The Biological Response to Ventricular Unloading

Introduction

Heart failure (HF) is one of the major medical burdens in our society. Complete recovery of the heart after insult has been an attractive goal for many clinicians and scientists for long time. Mechanical circulatory support (MCS) devices, such as left ventricular assist devices (LVADs), effectively unload the heart by reducing the pressure overload/increased afterload, ultimately resulting in a degree of myocardial reverse remodeling. Currently, the predominantly used LVADs are continuous-flow devices, for patients suffering from end-stage HF.

The important research advantages of the VAD population are that they present us with a translational research medium through which investigations for new anti-remodeling and regenerative therapies for HF can be studied. Particularly, tissue specimens obtained from bridge to transplant (BTT) patients before LVAD implantation (referred to as “pre”) and at the time of LVAD explantation (referred to as “post”) provide a platform to perform molecular, structural, and functional studies. This in turn could pave the way for understanding the mechanisms behind progressive myocardial dysfunction during HF and the pathways that regulate reverse cardiac remodeling.

A subset of patients who undergo LVAD-induced mechanical unloading show significant improvements in cardiac structure and function to the point where some of these patients can be weaned from the support and achieve sustained recovery (i.e., responders), while the others show no such sign of recovery (i.e., nonresponders). Comparing responders and nonresponders in several aspects such as cardiac hypertrophy regression, contractile dysfunction, metabolism, cell death, extracellular matrix remodeling, and others could be helpful in revealing signatures of myocardial recovery, which in turn could help design novel HF therapies. However, in order to unmask the mechanisms driving myocardial recovery, the fundamental biological effects on myocardial structure and function must be understood. This chapter attempts to compile the ongoing clinical, translational, and basic science research in the field of MCS and cardiovascular remodeling.

Cardiac hypertrophy-atrophy

Myocardial unloading with a pulsatile flow LVAD has been shown to induce regression of cardiac myocyte hypertrophy. Animal models with prolonged unloading of nonfailing, nonhypertrophic myocardium (heterotopic transplantation, LVAD, or mitral regurgitation ) suggested that mechanical unloading could lead to cardiac myocyte atrophy. Based on these data it was postulated that this phenomenon may also apply to hypertrophic and failing myocardium. This notion of prolonged LVAD unloading in HF patients causing regression of cardiac hypertrophy to the point of atrophy prevailed until structural, ultrastructural, microstructural, metabolic, molecular, and clinical functional data indicated that prolonged continuous-flow LVAD unloading does not induce myocardial atrophy and degeneration ( Fig. 9.1 ). Specifically, in two human HF studies, unloading by means of pulsatile flow LVAD support decreased cardiac myocyte size, but not to the levels below respective normal cardiac myocytes. Furthermore, light ( Fig. 9.2 ) and confocal ( Fig. 9.3 ) microscopy findings complemented by ultrastructural, metabolic, and molecular data did not identify any evidence suggesting cardiac myocyte atrophy or degeneration during continuous-flow LVAD support. These data support echocardiographic observations in LVAD patients that show regression of hypertrophy without atrophy. Ongoing investigations have examined the roles of several complex regulatory pathways as critical components of these structural changes, including cyclooxygenase-2–induced Akt phosphorylation, mitogen-activated protein kinase/Erk, and Akt kinase/glycogen synthase kinase 3β. Also, a recent study reported that Akt activation stimulates glucose utilization but impairs mitochondrial oxidative capacity independently of cardiac hypertrophy. Whether the primary stimulus for the regression of hypertrophy is related directly to mechanical unloading/stretch or to circulating systemic factors needs further investigation.

Contractile dysfunction, calcium handling, and cytoskeletal proteins

Contractile function of the heart is conducted through composite processes such as sympathetic and parasympathetic heart rate control, core conduction system activities (thin [actin, tropomyosin, and troponin] and thick [myosin] filaments), calcium regulatory pathways that orchestrate shortening and relaxation, and humoral substances like catecholamines. The failing heart has high energy demands that place considerable stress on the cardiac contractile machinery, which in turn diminishes myocardial contractility. The myocyte contractile defects observed in failing hearts were shown to be reversed after pulsatile-flow LVAD unloading, with improved shortening and relaxation in isolated myocytes and isolated strips of ventricular tissue. Improvements in LVAD-induced contractile dysfunction can be partially explained by changes in Ca ⁺⁺ handling, such as faster sarcolemmal Ca ⁺⁺ entry and shorter action potential durations, higher sarcoplasmic reticulum Ca ⁺⁺ content, improved abundance of sarcoplasmic reticulum (SR)/endoplasmic reticulum calcium ATPase, decreased abundance of Na ⁺ /Ca ⁺⁺ exchanger, and beneficial changes in L-type calcium channel and ryanodine receptor (RyR) function. There is an increase in RyR clusters that are not near sarcolemmal L-type calcium channels due to reduced density of the tubular system (t-system); hence, they are not contained by couplons. This shift of RyR clusters away from sarcolemmal L-type calcium channels can affect the excitation-contraction coupling for two reasons: first, these nonjunctional RyR clusters can result in decreased calcium release from SR; and second, they create asynchronous calcium release due to a reduced open probability, which does not correlate with the quick opening of junctional RyR clusters. A recent study demonstrated a direct relationship linking the distance between nonjunctional RyR clusters and the change in left ventricular (LV) ejection fraction (EF) in post-LVAD patients. The authors reported that the EF improved when the distance was less than 1 μm, whereas a distance that exceeded 1 μm was not associated with improvement in EF. An intact t-system at the time of LVAD implantation may serve as a predictor for cardiac recovery induced through unloading ( Fig. 9.4 ).

The action potential initiates the excitation-contraction cycle in ventricular cardiomyocytes. Depolarization starts with sodium (Na ⁺ ) channels activating the L-type Ca ⁺⁺ current that causes local Ca ⁺⁺ -induced Ca ⁺⁺ release (CICR) in the SR. The increased cytosolic Ca ⁺⁺ binds to myofilaments, causing contraction followed by diastolic Ca ⁺⁺ elimination, i.e., dissociation of Ca ⁺⁺ from the contractile filaments and the excess cytosolic Ca ⁺⁺ extruded via SR-Ca ⁺⁺ ATPase, Na ⁺ -Ca ⁺⁺ exchanger, and sarcolemmal Ca ⁺⁺ pump. Abnormalities in Ca ⁺⁺ regulation may lead to increased diastolic Ca ⁺⁺ , which is the hallmark of contractile dysfunction in end-stage chronic HF patients. Therapeutic measures targeting Ca ⁺⁺ homeostasis may facilitate the reverse remodeling in HF patients. Fischer et al. recently showed that increased SR-Ca ⁺⁺ leak correlated with deteriorating LV function after LVAD implantation, and this SR-Ca ⁺⁺ leak was significantly reduced by Ca ⁺⁺ calmodulin-dependent protein kinase II (CaMKII) inhibition. CaMKII hyperphosphorylates RYR2, leading to disturbed diastolic closure of RYR2, and has a cascading effect on the Ca ⁺⁺ homeostasis in HF patients. The observation that CaMKII inhibition reduces the SR-Ca ⁺⁺ leak in HF suggests that CaMKII inhibition may be a promising therapeutic target for cardiac remodeling after LVAD implantation. Furthermore, CICR in rodent HF models suggests that the mechanical unloading by heterotopic abdominal heart transplantation increases calcium transient amplitude and normalizes the calcium spark frequencies and L-type calcium channel activity in cardiomyocytes. The role of short-term continuous-flow LVAD unloading was studied in an ovine model of acute myocardial infarction (MI), where it was observed that mechanical unloading prevented cardiac remodeling and dysfunction. Compared to a non-LVAD control group, calcium handling proteins were not altered, thereby preserving calcium cycling and improving cardiac function. These recent investigations demonstrate that extensive structural remodeling of the t-system, along with its depletion and change in orientation, plays an important role in the development and progression of ventricular dysfunction.

The effect of mechanical unloading on cytoskeletal proteins has been studied in LVAD clinical studies. Birks et al. showed significant differences (between the time of implantation and explantation/transplantation) in the regulation of nonsarcomeric proteins (lamin A/C, spectrin), integrins (β1, β5, β6, α5, and α7), and sarcomeric proteins that changed only in the recovered group (β-actin, α-tropomyosin, α1-actinin, and α-filament A). They also reported that vinculin, Wiskott-Aldrich syndrome protein, p21-activated kinase, Rho, and Graf levels decreased in the recovery group and increased in the nonrecovery group. These observations warrant further investigations on the role of the previously reported proteins in cardiac reverse remodeling. Cardiac ankyrin repeat protein (CARP) has been associated with cardiac hypertrophy, acts as transcription cofactor under mechanical stress and pressure overload, and has a critical role in the Nkx2.5 transcriptional pathway that regulates the ventricular muscle gene expression in the developing heart. A recent report on the CARP revealed increased levels in HF that eventually returned to normal levels after LVAD support. They also reported that CARP levels were increased after transaortic constriction in mice lacking βII spectrin, which the authors previously reported as an essential nodal protein following its significant alteration in their human and animal HF studies. Together, these observations indicate the potentially important role of cytoskeletal proteins in the myocardial reverse remodeling processes and further investigations are warranted.

Cardiac metabolism and bioenergetics

The heart utilizes multiple energy substrates like fatty acids, glucose, ketone bodies, lactate, and amino acids to meet its enormous energy demand for sustaining normal contractile function. This versatile use of substrates to meet its metabolic demand (i.e., the metabolic flexibility of the heart) is impaired during HF. This metabolic remodeling is integral to HF development and progression, with the failing heart reverting to a fetal pattern of energy substrate metabolism, i.e., enhanced glycolysis with decrease in fatty acid oxidation. This decrease in fatty acid metabolism seems beneficial in cardiac ischemic insults leading to hypoxic conditions, where anaerobic respiratory processes like glycolysis are beneficial, but may be deleterious in other cardiac conditions. Mechanical unloading with LVAD may reverse the maladaptive metabolic switches of the failing heart and even activate cellular pathways of cardioprotection and repair. Studies on LVAD-induced mechanical unloading have shown (1) improved respiratory capacity and augmented nitric oxide–dependent control of mitochondrial respiration ; (2) normalization of cardiolipin, a lipid component of the mitochondrial membrane important for adenosine triphosphate (ATP) formation and substrate transport, suggesting improved mitochondrial structure and function ; (3) depletion of long-chain acyl carnitine levels suggesting impairment of mitochondrial FA oxidation in failing hearts and substrate versatility of the mammalian heart ; (4) increased concentration of several amino acids, such as alanine, leucine/isoleucine, glutamine/glutamic acid and citrulline ; and (5) significant upsurge in the levels of total cholesterol, high-density lipoproteins, low-density lipoproteins, and triglycerides after LVAD implantation. Also, this increased level of total cholesterol from baseline to 3 months and a higher absolute cholesterol level postimplantation (at 3 months) were strongly associated with decreased mortality rate. From all of the aforementioned studies, it is evident that LVAD unloading relieves a significant part of the metabolic distress in the failing heart.

One intriguing concept is that glycolysis, although less efficient in terms of generating ATP, could be a sufficient energy source due to the decreased myocardial energy demand during unloading. Recent investigations on metabolic changes during HF showed upregulation of glycolysis without a subsequent increase in pyruvate oxidation through the Krebs cycle. This mismatch between glycolysis upregulation and mitochondrial pyruvate oxidations was attributed to the impaired structure and function of mitochondria ( Fig. 9.5 ). These metabolic changes may also be associated with the shift toward ketone body metabolism during chronic HF. The earlier observations, along with post-LVAD alterations in the expression of several metabolism-related genes and proteins, require further investigation to elucidate the specific role of cardiac metabolic adaptations during myocardial reverse remodeling.