Early opening of sarcolemmal ATP-sensitive potassium channels is not a key step in PKC-mediated cardioprotection

Identification and characterisation of functional Kir6.1-containing ATP-sensitive potassium channels in the cardiac ventricular sarcolemmal membrane

BACKGROUND AND PURPOSE

The canonical Kir6.2/SUR2A ventricular KATP channel is highly ATP-sensitive and remains closed under normal physiological conditions. These channels activate only when prolonged metabolic compromise causes significant ATP depletion and then shortens the action potential to reduce contractile activity. Pharmacological activation of KATP channels is cardioprotective, but physiologically, it is difficult to understand how these channels protect the heart if they only open under extreme metabolic stress. The presence of a second KATP channel population could help explain this. Here, we characterise the biophysical and pharmacological behaviours of a constitutively active Kir6.1-containing KATP channel in ventricular cardiomyocytes.

EXPERIMENTAL APPROACH

Patch-clamp recordings from rat ventricular myocytes in combination with well-defined pharmacological modulators was used to characterise these newly identified K+ channels. Action potential recording, calcium (Fluo-4) fluorescence measurements and video edge detection of contractile function were used to assess functional consequences of channel modulation.

KEY RESULTS

Our data show a ventricular K+ conductance whose biophysical characteristics and response to pharmacological modulation were consistent with Kir6.1-containing channels. These Kir6.1-containing channels lack the ATP-sensitivity of the canonical channels and are constitutively active.

CONCLUSION AND IMPLICATIONS

We conclude there are two functionally distinct populations of ventricular KATP channels: constitutively active Kir6.1-containing channels that play an important role in fine-tuning the action potential and Kir6.2/SUR2A channels that activate with prolonged ischaemia to impart late-stage protection against catastrophic ATP depletion. Further research is required to determine whether Kir6.1 is an overlooked target in Comprehensive in vitro Proarrhythmia Assay (CiPA) cardiac safety screens.

KATP channels are regulators of programmed cell death and targets for the creation of novel drugs against ischemia/reperfusion cardiac injury

BACKGROUND

The use of percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI) is associated with a mortality rate of 5%-7%. It is clear that there is an urgent need to develop new drugs that can effectively prevent cardiac reperfusion injury. ATP-sensitive K+ (KATP ) channel openers (KCOs) can be classified as such drugs.

RESULTS

KCOs prevent irreversible ischemia and reperfusion injury of the heart. KATP channel opening promotes inhibition of apoptosis, necroptosis, pyroptosis, and stimulation of autophagy. KCOs prevent the development of cardiac adverse remodeling and improve cardiac contractility in reperfusion. KCOs exhibit antiarrhythmic properties and prevent the appearance of the no-reflow phenomenon in animals with coronary artery occlusion and reperfusion. Diabetes mellitus and a cholesterol-enriched diet abolish the cardioprotective effect of KCOs. Nicorandil, a KCO, attenuates major adverse cardiovascular event and the no-reflow phenomenon, reduces infarct size, and decreases the incidence of ventricular arrhythmias in patients with acute myocardial infarction.

CONCLUSION

The cardioprotective effect of KCOs is mediated by the opening of mitochondrial KATP (mitoKATP ) and sarcolemmal KATP (sarcKATP ) channels, triggered free radicals' production, and kinase activation.

Slowly activating voltage-gated potassium current potentiation by ML277 is a novel cardioprotective intervention

Cardiovascular disease is thought to account for nearly a third of deaths worldwide, with ischemic heart disease, including acute coronary syndromes such as myocardial infarction, accounting for 1.7 million deaths per year. There is a clear need for interventions to impart cardioprotection against ischemia. Here, we show that the slowly activating voltage-gated potassium current (IKs) potentiator ML277 imparts cardioprotection against ischemia in cellular and whole-heart models by modulating the action potential duration. In three different metabolic inhibition and reperfusion models, an increased contractile recovery and cell survival was observed with ML277, indicative of protection. Finally, ML277 reduced infarct size in an ex vivo Langendorff coronary ligation model, including if only applied on reperfusion. In conclusion, potentiation of the IKs with ML277 imparted a cardioprotection that was equivalent to the protection reported previously by ischemic preconditioning. These data suggest that IKs potentiation may be therapeutically useful in acute coronary syndromes.

Selective protein kinase C inhibition switches time-dependent glucose cardiotoxicity to cardioprotection

Hyperglycaemia at the time of myocardial infarction has an adverse effect on prognosis irrespective of a prior diagnosis of diabetes, suggesting glucose is the damaging factor. In ex vivo models of ischaemia, we demonstrated that deleterious effects of acutely elevated glucose are PKCα/β-dependent, and providing PKCα/β are inhibited, elevated glucose confers cardioprotection. Short pre-treatments with high glucose were used to investigate time-dependent glucose cardiotoxicity, with PKCα/β inhibition investigated as a potential mechanism to reverse the toxicity. Freshly isolated non-diabetic rat cardiomyocytes were exposed to elevated glucose to investigate the time-dependence toxic effects. High glucose challenge for >7.5 min was cardiotoxic, proarrhythmic and lead to contractile failure, whilst cardiomyocytes exposed to metabolic inhibition following 5-min high glucose, displayed a time-dependent protection lasting ∼15 min. This protection was further enhanced with PKCα/β inhibition. Cardioprotection was measured as a delay in contractile failure and K_ATP channel activation, improved contractile and Ca²⁺ transient recovery and increased cell survival. Finally, the effects of pre-ischaemic treatment with high glucose in a whole-heart coronary ligation protocol, where protection was evident with PKCα/β inhibition. Selective PKCα/β inhibition enhances protection suggesting glycaemic control with PKC inhibition as a potential cardioprotective therapeutics in myocardial infarction and elective cardiac surgery.

Functional Regulation of KATP Channels and Mutant Insight Into Clinical Therapeutic Strategies in Cardiovascular Diseases

ATP-sensitive potassium channels (K_ATP channels) play pivotal roles in excitable cells and link cellular metabolism with membrane excitability. The action potential converts electricity into dynamics by ion channel-mediated ion exchange to generate systole, involved in every heartbeat. Activation of the K_ATP channel repolarizes the membrane potential and decreases early afterdepolarization (EAD)-mediated arrhythmias. K_ATP channels in cardiomyocytes have less function under physiological conditions but they open during severe and prolonged anoxia due to a reduced ATP/ADP ratio, lessening cellular excitability and thus preventing action potential generation and cell contraction. Small active molecules activate and enhance the opening of the K_ATP channel, which induces the repolarization of the membrane and decreases the occurrence of malignant arrhythmia. Accumulated evidence indicates that mutation of K_ATP channels deteriorates the regulatory roles in mutation-related diseases. However, patients with mutations in K_ATP channels still have no efficient treatment. Hence, in this study, we describe the role of K_ATP channels and subunits in angiocardiopathy, summarize the mutations of the K_ATP channels and the functional regulation of small active molecules in K_ATP channels, elucidate the potential mechanisms of mutant K_ATP channels and provide insight into clinical therapeutic strategies.

Adaptation to oxidative stress at cellular and tissue level

Several in vitro and in vivo investigations have already proved that cells and tissues, when pre-exposed to low oxidative stress by different stimuli such as chemical, physical agents and environmental factors, display more resistance against subsequent stronger ischaemic injuries, resulting in an adaptive response known as ischaemic preconditioning (IPC). The aim of this review is to report the most recent knowledge about the complex adaptive mechanisms, including signalling transduction pathways, antioxidant systems, apoptotic and inflammation pathways, underlying cell protection against oxidative damage. In addition, an update about in vivo adaptation strategies in response to ischaemic/reperfusion episodes and brain trauma is also given.

ATP-Sensitive Potassium Channels Mediate the Cardioprotective Effect of Panax notoginseng Saponins against Myocardial Ischaemia-Reperfusion Injury and Inflammatory Reaction

Inflammatory response during myocardial ischemia reperfusion injury (MIRI) is essential for cardiac healing, while excessive inflammation extends the infarction and promotes adverse cardiac remodeling. Understanding the mechanism of these uncontrolled inflammatory processes has a significant impact during the MIRI therapy. Here, we found a critical role of ATP-sensitive potassium channels (K_ATP) in the inflammatory response of MIRI and its potential mechanism and explored the effects of Panax Notoginseng Saponins (PNS) during this possess. Rats underwent 40 min ischemia by occlusion of the left anterior descending (LAD) coronary artery and 60 min of reperfusion. PNS was treated at the corresponding time point before operation; 5-hydroxydecanoate (5-HD) and glybenclamide (Gly) (or Nicorandil (Nic)) were used as pharmacological blocker (or nonselective opener) of K_ATP. Cardiac function and pathomorphology were evaluated and a set of molecular signaling experiments was tested. K_ATP current density was measured by patch-clamp. Results revealed that in MIRI, PNS pretreatment restored cardiac function, reduced infarct size, and ameliorated inflammation through K_ATP. However, inhibiting K_ATP by 5-HD and Gly significantly reversed the effects, including NLRP3 inflammasome and inflammatory mediators IL-6, MPO, TNF-α, and MCP-1. Moreover, PNS inhibited the phosphorylation and nuclear translocation of NF-κB in I/R myocardium when the K_ATP was activated. Importantly, PNS promoted the expression of subunits and activation of K_ATP. The study uncovered K_ATP served as a new potential mechanism during PNS modulating MIRI-induced inflammation and promoting injured heart recovery. The manipulation of K_ATP could be a potential therapeutic approach for MIRI and other inflammatory diseases.

A novel form of glycolytic metabolism-dependent cardioprotection revealed by PKCα and β inhibition

KEY POINTS

Acute hyperglycaemia at the time of a heart attack worsens the outcome for the patient. Acute hyperglycaemia is not limited to diabetic patients and can be due to a stress response in non-diabetics. This study suggests that the damaging cardiac effects of hyperglycaemia can be reversed by selective PKC inhibition. If PKCα/β isoforms are inhibited, then high glucose itself becomes protective against ischaemic damage. Selective PKC inhibition may therefore be a useful therapeutic tool to limit the damage that can occur during a heart attack by stress-induced hyperglycaemia.

ABSTRACT

Hyperglycaemia has a powerful association with adverse prognosis for patients with acute coronary syndromes (ACS). Previous work shows that high glucose prevents ischaemic preconditioning and causes electrical and mechanical disruption via protein kinase C α/β (PKCα/β) activation. The present study aimed to: (i) determine whether the adverse clinical association of hyperglycaemia in ACS can be replicated in preclinical cellular models of ACS and (ii) determine the importance of PKCα/β activation to the deleterious effect of glucose. Freshly isolated rat, guinea pig or rabbit cardiomyocytes were exposed to simulated ischaemia after incubation in the presence of normal (5 mm) or high (20 mm) glucose in the absence or presence of small molecule or tat-peptide-linked PKCαβ inhibitors. In each of the four conditions, the following hallmarks of cardioprotection were recorded using electrophysiology or fluorescence imaging: cardiomyocyte contraction and survival, action potential stability and time to failure, intracellular calcium and ATP, mitochondrial depolarization, ischaemia-sensitive leak current, and time to K_ir 6.2 opening. High glucose alone resulted in decreased cardiomyocyte contraction and survival; however, it also imparted cardioprotection in the presence of PKCα/β inhibitors. This cardioprotective phenotype displayed improvements in all of the measured parameters and decreased myocardium damage during whole heart coronary ligation experiments. High glucose is deleterious to cellular and whole-heart models of simulated ischaemia, in keeping with the clinical association of hyperglycaemia with an adverse outcome in ACS. PKCαβ inhibition revealed high glucose to show a cardioprotective phenotype in this setting. The results of the present study suggest the potential for the therapeutic application of PKCαβ inhibition in ACS associated with hyperglycaemia.

Ischemic postconditioning and pinacidil suppress calcium overload in anoxia-reoxygenation cardiomyocytes via down-regulation of the calcium-sensing receptor

Ischemic postconditioning (IPC) and ATP sensitive potassium channel (KATP) agonists (e.g. pinacidil and diazoxide) postconditioning are effective methods to defeat myocardial ischemia-reperfusion (I/R) injury, but their specific mechanisms of reducing I/R injury are not fully understood. We observed an intracellular free calcium ([Ca²⁺]_i) overload in Anoxia/reoxygenation (A/R) cardiomyocytes, which can be reversed by KATP agonists diazoxide or pinacidil. The calcium-sensing receptor (CaSR) regulates intracellular calcium homeostasis. CaSR was reported to be involved in the I/R-induced apoptosis in rat cardiomyocytes. We therefore hypothesize that IPC and pinacidil postconditioning (PPC) reduce calcium overload in I/R cardiomyocytes by the down-regulation of CaSR. A/R model was established with adult rat caridomyocyte. mRNA and protein expression of CaSR were detected, IPC, PPC and KATP’s effects on [Ca²⁺]_i concentration was assayed too. IPC and PPC ameliorated A/R insult induced [Ca²⁺]_i overload in cardiomyocytes. In addition, they down-regulated the mRNA and protein level of CaSR as we expected. CaSR agonist spermine and KATP blocker glibenclamide offset IPC’s effects on CaSR expression and [Ca²⁺]_i modulation. Our data indicate that CaSR down-regulation contributes to the mitigation of calcium overload in A/R cardiomyocytes, which may partially represents IPC and KATP’s myocardial protective mechanism under I/R circumstances.

Distinct and complementary roles for α and β isoenzymes of PKC in mediating vasoconstrictor responses to acutely elevated glucose

BACKGROUND AND PURPOSE

We investigated the hypothesis that elevated glucose increases contractile responses in vascular smooth muscle and that this enhanced constriction occurs due to the glucose-induced PKC-dependent inhibition of voltage-gated potassium channels.

EXPERIMENTAL APPROACH

Patch-clamp electrophysiology in rat isolated mesenteric arterial myocytes was performed to investigate the glucose-induced inhibition of voltage-gated potassium (Kv ) current. To determine the effects of glucose in whole vessel, wire myography was performed in rat mesenteric, porcine coronary and human internal mammary arteries.

KEY RESULTS

Glucose-induced inhibition of Kv was PKC-dependent and could be pharmacologically dissected using PKC isoenzyme-specific inhibitors to reveal a PKCβ-dependent component of Kv inhibition dominating between 0 and 10 mM glucose with an additional PKCα-dependent component becoming evident at concentrations greater than 10 mM. These findings were supported using wire myography in all artery types used, where contractile responses to vessel depolarization and vasoconstrictors were enhanced by increasing bathing glucose concentration, again with evidence for distinct and complementary PKCα/PKCβ-mediated components.

CONCLUSIONS AND IMPLICATIONS

Our results provide compelling evidence that glucose-induced PKCα/PKCβ-mediated inhibition of Kv current in vascular smooth muscle causes an enhanced constrictor response. Inhibition of Kv current causes a significant depolarization of vascular myocytes leading to marked vasoconstriction. The PKC dependence of this enhanced constrictor response may present a potential therapeutic target for improving microvascular perfusion following percutaneous coronary intervention after myocardial infarction in hyperglycaemic patients.