Opening of the inward rectifier potassium channel alleviates maladaptive tissue repair following myocardial infarction

mTORC1 and SGLT2 Inhibitors-A Therapeutic Perspective for Diabetic Cardiomyopathy

Diabetic cardiomyopathy is a critical diabetes-mediated co-morbidity characterized by cardiac dysfunction and heart failure, without predisposing hypertensive or atherosclerotic conditions. Metabolic insulin resistance, promoting hyperglycemia and hyperlipidemia, is the primary cause of diabetes-related disorders, but ambiguous tissue-specific insulin sensitivity has shed light on the importance of identifying a unified target paradigm for both the glycemic and non-glycemic context of type 2 diabetes (T2D). Several studies have indicated hyperactivation of the mammalian target of rapamycin (mTOR), specifically complex 1 (mTORC1), as a critical mediator of T2D pathophysiology by promoting insulin resistance, hyperlipidemia, inflammation, vasoconstriction, and stress. Moreover, mTORC1 inhibitors like rapamycin and their analogs have shown significant benefits in diabetes and related cardiac dysfunction. Recently, FDA-approved anti-hyperglycemic sodium-glucose co-transporter 2 inhibitors (SGLT2is) have gained therapeutic popularity for T2D and diabetic cardiomyopathy, even acknowledging the absence of SGLT2 channels in the heart. Recent studies have proposed SGLT2-independent drug mechanisms to ascertain their cardioprotective benefits by regulating sodium homeostasis and mimicking energy deprivation. In this review, we systematically discuss the role of mTORC1 as a unified, eminent target to treat T2D-mediated cardiac dysfunction and scrutinize whether SGLT2is can target mTORC1 signaling to benefit patients with diabetic cardiomyopathy. Further studies are warranted to establish the underlying cardioprotective mechanisms of SGLT2is under diabetic conditions, with selective inhibition of cardiac mTORC1 but the concomitant activation of mTORC2 (mTOR complex 2) signaling.

Activation of Kir2.1 improves myocardial fibrosis by inhibiting Ca 2+ overload and the TGF-β1/Smad signaling pathway

The inwardly rectifying potassium channel Kir2.1 is closely associated with many cardiovascular diseases. However, the effect and mechanism of Kir2.1 in diabetic cardiomyopathy remain unclear. In vivo, we use STZ to establish the model, and ventricular structural changes, myocardial inflammatory infiltration, and myocardial fibrosis severity are detected by echocardiography, histological staining, immunohistochemistry, and western blot analysis, respectively. In vitro, a myocardial fibrosis model is established with high glucose. The Kir2.1 current amplitude, intracellular calcium concentration, fibrosis-related proteins, and TGF-β1/Smad pathway proteins are detected by whole-cell patch clamp, calcium probes, western blot analysis, and immunofluorescence, respectively. The in vivo results show that compared to diabetic cardiomyopathy, zacopride (a Kir2.1 selective agonist) significantly reduces the left ventricular systolic diameter and diastolic diameter, increases the left ventricular ejection fraction and left ventricular short-axis shortening, improves the degree of cell necrosis, and reduces the expression of myocardial interstitial fibrosis protein and collagen fibre deposition area. The in vitro results show that the current amplitude and protein expression of Kir2.1 are both decreased in the high glucose-induced myocardial fibrosis model. Additionally, zacopride significantly upregulates the expression of Kir2.1 and inhibits the expressions of the fibrosis-related proteins α-SMA, collagen I, and collagen III. Activation of Kir2.1 reduces the intracellular calcium concentration and inhibits the protein expressions of TGF-β1 and p-Smad 2/3. Activation of Kir2.1 can improve myocardial fibrosis induced by diabetic cardiomyopathy, and the possible mechanism may be related to inhibiting Ca ²⁺ overload and the TGF-β1/Smad signaling pathway.

Tetramisole is a new IK1 channel agonist and exerts IK1 -dependent cardioprotective effects in rats

Cardiac ischemia, hypoxia, arrhythmias, and heart failure share the common electrophysiological changes featured by the elevation of intracellular Ca2+ (Ca2+ overload) and inhibition of the inward rectifier potassium (IK1 ) channel. IK1 channel agonists have been considered a new type of anti-arrhythmia and cardioprotective agents. We predicted using a drug repurposing strategy that tetramisole (Tet), a known anthelminthic agent, was a new IK1 channel agonist. The present study aimed to experimentally identify the above prediction and further demonstrate that Tet has cardioprotective effects. Results of the whole-cell patch clamp technique showed that Tet at 1-100 μmol/L enhanced IK1 current, hyperpolarized resting potential (RP), and shortened action potential duration (APD) in isolated rat cardiomyocytes, while without effects on other ion channels or transporters. In adult Sprague-Dawley (SD) rats in vivo, Tet showed anti-arrhythmia and anticardiac remodeling effects, respectively, in the coronary ligation-induced myocardial infarction model and isoproterenol (Iso, i.p., 3 mg/kg/day, 10 days) infusion-induced cardiac remodeling model. Tet also showed anticardiomyocyte remodeling effect in Iso (1 μmol/L) infused adult rat ventricular myocytes or cultured H9c2 (2-1) cardiomyocytes. Tet at 0.54 mg/kg in vivo or 30 μmol/L in vitro showed promising protections on acute ischemic arrhythmias, myocardial hypertrophy, and fibrosis. Molecular docking was performed and identified the selective binding of Tet with Kir2.1. The cardioprotection of Tet was associated with the facilitation of IK1 channel forward trafficking, deactivation of PKA signaling, and inhibition of intracellular calcium overload. Enhancing IK1 may play dual roles in anti-arrhythmia and antiventricular remodeling mediated by restoration of Ca2+ homeostasis.

Towards the Development of AgoKirs: New Pharmacological Activators to Study Kir2.x Channel and Target Cardiac Disease

Inward rectifier potassium ion channels (I_K1-channels) of the K_ir2.x family are responsible for maintaining a stable negative resting membrane potential in excitable cells, but also play a role in processes of non-excitable tissues, such as bone development. I_K1-channel loss-of-function, either congenital or acquired, has been associated with cardiac disease. Currently, basic research and specific treatment are hindered by the absence of specific and efficient K_ir2.x channel activators. However, twelve different compounds, including approved drugs, show off-target I_K1 activation. Therefore, these compounds contain valuable information towards the development of agonists of K_ir channels, AgoKirs. We reviewed the mechanism of I_K1 channel activation of these compounds, which can be classified as direct or indirect activators. Subsequently, we examined the most viable starting points for rationalized drug development and possible safety concerns with emphasis on cardiac and skeletal muscle adverse effects of AgoKirs. Finally, the potential value of AgoKirs is discussed in view of the current clinical applications of potentiators and activators in cystic fibrosis therapy.

IK1 channel agonist zacopride suppresses ventricular arrhythmias in conscious rats with healing myocardial infarction

AIMS

Arrhythmogenesis of chronic myocardial infarction (MI) is associated with the prolongation of action potential, reduction of inward rectifier potassium (I_K1, Kir) channels and hyper-activity of Calcium/calmodulin-dependent kinase II (CaMKII) in cardiomyocytes. Zacopride, a selective I_K1 agonist, was applied to clarify the cardioprotection of I_K1 agonism via a CaMKII signaling on arrhythmias post-MI.

METHODS

Male SD rats were implanted wireless transmitter in the abdominal cavity and subjected to left main coronary artery ligation or sham operation. The telemetric ECGs were monitored per day throughout 4 weeks. At the endpoint, isoproterenol (1.28 mg/kg, i.v.) was administered for provocation test. The expressions of Kir2.1 (dominant subunit of I_K1 in ventricle) and CaMKII were detected by Western-blotting.

KEY FINDINGS

In the telemetric rats post-MI, zacopride significantly reduced the episodes of atrioventricular conduction block (AVB), premature ventricular contraction (PVC), ventricular tachycardia (VT) and ventricular fibrillation (VF), without significant effect on superventricular premature contraction (SPVC). In provocation test, zacopride suppressed the onset of ventricular arrhythmias in conscious PMI or sham rats. The expression of Kir2.1 was significantly downregulated and p-CaMKII was upregulated post-MI, whereas both were restored by zacopride treatment.

SIGNIFICANCE

I_K1/Kir2.1 might be an attractive target for pharmacological controlling of lethal arrhythmias post MI.

IK1 Channel Agonist Zacopride Alleviates Cardiac Hypertrophy and Failure via Alterations in Calcium Dyshomeostasis and Electrical Remodeling in Rats

Intracellular Ca²⁺ overload, prolongation of the action potential duration (APD), and downregulation of inward rectifier potassium (I_K1) channel are hallmarks of electrical remodeling in cardiac hypertrophy and heart failure (HF). We hypothesized that enhancement of I_K1 currents is a compensation for I_K1 deficit and a novel modulation for cardiac Ca²⁺ homeostasis and pathological remodeling. In adult Sprague-Dawley (SD) rats in vivo, cardiac hypertrophy was induced by isoproterenol (Iso) injection (i.p., 3 mg/kg/d) for 3, 10, and 30 days. Neonatal rat ventricular myocytes (NRVMs) were isolated from 1 to 3 days SD rat pups and treated with 1 μmol/L Iso for 24 h in vitro. The effects of zacopride, a selective I_K1/Kir2.1 channel agonist, on cardiac remodeling/hypertrophy were observed in the settings of 15 μg/kg in vivo and 1 μmol/L in vitro. After exposing to Iso for 3 days and 10 days, rat hearts showed distinct concentric hypertrophy and fibrosis and enhanced pumping function (P < 0.01 or P < 0.05), then progressed to dilatation and dysfunction post 30 days. Compared with the age-matched control, cardiomyocytes exhibited higher cytosolic Ca²⁺ (P < 0.01 or P < 0.05) and lower SR Ca²⁺ content (P < 0.01 or P < 0.05) all through 3, 10, and 30 days of Iso infusion. The expressions of Kir2.1 and SERCA2 were downregulated, while p-CaMKII, p-RyR2, and cleaved caspase-3 were upregulated. Iso-induced electrophysiological abnormalities were also manifested with resting potential (RP) depolarization (P < 0.01), APD prolongation (P < 0.01) in adult cardiomyocytes, and calcium overload in cultured NRVMs (P < 0.01). Zacopride treatment effectively retarded myocardial hypertrophy and fibrosis, preserved the expression of Kir2.1 and some key players in Ca²⁺ homeostasis, normalized the RP (P < 0.05), and abbreviated APD (P < 0.01), thus lowered cytosolic [Ca^{2 +}]_i (P < 0.01 or P < 0.05). I_K1channel blocker BaCl₂ or chloroquine largely reversed the cardioprotection of zacopride. We conclude that cardiac electrical remodeling is concurrent with structural remodeling. By enhancing cardiac I_K1, zacopride prevents Iso-induced electrical remodeling around intracellular Ca²⁺ overload, thereby attenuates cardiac structural disorder and dysfunction. Early electrical interventions may provide protection on cardiac remodeling.