remained in bed for 12 wk; there was no change in LV mass for the control subjects (153.0 ± 12.2 vs. 153.4 ± 12.1 g, P = 0.81). Mean wall thickness decreased (4 ± 2.5%, P = 0.01) after 6 wk of bed rest associated with the decrease in LV mass, suggesting a physiological remodeling with respect to altered load. LV end-diastolic volume decreased by 14 ± 1.7% ( P = 0.002) after 2 wk of bed rest and changed minimally thereafter. After 6 wk of bed rest, RV free wall mass decreased by 10 ± 2.7% ( P = 0.06) and RV end-diastolic volume by 16 ± 7.9% ( P = 0.06). After spaceflight, LV mass decreased by 12 ± 6.9% ( P = 0.07). In conclusion, cardiac atrophy occurs during prolonged (6 wk) horizontal bed rest and may also occur after short-term spaceflight. We suggest that cardiac atrophy is due to a physiological adaptation to reduced myocardial load and work in real or simulated microgravity and demonstrates the plasticity of cardiac muscle under different loading conditions.

Cardiac-specific attenuation of natriuretic peptide A receptor activity accentuates adverse cardiac remodeling and mortality in response to pressure overload

Atrial (ANP) and brain (BNP) natriuretic peptides are hormones of myocardial cell origin. These hormones bind to the natriuretic peptide A receptor (NPRA) throughout the body, stimulating cGMP production and playing a key role in blood pressure control. Because NPRA receptors are present on cardiomyocytes, we hypothesized that natriuretic peptides may have direct autocrine or paracrine effects on cardiomyocytes or adjacent cardiac cells. Because both natriuretic peptides and NPRA gene expression are upregulated in states of pressure overload, we speculated that the effects of the natriuretic peptides on cardiac structure and function would be most apparent after pressure overload. To attenuate cardiomyocyte NPRA activity, transgenic mice with cardiac specific expression of a dominant-negative (DN-NPRA) mutation (HCAT D 893A) in the NPRA receptor were created. Cardiac structure and function were assessed (avertin anesthesia) in the absence and presence of pressure overload produced by suprarenal aortic banding. In the absence of pressure overload, basal and BNP-stimulated guanylyl cyclase activity assessed in cardiac membrane fractions was reduced. However, systolic blood pressure, myocardial cGMP, log plasma ANP levels, and ventricular structure and function were similar in wild-type (WT-NPRA) and DN-NPRA mice. In the presence of pressure overload, myocardial cGMP levels were reduced, and ventricular hypertrophy, fibrosis, filling pressures, and mortality were increased in DN-NPRA compared with WT-NPRA mice. In addition to their hormonal effects, endogenous natriuretic peptides exert physiologically relevant autocrine and paracrine effects via cardiomyocyte NPRA receptors to modulate cardiac hypertrophy and fibrosis in response to pressure overload.

Atrial BNP endocrine function during chronic unloading of the normal canine heart

The goal of the study was to define the effect of chronic unloading of the normal heart on atrial endocrine function with a focus on brain natriuretic peptide (BNP), specifically addressing the role of load and neurohumoral stimulation. Although produced primarily by atrial myocardium in the normal heart, controversy persists with regard to load-dependent vs. neurohumoral mechanisms controlling atrial BNP synthesis and storage. We used a unique canine model of chronic unloading of the heart produced by thoracic inferior vena caval constriction (TIVCC), which also resulted in activation of plasma endothelin (ET-1), ANG II, and norepinephrine (NE), known activators of BNP synthesis, compared with sham. TIVCC was produced by banding of the inferior vena cava for 10 days (n = 6), whereas in control (n = 5) the band was not constricted (sham). In a third group (n = 7), the band was released on day 11, thus acutely reloading the heart. Chronic TIVCC decreased cardiac output and right atrial pressure with a decrease in atrial mass index consistent with atrial atrophy. Atrial BNP mRNA decreased compared with sham. Immunoelectron microscopy revealed an increase in BNP in atrial granules consistent with increased storage. Acute reloading increased cardiac filling pressures and resulted in an increase in plasma BNP. We conclude that chronic unloading of the normal heart results in atrial atrophic remodeling and in suppression of atrial BNP mRNA despite intense stimulation by ET, ANG II, and NE, underscoring the primacy of load in the control of atrial endocrine function and structure.

Myocardial Hemodynamics, Physiology, and Perfusion With an Axial Flow Left Ventricular Assist Device in the Calf

The Jarvik 2000 axial flow left ventricular assist device (LVAD) is used clinically as a bridge to transplantation or as destination therapy in end-stage heart disease. The effect of the pump's continuous flow output on myocardial and end-organ blood flow has not been studied experimentally. To address this, the Jarvik 2000 pump was implanted in eight calves and then operated at speeds ranging from 8,000 to 12,000 rpm. Micromanometry, echocardiography, and blood oxygenation measurements were used to assess changes in hemodynamics, cardiac dimensions, and myocardial metabolism, respectively, at different speeds as compared with baseline (pump off, 0 rpm) in this experimental model. Microsphere studies were performed to assess the effects on heart, kidney, and brain perfusion at different speeds. The Jarvik 2000 pump unloaded the left ventricle and reduced end-diastolic pressures and left ventricular dimensions, particularly at higher pump speeds. The ratio of myocardial oxygen consumption to coronary blood flow and the ratio of subendocardial to subepicardial blood flow remained constant. Optimal adjustment of pump speed and volume status allowed opening of the aortic valve and contribution of the native left ventricle to cardiac output, even at the maximum pump speed. Neither brain nor kidney microcirculation was adversely affected at any pump speed. We conclude that the Jarvik 2000 pump adequately unloads the left ventricle without compromising myocardial metabolism or end-organ perfusion.

Assessment of myocardial recovery in a patient with acute myocarditis supported with a left ventricular assist device: a case report

Acute myocarditis may present with profound hemodynamic compromise; however, spontaneous resolution of the inflammatory process may occur in up to half of such patients. In patients with fulminant myocarditis, mechanical circulatory support may serve as a bridge to myocardial recovery. In this report we describe a 35-year-old man with acute myocarditis who required left ventricular assist device support as a bridge to recovery, and suggest a method for determining the suitability and timing of device explantation. A combination of echocardiography, right heart catheterization, exercise testing and serial endomyocardial biopsies was used to determine the resolution of myocarditis, recovery of myocardial function and timing for device explantation. Successful device explantation was performed after 37 days of device support. Further study is required to assess the role of ventricular assist devices in combination with immunosuppressive therapy in the management of fulminant myocarditis.

Physiologic and hemodynamic basis of ventricular assist devices

Heart failure is a particularly complex disorder with etiology that is primary in nature or secondary to other systemic diseases, including hypertension, diabetes, and atherosclerosis. The pathogenesis appears to result, in part, from extensive abnormal interactions among tissues, such as the heart, vasculature, kidney, lungs, and sympathetic nervous system. Improvements in understanding this complex disorder, particularly factors that contribute to cardiac cell cycle alterations, gene activation and re-expression resulting in cardiac remodeling and, eventually, maladaption are paramount. Clinical experience with the current generation of mechanical blood pumps continues to be promising; nonetheless, these devices are not the definitive therapy for all patients with heart failure. The next generation of devices capable of mimicking many of the native heart pump attributes, such as responsiveness to preload, afterload, contractility, and beat rate, will broaden the use of this technology. In addition to solving the fundamental engineering challenges (size, energy supply, biocompatibility, durability, and portability), implantable heart pumps that are physiologically adaptive would enhance the treatment strategies for prolonged chronic support. The ultimate measure of device mediated success is to show improvements that extend beyond a favorable hemodynamic profile and include nutritional status and metabolic and neurohormonal levels and must demonstrate improved exercise tolerance and a better quality of life.

Heterotopic transplantation as a model to study functional recovery of unloaded failing hearts

OBJECTIVES

Recent studies have demonstrated cardiac improvement in patients supported with a ventricular assist device, suggesting that reverse remodeling and myocardial recovery are possible. We developed an animal model of cardiac unloading by adapting a heterotopic transplantation technique and used it to examine the pattern of functional recovery in the left ventricle of the failing heart.

METHODS

Heart failure was induced in adult New Zealand rabbits by coronary artery ligation with subsequent myocardial infarction. Animals undergoing sham operation served as a control group. After 4 weeks or 3 months, failing hearts were transplanted into the necks of recipient rabbits. A left ventricular latex balloon connected to subcutaneous tubing allowed repeated physiologic analysis on days 1 and after transplantation and then every 5 days until day 30.

RESULTS

Contractility (left ventricular dP/dt(max)) and relaxation (left ventricular dP/dt(min)) were significantly lower in transplanted postinfarction hearts as compared to control hearts immediately after transplantation. Both left ventricular dP/dt(max) and left ventricular dP/dt(min) responses to increased preload and to beta-adrenergic stimulation progressively improved to a significantly higher level after 30 days of left ventricular unloading for the hearts that were transplanted 4 weeks after myocardial infarction. However, this functional improvement was not detected in failing hearts transplanted 3 months after infarction.

CONCLUSIONS

This model of cardiac unloading appears at least partially to mimic conditions of ventricular assist devices. If performed early in the development of heart failure, it permits improvement of contractile dysfunction and restoration of cardiac responsiveness to mechanical and beta-adrenergic stimulation. Therefore this model may constitute a novel alternative in the study of reverse remodeling in unloaded failing hearts.

Predominant activation of endothelin-dependent cardiac hypertrophy by norepinephrine in rat left ventricle

Locally released endothelin (ET)-1 has been recently identified as an important mediator of cardiac hypertrophy. It is still unclear, however, which primary stimulus specifically activates ET-dependent signaling pathways. We therefore examined in adult rats (n = 51) the effects of a selective ET(A) receptor antagonist in experimental models of cardiac hypertrophy, in which myocardial growth is predominantly initiated by a single primary stimulus. Rats were exposed to mechanical overload (ascending aortic stenosis), increased levels of circulating ANG II (ANG II infusion combined with hydralazine), or adrenergic stimulation (infusion of norepinephrine in a subpressor dose) for 7 days. All experimental treatments significantly increased left ventricular weight/body weight ratios compared with untreated rats, whereas systolic left ventricular peak pressure was increased only after ascending aortic stenosis. ET(A) receptor blockade exclusively reduced norepinephrine-induced cardiac hypertrophy and atrial natriuretic peptide gene expression. Blood pressure levels and heart rates remained unaffected during ET(A) receptor blockade in all experimental groups. These data indicate that in rat left ventricle, the ET-dependent signaling pathway leading to early development of cardiac hypertrophy and fetal gene expression is primarily activated by norepinephrine.