Prolonged leptin treatment increases transient outward K⁺ current via upregulation of Kv4.2 and Kv4.3 channel subunits in adult rat ventricular myocytes

Association of attenuated leptin signaling pathways with impaired cardiac function under prolonged high-altitude hypoxia

Cardiovascular function and adipose metabolism were markedly influenced under high altitudes. However, the interplay between adipokines and heart under hypoxia remains to be elucidated. We aim to explore alterations of adipokines and underlying mechanisms in regulating cardiac function under high altitudes. We investigated the cardiopulmonary function and five adipokines in Antarctic expeditioners at Kunlun Station (4,087 m) for 20 days and established rats exposed to hypobaric hypoxia (5,000 m), simulating Kunlun Station. Antarctic expeditioners exhibited elevated heart rate, blood pressure, systemic vascular resistance, and decreased cardiac pumping function. Plasma creatine phosphokinase-MB (CK-MB) and platelet-endothelial cell adhesion molecule-1 (sPecam-1) increased, and leptin, resistin, and lipocalin-2 decreased. Plasma leptin significantly correlated with altered cardiac function indicators. Additionally, hypoxic rats manifested impaired left ventricular systolic and diastolic function, elevated plasma CK-MB and sPecam-1, and decreased plasma leptin. Chronic hypoxia for 14 days led to increased myocyte hypertrophy, fibrosis, apoptosis, and mitochondrial dysfunction, coupled with reduced protein levels of leptin signaling pathways in myocardial tissues. Cardiac transcriptome analysis revealed leptin was associated with downregulated genes involved in rhythm, Na+/K+ transport, and cell skeleton. In conclusion, chronic hypoxia significantly reduced leptin signaling pathways in cardiac tissues along with significant pathological changes, thus highlighting the pivotal role of leptin in regulation of cardiac function under high altitudes.

Impact of Obesity on Atrial Fibrillation Pathogenesis and Treatment Options

Atrial fibrillation (AF) is the most common cardiac arrhythmia. AF increases the risk of stroke, heart failure, dementia, and hospitalization. Obesity significantly increases AF risk, both directly and indirectly, through related conditions, like hypertension, diabetes, and heart failure. Obesity-driven structural and electrical remodeling contribute to AF via several reported mechanisms, including adiposity, inflammation, fibrosis, oxidative stress, ion channel alterations, and autonomic dysfunction. In particular, expanding epicardial adipose tissue during obesity has been suggested as a key driver of AF via paracrine signaling and direct infiltration. Weight loss has been shown to reverse these changes and reduce AF risk and recurrence after ablation. However, studies on how obesity affects pharmacologic or interventional AF treatments are limited. In this review, we discuss mechanisms by which obesity mediates AF and treatment outcomes, aiming to provide insight into obesity-drug interactions and guide personalized treatment for this patient subgroup.

GSK3β Modulates Timing-Dependent Long-Term Depression Through Direct Phosphorylation of Kv4.2 Channels

Spike timing-dependent plasticity (STDP) is a form of activity-dependent remodeling of synaptic strength that underlies memory formation. Despite its key role in dictating learning rules in the brain circuits, the molecular mechanisms mediating STDP are still poorly understood. Here, we show that spike timing-dependent long-term depression (tLTD) and A-type K+ currents are modulated by pharmacological agents affecting the levels of active glycogen-synthase kinase 3 (GSK3) and by GSK3β knockdown in layer 2/3 of the mouse somatosensory cortex. Moreover, the blockade of A-type K+ currents mimics the effects of GSK3 up-regulation on tLTD and occludes further changes in synaptic strength. Pharmacological, immunohistochemical and biochemical experiments revealed that GSK3β influence over tLTD induction is mediated by direct phosphorylation at Ser-616 of the Kv4.2 subunit, a molecular determinant of A-type K+ currents. Collectively, these results identify the functional interaction between GSK3β and Kv4.2 channel as a novel mechanism for tLTD modulation providing exciting insight into the understanding of GSK3β role in synaptic plasticity.

Preventive Leptin Administration Protects Against Sepsis Through Improving Hypotension, Tachycardia, Oxidative Stress Burst, Multiple Organ Dysfunction, and Increasing Survival

Sepsis syndrome is the most important cause of mortality in critically ill patients admitted to intensive care units (ICUs). However, current therapies for its prevention and treatment are still unsatisfactory, and the mortality rate is still high. Non-septic ICU patients are vulnerable to acquire sepsis syndrome. Thus, a preventive treatment for this population is needed. During sepsis syndrome and endotoxemia, severe hypotension, tachycardia, oxidative and immune response increase, multiple organ dysfunction syndrome (MODS) and decreased survival are observed. Leptin administration protects against negative effects of sepsis syndrome and endotoxemia. Furthermore, it is has been reported that leptin elevates blood pressure mediated by sympathetic nervous system activation. However, whether leptin administration before sepsis induction mediates its protective effects during sepsis through blood pressure regulation is not known. Therefore, we investigated whether pre-treatment of leptin improves blood pressure and MODS, resulting in survival increase during endotoxemia. The results showed that leptin administration before endotoxemia induction reduced both the hypotension and tachycardia characteristically observed during endotoxemia. Notably, this protective effect was observed early and late in the course of endotoxemia. Endotoxemia-induced MODS decreased in leptin-treated rats, which was reflected in normal values for liver and kidney function, inhibition of muscle mass wasting and maintenance of glycemia. Furthermore, leptin pre-treatment decreased the oxidative stress burst in blood and blunted the increased pro-inflammatory cytokines TNF-α, IL-1β, and IL-6 observed during endotoxemia. Remarkably, according to the leptin-induced increase in survival, leptin pre-administration decreased the risk for death associated with sepsis syndrome at early and late times after endotoxemia induction. These results show a potential preventive therapy against sepsis syndrome and endotoxemia in vulnerable patients, based in the beneficial actions of leptin.

Calcitriol, the Bioactive Metabolite of Vitamin D, Increases Ventricular K+ Currents in Isolated Mouse Cardiomyocytes

Calcitriol, the bioactive metabolite of vitamin D, interacts with the ubiquitously expressed nuclear vitamin D receptor (VDR) to induce genomic effects, but it can also elicit rapid responses via membrane-associated VDR through mechanisms that are poorly understood. The down-regulation of K⁺ currents is the main origin of electrophysiological remodeling in pathological hypertrophy and heart failure (HF), which can contribute to action potential prolongation and subsequently increase the risk of triggered arrhythmias. Adult mouse ventricular myocytes were isolated and treated with 10 nM calcitriol or vehicle for 15–30 min. In some experiments, cardiomyocytes were pretreated with the Akt inhibitor triciribine. In the adult mouse ventricle, outward K⁺ currents involved in cardiac repolarization are comprised of three components: the fast transient outward current (I_tof), the ultrarapid delayed rectifier K⁺ current (I_kur), and the non-inactivating steady-state outward current (I_ss). K⁺ currents were investigated using the whole-cell or the perforated patch-clamp technique and normalized to cell capacitance to obtain current densities. Calcitriol treatment of cardiomyocytes induced an increase in the density of I_tof and I_kur, which was lost in myocytes isolated from VDR-knockout mice. In addition, calcitriol activated Akt in cardiomyocytes and pretreatment with triciribine prevented the calcitriol-induced increase of outward K⁺ currents. In conclusion, we demonstrate that calcitriol via VDR and Akt increases both I_tof and I_kur densities in mouse ventricular cardiomyocytes. Our findings may provide new mechanistics clues for the cardioprotective role of this hormone in the heart.

Beneficial effects of leptin treatment in a setting of cardiac dysfunction induced by transverse aortic constriction in mouse

KEY POINTS

Leptin, is a 16 kDa pleiotropic peptide not only primarily secreted by adipocytes, but also produced by other tissues, including the heart. Controversy exists regarding the adverse and beneficial effects of leptin on the heart We analysed the effect of a non-hypertensive dose of leptin on cardiac function, [Ca²⁺ ]_i handling and cellular electrophysiology, which participate in the genesis of pump failure and related arrhythmias, both in control mice and in mice subjected to chronic pressure-overload by transverse aorta constriction. We find that leptin activates mechanisms that contribute to cardiac dysfunction under physiological conditions. However, after the establishment of pressure overload, an increase in leptin levels has protective cardiac effects with respect to rescuing the cellular heart failure phenotype. These beneficial effects of leptin involve restoration of action potential duration via normalization of transient outward potassium current and sarcoplasmic reticulum Ca²⁺ content via rescue of control sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase levels and ryanodine receptor function modulation, leading to normalization of Ca²⁺ handling parameters.

ABSTRACT

Leptin, is a 16 kDa pleiotropic peptide not only primary secreted by adipocytes, but also produced by other tissues, including the heart. Evidence indicates that leptin may have either adverse or beneficial effects on the heart. To obtain further insights, in the present study, we analysed the effect of leptin treatment on cardiac function, [Ca²⁺ ]_i handling and cellular electrophysiology, which participate in the genesis of pump failure and related arrhythmias, both in control mice and in mice subjected to chronic pressure-overload by transverse aorta constriction (TAC). Three weeks after surgery, animals received either leptin (0.36 mg kg^-1  day^-1 ) or vehicle via osmotic minipumps for 3 weeks. Echocardiographic measurements showed that, although leptin treatment was deleterious on cardiac function in sham, leptin had a cardioprotective effect following TAC. [Ca²⁺ ]_i transient in cardiomyocytes followed similar pattern. Patch clamp experiments showed prolongation of action potential duration (APD) in TAC and leptin-treated sham animals, whereas, following TAC, leptin reduced the APD towards control values. APD variations were associated with decreased transient outward potassium current and Kv4.2 and KChIP2 protein expression. TAC myocytes showed a higher incidence of triggered activities and spontaneous Ca²⁺ waves. These proarrhythmic manifestations, related to Ca²⁺ /calmodulin-dependent protein kinase II and ryanodine receptor phosphorylation, were reduced by leptin. The results of the present study demonstrate that, although leptin treatment was deleterious on cardiac function in control animals, leptin had a cardioprotective effect following TAC, normalizing cardiac function and reducing arrhythmogeneity at the cellular level.

Leptin-mediated ion channel regulation: PI3K pathways, physiological role, and therapeutic potential

Leptin is produced by adipose tissue and identified as a "satiety signal," informing the brain when the body has consumed enough food. Specific areas of the hypothalamus express leptin receptors (LEPRs) and are the primary site of leptin action for body weight regulation. In response to leptin, appetite is suppressed and energy expenditure allowed. Beside this hypothalamic action, leptin targets other brain areas in addition to neuroendocrine cells. LEPRs are expressed also in the hippocampus, neocortex, cerebellum, substantia nigra, pancreatic β-cells, and chromaffin cells of the adrenal gland. It is intriguing how leptin is able to activate different ionic conductances, thus affecting excitability, synaptic plasticity and neurotransmitter release, depending on the target cell. Most of the intracellular pathways activated by leptin and directed to ion channels involve PI3K, which in turn phosphorylates different downstream substrates, although parallel pathways involve AMPK and MAPK. In this review we will describe the effects of leptin on BK, KATP, KV, CaV, TRPC, NMDAR and AMPAR channels and clarify the landscape of pathways involved. Given the ability of leptin to influence neuronal excitability and synaptic plasticity by modulating ion channels activity, we also provide a short overview of the growing potentiality of leptin as therapeutic agent for treating neurological disorders.

Leptin modulates electrophysiological characteristics and isoproterenol-induced arrhythmogenesis in atrial myocytes

Background

Obesity is an important risk factor for atrial fibrillation (AF). Leptin is an important adipokine. However, it is not clear whether leptin directly modulates the electrophysiological characteristics of atrial myocytes.

Results

Whole cell patch clamp and indo-1 fluorescence were used to record the action potentials (APs) and ionic currents in isolated rabbit left atrial (LA) myocytes incubated with and without (control) leptin (100 nM) for 1 h to investigate the role of leptin on atrial electrophysiology. Leptin-treated LA myocytes (n = 19) had longer 20% of AP duration (28 ± 3 vs. 21 ± 2 ms, p < 0.05), but similar 50% of AP duration (51 ± 4 vs. 50 ± 3 ms, p > 0.05), and 90% of AP duration (89 ± 5 vs. 94 ± 4 ms, p > 0.05), as compared to the control (n = 22). In the presence of isoproterenol (10 nM), leptin-treated LA myocytes (n = 21) showed a lower incidence (19% vs. 54.2%, p < 0.05) of delayed afterdepolarization (DAD) than the control (n = 24). Leptin-treated LA myocytes showed a larger sodium current, but a smaller ultra-rapid delayed rectifier potassium current, and sodium-calcium exchanger current than the control. Leptin-treated and control LA myocytes exhibited a similar late sodium current, inward rectifier potassium current, transient outward current and L-type calcium current. In addition, the leptin-treated LA myocytes (n = 38) exhibited a smaller intracellular Ca²⁺ transient (0.21 ± 0.01 vs. 0.26 ± 0.01 R410/485, p < 0.05) and sarcoplasmic reticulum Ca²⁺ content (0.35 ± 0.02 vs. 0.43 ± 0.03 R410/485, p < 0.05) than the control LA myocytes (n = 42).

Conclusions

Leptin regulates the LA electrophysiological characteristics and attenuates isoproterenol-induced arrhythmogenesis.