ATP-sensitive potassium channels are altered in ventricular myocytes from diabetic rats

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.

Ischemic preconditioning: Interruption of various disorders

Ischemic heart diseases are the leading cause of morbidity and mortality worldwide. Reperfusion of an ischemic heart is necessary to regain the normal functioning of the heart. However, abrupt reperfusion of an ischemic heart elicits a cascade of adverse events that leads to injury of the myocardium, i.e., ischemia-reperfusion injury. An endogenous powerful strategy to protect the ischemic heart is ischemic preconditioning, in which the myocardium is subjected to short periods of sublethal ischemia and reperfusion before the prolonged ischemic insult. However, it should be noted that the cardioprotective effect of preconditioning is attenuated in some pathological conditions. The aim of this article is to review present knowledge on how menopause and some metabolic disorders such as diabetes and hyperlipidemia affect myocardial ischemic preconditioning and the mechanisms involved.

Cardioprotection from emulsified isoflurane postconditioning is lost in rats with streptozotocin-induced diabetes due to the impairment of Brg1/Nrf2/STAT3 signalling

IsoPostC confers cardioprotection against myocardial IRI in non-diabetic rats but loses its effectiveness in diabetes, which may be mainly due to the impairment/reduction of Brg1/Nrf2/STAT3.

The Influence of Diabetes Mellitus in Myocardial Ischemic Preconditioning

Ischemic preconditioning (IP) is a powerful mechanism of protection discovered in the heart in which ischemia paradoxically protects the myocardium against other ischemic insults. Many factors such as diseases and medications may influence IP expression. Although diabetes poses higher cardiovascular risk, the physiopathology underlying this condition is uncertain. Moreover, although diabetes is believed to alter intracellular pathways related to myocardial protective mechanisms, it is still controversial whether diabetes may interfere with ischemic preconditioning and whether this might influence clinical outcomes. This review article looks at published reports with animal models and humans that tried to evaluate the possible influence of diabetes in myocardial ischemic preconditioning.

Mechanisms of sudden cardiac death: oxidants and metabolism

Ventricular arrhythmia is the leading cause of sudden cardiac death (SCD). Deranged cardiac metabolism and abnormal redox state during cardiac diseases foment arrhythmogenic substrates through direct or indirect modulation of cardiac ion channel/transporter function. This review presents current evidence on the mechanisms linking metabolic derangement and excessive oxidative stress to ion channel/transporter dysfunction that predisposes to ventricular arrhythmias and SCD. Because conventional antiarrhythmic agents aiming at ion channels have proven challenging to use, targeting arrhythmogenic metabolic changes and redox imbalance may provide novel therapeutics to treat or prevent life-threatening arrhythmias and SCD.

Remote ischemic preconditioning improves outcome at 6 years after elective percutaneous coronary intervention: the CRISP stent trial long-term follow-up

BACKGROUND

Postprocedural myocardial infarction (type 4a) has been shown to be an adverse prognostic indicator after elective percutaneous coronary intervention (PCI). The Cardiac Remote Ischemic Preconditioning in Coronary Stenting (CRISP Stent) study demonstrated that remote ischemic preconditioning reduced procedural symptoms, ECG ST-segment deviation, and cardiac troponin I release after elective PCI and reduced the major adverse cardiac and cerebral event (MACCE) rate at 6 months. We were interested to confirm if this early benefit in MACCE rate in the remote ischemic preconditioning group was sustained long-term.

METHODS AND RESULTS

Patients were telephoned by researchers blinded to the randomization details. MACCE, defined as all-cause mortality, nonfatal myocardial infarction, transient ischemic attack or stroke, and heart failure requiring hospital admission, were adjudicated by case note and national database review. One hundred ninety-two (89.3%) of the 225 patients with elective PCI randomized in the original study were available for long-term follow-up (mean time to event or last follow-up: 1579.7±603.6 days). There were a total of 59 (30.7%) MACCEs. Patients with an MACCE had a higher mean cardiac troponin I after PCI (±SD): 2.07±6.99 versus 0.91±2.07 ng/mL (P=0.05). The MACCE rate at 6 years remained lower in the remote ischemic preconditioning group (hazard ratio, 0.58; 95% confidence interval, 0.35-0.97; P=0.039; absolute risk reduction=0.13 and number needed to treat=8 to prevent the MACCE at 6 years).

CONCLUSIONS

Remote ischemic preconditioning reduces the incidence of postprocedural cardiac troponin I after elective PCI and confers an MACCE-free survival benefit at both short- and long-term follow-up.

CLINICAL TRIAL REGISTRATION

URL: http://www.ukcrn.org.uk. Unique identifier: UKCRN 4074.

The diabetic heart: too sweet for its own good?

Diabetes mellitus is a major risk factor for ischemic heart disease (IHD). Patients with diabetes and IHD experience worse clinical outcomes, suggesting that the diabetic heart may be more susceptible to ischemia-reperfusion injury (IRI). In contrast, the animal data suggests that the diabetic heart may be either more, equally, or even less susceptible to IRI. The conflicting animal data may be due to the choice of diabetic and/or IRI animal model. Ischemic conditioning, a phenomenon in which the heart is protected against IRI by one or more brief nonlethal periods of ischemia and reperfusion, may provide a novel cardioprotective strategy for the diabetic heart. Whether the diabetic heart is amenable to ischemic conditioning remains to be determined using relevant animal models of IRI and/or diabetes. In this paper, we review the limitations of the current experimental models used to investigate IRI and cardioprotection in the diabetic heart.

Preconditioning the diabetic human myocardium

Our objective was to determine whether human diabetic myocardium is amenable to the cardioprotective actions of ischaemic preconditioning. Human right atrial appendages were harvested from diabetic and non-diabetic patients undergoing elective coronary artery bypass graft surgery. The atrial trabeculae were isolated and subjected to 90 min. of hypoxia followed by 120 min. of reoxygenation, following which the percentage recovery of baseline contractile function was determined. The atrial trabeculae were randomized to: (i) controls (groups 1 and 3); (ii) standard hypoxic preconditioning (HPC) protocol consisting of 4 min. of hypoxia/16 min. of reoxygenation before the 90 min. index hypoxic period (groups 2 and 4); (iii) Prolonged HPC protocol consisting of: 7 min. of hypoxia /16 min. of reoxygenation before the index hypoxic period (group 5). In addition, basal levels of Akt phosphorylation were determined in right atrial appendages harvested from non-diabetic patients and diabetic patients to determine whether PI3K-Akt signalling is down-regulated in the diabetic heart. Standard HPC improved baseline contractile function in human atrial trabeculae harvested from non-diabetic patients (52.4 +/- 3.8% with HPC versus 30.0 +/- 3.2% in control: P = 0.001; N = 6/group), but not in atrial trabeculae isolated from diabetic patients (22.6 +/- 3.3% with HPC versus 28.5 +/- 1.9% in control: P > 0.05; N = 6/group). However, the prolonged HPC protocol did improve baseline contractile function in atrial trabeculae harvested from diabetic patients (42.0 +/- 2.4% with HPC versus 28.5 +/- 1.9% in control: P= 0.001; N > or = 6/group). Western blot analysis demonstrated lower levels of phosphorylated Akt in diabetic myocardium compared to non-diabetic myocardium (0.13 +/- 0.03 arbitrary units versus 0.39 +/- 0.11 arbitrary units: P= 0.047; N > or = 4/group). From the data obtained it appears that the threshold for preconditioning the diabetic myocardium is elevated which may be related to the down-regulation of the PI3K-Akt pathway.

Ischemic tolerance of rat hearts in acute and chronic phases of experimental diabetes

Different from clinical studies of diabetes mellitus (DM), experimental data reveal both, higher and lower vulnerability of the heart to ischemic injury. We have previously demonstrated an enhanced resistance to ischemia-induced arrhythmias in isolated rat hearts in the acute phase of DM. Our objectives were thus to extend our knowledge to the effects of DM of different duration on myocardial infarction, in conjunction with susceptibility to arrhythmias, in the in vivo model. DM was induced by streptozotocin (45 mg/kg, i.v.) and following 1 week (acute phase) and 8 weeks (chronic phase), anesthetized open-chest diabetic and age-matched control rats were subjected to 30-min regional ischemia (occlusion of LAD coronary artery) followed by 4-h reperfusion for the evaluation of the infarct size (tetrazolium staining). In the control rats, ventricular tachycardia (VT) represented 45.4% of total arrhythmias and occurred in 90% of the animals. In the acute phase of DM, arrhythmia profile was similar to that in the control animals, and the incidence and severity of arrhythmias were not enhanced. On the other hand, the size of infarct area normalized to the size of area at risk was significantly smaller in the diabetics than in the controls (47.2 +/- 2.8 vs. 70.2 +/- 2.1%, respectively; p < 0.05). In the chronic phase, only 17.7% of arrhythmias occurred as VT in 44% of the diabetics (p < 0.05 vs. controls). Severity of arrhythmias was also lower (arrhythmia score: 2.1 +/- 0.3 vs. 2.9 +/- 0.3 in the controls, respectively; p < 0.05). This effect was not due to a smaller infarct size, since the latter did not differ from that in the controls.

IN CONCLUSION

diabetic rat hearts exhibit rather lower, than higher sensitivity to ischemia. In acute phase of DM, diabetic hearts are more resistant to irreversible cell damage, whereas in the chronic phase they exhibit reduced susceptibility to arrhythmias; these discrepancies might reflect different pathogenesis of arrhythmias and myocardial infarction.

Effects of arrhythmogenic lipid metabolites on the L-type calcium current of diabetic vs. non-diabetic rat hearts

Accumulation of lipid metabolites, such as palmitoylcarnitine and lysophosphatidylcholine, is thought to be a major contributor to the development of cardiac arrhythmias during myocardial ischemia. This arrhythmogenicity is likely due to the effects of these metabolites on various ion channels. Diabetic hearts have been shown to accumulate much higher concentrations of these lipid metabolites during ischemia, which may be an important factor in the enhanced incidence of arrhythmias in diabetic hearts. However, it is not known whether these metabolites have similar effects on the ion channels of diabetic hearts as in non-diabetic hearts. Previous studies on myocytes from non-diabetic hearts have reported either enhancement or inhibition of L-type calcium current (I(Ca)) by these lipid metabolites. Thus, it is not clear whether the effects of palmitoylcarnitine and/or lysophosphatidlycholine on I(Ca) contribute to the enhanced arrhythmogenicity of diabetic hearts or protect against arrhythmias. We determined the effect of exogenous palmitoylcarnitine and lysophosphatidylcholine on the (I(Ca)) in ventricular myocytes from streptozotocin-diabetic and non-diabetic rat hearts under identical conditions. We found that palmitoylcarnitine and lysophosphatidylcholine exhibited a dose-dependent inhibition of I(Ca), which was virtually identical in diabetic and non-diabetic cardiac myocytes. Thus, we conclude that these arrhythmogenic lipid metabolites have similar actions on calcium channels in diabetic and non-diabetic hearts. Therefore, the greater susceptibility of diabetic hearts to arrhythmias during myocardial ischemia is not due to an altered sensitivity of the L-type calcium channels to lipid metabolites, but may be explained, in large part, by the greater accumulation of these metabolites during ischemia.

Acute diabetes modulates response to ischemia in isolated rat heart

Diabetic hearts are suggested to exhibit either increased or lower sensitivity to ischemia. Detrimental effects of prolonged ischemia can be attenuated by preconditioning, however, relatively little is known about its effects in the diseased myocardium. This study was designed to test the susceptibility to ischemia-induced arrhythmias and the effect of preconditioning in the diabetic heart. Rats were made diabetic with streptozotocin (45 mg/kg, i.v.). After 1 week, isolated Langendorff-perfused hearts were subjected to 30 min occlusion of LAD coronary artery without or with preceding preconditioning induced by one cycle of 5 min ischemia and 10 min reperfusion. Glycogen and lactate contents were estimated in the preconditioned and non-preconditioned hearts before and after ischemia. Diabetic hearts were more resistant to ischemia-induced arrhythmias: incidence of ventricular tachycardia (VT) decreased to 42% and only transient ventricular fibrillation (VF) occurred in 17% of the hearts as compared to the non-diabetic controls (VT 100% and VF 70% including sustained VF 36%; p < 0.05). Preconditioning effectively suppressed the incidence and severity of arrhythmias (VT 33%, VF 0%) in the normal hearts. However, this intervention did not confer any additional protection in the diabetic hearts. Despite higher glycogen content in the diabetic myocardium and greater glycogenolysis during ischemia, production of lactate in these hearts was significantly lower than in the controls. Preconditioning caused a substantial decrease in the accumulation of lactate in the normal hearts, whereby in the diabetic hearts, this intervention did not cause any further reduction in the level of lactate. In conclusion, diabetic rat hearts exhibit lower susceptibility to ischemic injury and show no additional response to preconditioning. Reduced production of glycolytic metabolites during ischemia can account for the enhanced resistance of diabetic hearts to ischemia as well as for the lack of further protection by preconditioning.

Altered ATP sensitivity of ATP-dependent K+ channels in diabetic rat hearts

The effects of streptozotocin-induced diabetes (5-7 days or 7 wk) on cardiac ATP-sensitive potassium channels (KATP channels) were investigated with the use of single-channel and action potential recordings from dissociated ventricular myocytes isolated from control and diabetic rat hearts. In inside-out patches from diabetic myocytes (5-7 days), the IC50 for ATP inhibition was 82 +/- 7.2 microM (mean +/- SE, n = 8), twice that in controls (43 +/- 3.6 microM, n = 12). For 7-wk diabetic rats, the IC50 was 75 +/- 2.3 microM (n = 6). Increasing internal ADP concentration attenuated ATP-induced inhibition in both controls and diabetics. On reducing the internal pH from 7.4 to 6.8, both control and diabetic myocytes showed a 1.7-fold increase in the IC50 for ATP inhibition. No differences were observed in either intraburst kinetics or unitary conductance of single channels from control and diabetic myocytes. In diabetic myocytes, action potential duration at 90% repolarization (APD90) was longer and more variable than in controls and was significantly shortened by application of the KATP channel opener cromakalim (50 microM). Cromakalim scarcely affected APD90 in controls. Computer simulation of the longer diabetic APD90 required a lower background conductance during the plateau phase in addition to small, measured changes in the delayed rectifier current, transient outward current, and ATP-sensitive K+ current (KATP current, IKATP). The simulations reproduced the enhanced sensitivity of the diabetic APD90 to changes in IKATP. These results have important implications for cardiac function in diabetics and their treatment by sulfonylureas.