New Insights into the Development and Morphogenesis of the Cardiac Purkinje Fiber Network: Linking Architecture and Function

The rapid propagation of electrical activity through the ventricular conduction system (VCS) controls spatiotemporal contraction of the ventricles. Cardiac conduction defects or arrhythmias in humans are often associated with mutations in key cardiac transcription factors that have been shown to play important roles in VCS morphogenesis in mice. Understanding of the mechanisms of VCS development is thus crucial to decipher the etiology of conduction disturbances in adults. During embryogenesis, the VCS, consisting of the His bundle, bundle branches, and the distal Purkinje network, originates from two independent progenitor populations in the primary ring and the ventricular trabeculae. Differentiation into fast-conducting cardiomyocytes occurs progressively as ventricles develop to form a unique electrical pathway at late fetal stages. The objectives of this review are to highlight the structure–function relationship between VCS morphogenesis and conduction defects and to discuss recent data on the origin and development of the VCS with a focus on the distal Purkinje fiber network.

Temperature effects on the contractile performance and efficiency of oxidative muscle from a eury- vs. stenothermal salmonid

We compared the thermal sensitivity of oxidative muscle function between the eurythermal Atlantic salmon (Salmo salar) and the more stenothermal Arctic char (Salvelinus alpinus; which prefers cooler waters). Power output was measured in red skeletal muscle strips and myocardial trabeculae, and efficiency (net work/energy consumed) was measured for trabeculae, from cold (6oC) and warm (15oC) acclimated fish at temperatures from 2-26oC. The mass-specific net power produced by char red muscle was greater than in salmon, by 2-5 fold depending on test temperature. Net power first increased, then decreased, when the red muscle of 6oC-acclimated char was exposed to increasing temperature. Acclimation to 15oC significantly impaired mass-specific power in char (by ∼40-50%) from 2 to 15oC, but lessened its relative decrease between 15 and 26oC. In contrast, maximal net power increased, and then plateaued, with increasing temperature in salmon from both acclimation groups. Increasing test temperature resulted in a ∼3-5 fold increase in maximal net power produced by ventricular trabeculae in all groups, and this effect was not influenced by acclimation temperature. Nonetheless, lengthening power was higher in trabeculae from warm acclimated char, and char trabeculae could not contract as fast as those from salmon. Finally, the efficiency of myocardial net work was approximately 2-fold greater in 15oC acclimated salmon than char (∼15 vs. 7%), and highest at 20oC in salmon. This study provides several mechanistic explanations as to their inter-specific difference in upper thermal tolerance, and potentially why southern char populations are being negatively impacted by climate change.

Contraction–relaxation coupling is unaltered by exercise training and infarction in isolated canine myocardium

The two main phases of the mammalian cardiac cycle are contraction and relaxation; however, whether there is a connection between them in humans is not well understood. Routine exercise has been shown to improve cardiac function, morphology, and molecular signatures. Likewise, the acute and chronic changes that occur in the heart in response to injury, disease, and stress are well characterized, albeit not fully understood. In this study, we investigated how exercise and myocardial injury affect contraction–relaxation coupling. We retrospectively analyzed the correlation between the maximal speed of contraction and the maximal speed of relaxation of canine myocardium after receiving surgically induced myocardial infarction, followed by either sedentary recovery or exercise training for 10–12 wk. We used isolated right ventricular trabeculae, which were electrically paced at different lengths, frequencies, and with increasing β-adrenoceptor stimulation. In all conditions, contraction and relaxation were linearly correlated, irrespective of injury or training history. Based on these results and the available literature, we posit that contraction–relaxation coupling is a fundamental myocardial property that resides in the structural arrangement of proteins at the level of the sarcomere and that this may be regulated by the actions of cardiac myosin binding protein C (cMyBP-C) on actin and myosin.

Selective beta-3 adrenergic receptor blockade increases contractility of human ventricular trabeculae from HFrEF donors

Background Alterations of the beta-adrenergic system have been extensively described in the setting of heart failure (HF). Upregulation of beta-3 adrenergic receptor (β3-AdrR) expression in human failing hearts depresses myocardial contractility and, during an acute decompensation event, can be considered a maladaptive compensatory mechanism that exacerbates cardiac dysfunction. APD418 is a selective β3-AdrR antagonist currently in development for patients who have acute heart failure with reduced ejection fraction (HFrEF). APD418 is designed to improve myocardial contractility by selectively antagonizing the β3-AdrR and thereby avoiding the cAMP/Ca2+ signaling pathway stimulated by current inotropes. Purpose This study evaluated the effect of a selective β3-AdrR antagonist (APD418) on contractile responses in explanted human ventricular trabeculae obtained from normal and HFrEF hearts. Methods Left ventricular trabeculae from normal and HFrEF donors were electrically stimulated (1 Hz) ex-vivo to analyze force generated during contractions. First, BRL37344, a selective β3-AdrR agonist, was applied at increasing concentrations (0.01–10 μM) to confirm β3-AdrR mediated negative inotropy in human myocardial tissue. To test the effect of a selective β3-AdrR antagonist on contractile force, myocardial tissue was pre-treated with APD418 or vehicle for 5 minutes, followed by treatment with non-selective β-AdrR agonists isoproterenol (10 nM) or norepinephrine (5 μM). Results In heart tissue from normal donors, the β3-AdrR agonist BRL37344 did not affect contractile function at 0.01 and 0.1 μM. However, in heart tissue from HFrEF donors, BRL37344 induced a significant decrease in contractility at 0.01, 0.1 and 1 μM (85.9±1.8% with 0.1 μM BRL37344 vs 104.1±2.9% with vehicle). Selective blockade of β3-AdrR with APD418 had no effect on force of contraction induced by norepinephrine in cardiac tissue from normal donors. In contrast, APD418 potentiated the force of contraction induced by either isoproterenol (49.1±20.6% increase with 0.1 μM APD418 compared to baseline) or norepinephrine (26.5±4.9% increase with 0.01 μM APD418 compared to baseline) in cardiac tissue from HFrEF patients. Conclusion This is, to our knowledge, the first evidence showing that selective blockade of β3-AdrR increases contractility of human ventricular trabeculae from HFrEF donors and suggests that further studies evaluating the therapeutic benefit of APD418 in patients with HFrEF are warranted. Funding Acknowledgement Type of funding source: Private company. Main funding source(s): Arena Pharmaceuticals

Cardiac myosin regulatory light chain kinase modulates cardiac contractility by phosphorylating both myosin regulatory light chain and troponin I

Heart muscle contractility and performance are controlled by posttranslational modifications of sarcomeric proteins. Although myosin regulatory light chain (RLC) phosphorylation has been studied extensively in vitro and in vivo, the precise role of cardiac myosin light chain kinase (cMLCK), the primary kinase acting upon RLC, in the regulation of cardiomyocyte contractility remains poorly understood. In this study, using recombinantly expressed and purified proteins, various analytical methods, in vitro and in situ kinase assays, and mechanical measurements in isolated ventricular trabeculae, we demonstrate that human cMLCK is not a dedicated kinase for RLC but can phosphorylate other sarcomeric proteins with well-characterized regulatory functions. We show that cMLCK specifically monophosphorylates Ser23 of human cardiac troponin I (cTnI) in isolation and in the trimeric troponin complex in vitro and in situ in the native environment of the muscle myofilament lattice. Moreover, we observed that human cMLCK phosphorylates rodent cTnI to a much smaller extent in vitro and in situ, suggesting species-specific adaptation of cMLCK. Although cMLCK treatment of ventricular trabeculae exchanged with rat or human troponin increased their cross-bridge kinetics, the increase in sensitivity of myofilaments to calcium was significantly blunted by human TnI, suggesting that human cTnI phosphorylation by cMLCK modifies the functional consequences of RLC phosphorylation. We propose that cMLCK-mediated phosphorylation of TnI is functionally significant and represents a critical signaling pathway that coordinates the regulatory states of thick and thin filaments in both physiological and potentially pathophysiological conditions of the heart.