Rationale:: Downregulation of the pacemaking ion channel, HCN4 (hyperpolarization-activated cyclic nucleotide gated channel 4), and the corresponding ionic current, I _f , underlies exercise training–induced sinus bradycardia in rodents. If this occurs in humans, it could explain the increased incidence of bradyarrhythmias in veteran athletes, and it will be important to understand the underlying processes. Objective:: To test the role of HCN4 in the training-induced bradycardia in human athletes and investigate the role of microRNAs (miRs) in the repression of HCN4. Methods and Results:: As in rodents, the intrinsic heart rate was significantly lower in human athletes than in nonathletes, and in all subjects, the rate-lowering effect of the HCN selective blocker, ivabradine, was significantly correlated with the intrinsic heart rate, consistent with HCN repression in athletes. Next-generation sequencing and quantitative real-time reverse transcription polymerase chain reaction showed remodeling of miRs in the sinus node of swim-trained mice. Computational predictions highlighted a prominent role for miR-423-5p. Interaction between miR-423-5p and HCN4 was confirmed by a dose-dependent reduction in HCN4 3′-untranslated region luciferase reporter activity on cotransfection with precursor miR-423-5p (abolished by mutation of predicted recognition elements). Knockdown of miR-423-5p with anti-miR-423-5p reversed training-induced bradycardia via rescue of HCN4 and I _f . Further experiments showed that in the sinus node of swim-trained mice, upregulation of miR-423-5p (intronic miR) and its host gene, NSRP1, is driven by an upregulation of the transcription factor Nkx2.5. Conclusions:: HCN remodeling likely occurs in human athletes, as well as in rodent models. miR-423-5p contributes to training-induced bradycardia by targeting HCN4. This work presents the first evidence of miR control of HCN4 and heart rate. miR-423-5p could be a therapeutic target for pathological sinus node dysfunction in veteran athletes.

Induced pluripotent stem cells (iPSCs) were first established from differentiated somatic cells by gene introduction of key transcription factors, OCT4, SOX2, KLF4, and c-MYC, over a decade ago. Although iPSCs can be applicable for regenerative medicine, disease modeling and drug screening, several issues associated with the utilization of iPSCs such as low reprogramming efficiency and the risk of tumorigenesis, still need to be resolved. In addition, the molecular mechanisms involved in the somatic cell reprogramming to pluripotency are yet to be elucidated. Compared with their somatic counterparts, pluripotent stem cells, including embryonic stem cells and iPSCs, exhibit a high rate of glycolysis akin to aerobic glycolysis in cancer cells. This is known as the Warburg effect and is essential for maintaining stem cell properties. This unique glycolytic metabolism in iPSCs can provide energy and drive the pentose phosphate pathway, which is the preferred pathway for rapid cell proliferation. During reprogramming, somatic cells undergo a metabolic shift from oxidative phosphorylation (OXPHOS) to glycolysis trigged by a transient OXPHOS burst, resulting in the initiation and progression of reprogramming to iPSCs. Metabolic intermediates and mitochondrial functions are also involved in the epigenetic modification necessary for the process of iPSC reprogramming. Among the key regulatory molecules that have been reported to be involved in metabolic shift so far, hypoxia-inducible factor 1 (HIF1) controls the transcription of many target genes to initiate metabolic changes in the early stage and maintains glycolytic metabolism in the later phase of reprogramming. This review summarizes the current understanding of the unique metabolism of pluripotent stem cells and the metabolic shift during reprogramming, and details the relevance of HIF1 in the metabolic shift.

Bradyarrhythmias are an important cause of mortality in heart failure and previous studies indicate a mechanistic role for electrical remodelling of the key pacemaking ion channel HCN4 in this process. Here we show that, in a mouse model of heart failure in which there is sinus bradycardia, there is upregulation of a microRNA (miR-370-3p), downregulation of the pacemaker ion channel, HCN4, and downregulation of the corresponding ionic current,I_f, in the sinus node. In vitro, exogenous miR-370-3p inhibits HCN4 mRNA and causes downregulation of HCN4 protein, downregulation ofI_f, and bradycardia in the isolated sinus node. In vivo, intraperitoneal injection of an antimiR to miR-370-3p into heart failure mice silences miR-370-3p and restores HCN4 mRNA and protein andI_fin the sinus node and blunts the sinus bradycardia. In addition, it partially restores ventricular function and reduces mortality. This represents a novel approach to heart failure treatment.

Rationale: Athletes present with atrioventricular node dysfunction manifesting as atrioventricular block. This can necessitate electronic pacemaker implantation, known to be more frequent in athletes with a long training history. Objective: Atrioventricular block in athletes is attributed to high vagal tone. Here, we investigated the alternative hypothesis that electrical remodeling of the atrioventricular node is responsible. Methods and Results: Radiotelemetry ECG data and atrioventricular node biopsies were collected in sedentary and trained Standardbred racehorses, a large-animal model of the athlete’s heart. Trained horses presented with longer PR intervals (that persisted under complete autonomic block) versus sedentary horses, concomitant with reduced expression of key ion channels involved in atrioventricular node conduction: L-type Ca ²⁺ channel subunit Ca _V 1.2 and HCN4 (hyperpolarization-activated cyclic nucleotide-gated channel 4). Atrioventricular node electrophysiology was explored further in mice; prolongation of the PR interval (in vivo and ex vivo), Wenckebach cycle length, and atrioventricular node refractory period were observed in mice trained by swimming versus sedentary mice. Transcriptional profiling in laser-capture microdissected atrioventricular node revealed striking reduction in pacemaking ion channels in trained mice, translating into protein downregulation of Ca _V 1.2 and HCN4. Correspondingly, patch-clamp recordings in isolated atrioventricular node myocytes demonstrated a training-induced reduction in I _{Ca, L} and I _f density that likely contributed to the observed lower frequency of action potential firing in trained cohorts. MicroRNA (miR) profiling and in vitro studies revealed miR-211-5p and miR-432 as direct regulators of Ca _V 1.2 and HCN4. In vivo miRs suppression or detraining restored training-induced PR prolongation and ion channel remodeling. Conclusions: Training-induced atrioventricular node dysfunction is underscored by likely miR-mediated transcriptional remodeling that translates into reduced current density of key ionic currents involved in impulse generation and conduction. We conclude that electrical remodeling is a key mechanism underlying atrioventricular block in athletes.

Background The effects of pimobendan on left atrial pressure (LAP) in dogs with mitral valve disease (MR) have not been documented in a quantitative manner. Objective The objective was to document and study the short‐term effects of pimobendan on LAP and echocardiographic parameters in MR dogs. Animals Eight healthy Beagle dogs weighing 10.0–14.7 kg (3 males and 5 females; aged 2 years) were used. Methods Experimental, cross‐over, and interventional study. Dogs with surgically induced MR received pimobendan at either 0.25 mg/kg or 0.50 mg/kg PO q12h for 7 days and then, after a 7‐day wash‐out period, the other dosage. LAP was measured for 30 minutes at baseline and again on days 1, 2, 4, and 7 of pimobendan administration. Results Mean LAP was significantly decreased after the administration of 0.25 mg/kg (15.81 ± 5.44 mmHg to 12.67 ± 5.71 mmHg, P < .001) and 0.50 mg/kg (15.76 ± 5.45 mmHg to 10.77 ± 5.23 mmHg, P < .001). Also, the 0.50 mg/kg group led to a significantly lower LAP (P < .01) compared with the 0.25 mg/kg group. Significant reduction was seen for the first time 4 days after the administration of 0.25 mg/kg and a day after the administration of 0.50 mg/kg. Conclusions and Clinical Importance Pimobendan decreased LAP in a dose‐dependent manner in dogs with acute MR caused by experimental chordal rupture. This study did not evaluate adverse effects of high‐dose pimobendan, and additional studies in clinical patients are warranted.

In cardiomyocytes, intracellular calcium (Ca²⁺) transients are elicited by electrical and receptor stimulations, leading to muscle contraction and gene expression, respectively. Although such elevations of Ca²⁺levels ([Ca²⁺]) also occur in the nucleus, the precise mechanism of nuclear [Ca²⁺] regulation during different kinds of stimuli, and its relationship with cytoplasmic [Ca²⁺] regulation are not fully understood. To address these issues, we used a new region-specific fluorescent protein-based Ca²⁺ indicator, GECO, together with the conventional probe Fluo-4 AM. We confirmed that nuclear Ca²⁺ transients were elicited by both electrical and receptor stimulations in neonatal mouse ventricular myocytes. Kinetic analysis revealed that electrical stimulation-elicited nuclear Ca²⁺ transients are slower than cytoplasmic Ca²⁺ transients, and chelating cytoplasmic Ca²⁺ abolished nuclear Ca²⁺ transients, suggesting that nuclear Ca²⁺ are mainly derived from the cytoplasm during electrical stimulation. On the other hand, receptor stimulation such as with insulin-like growth factor-1 (IGF-1) preferentially increased nuclear [Ca²⁺] compared to cytoplasmic [Ca²⁺]. Experiments using inhibitors revealed that electrical and receptor stimulation-elicited Ca²⁺ transients were mainly mediated by ryanodine receptors and inositol 1,4,5-trisphosphate receptors (IP3Rs), respectively, suggesting different mechanisms for the two signals. Furthermore, IGF-1-elicited nuclear Ca²⁺ transient amplitude was significantly lower in myocytes lacking neuronal Ca²⁺ sensor-1 (NCS-1), a Ca²⁺ binding protein implicated in IP₃R-mediated pathway in the heart. Moreover, IGF-1 strengthened the interaction between NCS-1 and IP₃R. These results suggest a novel mechanism for receptor stimulation-induced nuclear [Ca²⁺] regulation mediated by IP3R and NCS-1 that may further fine-tune cardiac Ca²⁺ signal regulation.