Helen Maddock

Inhibiting mitochondrial permeability transition pore opening: a new paradigm for myocardial preconditioning?

Objective: We propose that ischemic preconditioning (IPC) and mitochondrial KATP channel activation protect the myocardium by inhibiting mitochondrial permeability transition pore (MPTP) opening at reperfusion. Methods: Isolated rat hearts were subjected to 35 min ischemia/120 min reperfusion and assigned to the following groups: (1) control; (2) IPC of 2x5 min each of preceding global ischemia; (3,4,5) 0.2 µmol/l cyclosporin A (CsA, which inhibits MPTP opening), 5 µmol/l FK506 (which inhibits the phosphatase calcineurin without inhibiting MPTP opening), or 20 µmol/l atractyloside (Atr, a MPTP opener) given at reperfusion; (6,7) pre-treatment with 30 µmol/l diazoxide (Diaz, a mitochondrial KATP channel opener) or 200 nmol/l 2 chloro-N6-cyclopentyl-adenosine (CCPA, an adenosine A1 receptor agonist); (8) IPC+Atr; (9) Diaz+Atr; (10) CCPA+Atr. The effect of mitochondrial KATP channel activation on calcium-induced MPTP opening in isolated calcein-loaded mitochondria was also assessed. Results: IPC, CsA when given at reperfusion, and pre-treatment with diazoxide or CCPA all limited infarct size (19.9±2.6% in IPC; 24.6±1.9% in CsA, 18.0±1.7% in Diaz, 20.4±3.3% in CCPA vs. 44.7±2.0% in control, P<0.0001). Opening the MPTP with atractyloside at reperfusion abolished this cardio-protective effect (47.7±1.8% in IPC+Atr, 42.3±3.2% in Diaz+Atr, 51.2±1.6% in CCPA+Atr). Atractyloside and FK506, given at reperfusion, did not influence infarct size (45.7±2.1% in Atr and 43.1±3.6% in FK506 vs. 44.7±2.0% in control, P = NS). Diazoxide (30 µmol/l) was shown to reduce calcium-induced MPTP opening by 52.5±8.0% in calcein-loaded mitochondria. 5-Hydroxydecanoic acid (100 µmol/l) was able to abolish the cardio-protective effects of both diazoxide and IPC. Conclusion: One interpretation of these data is that IPC and mitochondrial KATP channel activation may protect the myocardium by inhibiting MPTP opening at reperfusion.

Glimepiride, a Novel Sulfonylurea, Does Not Abolish Myocardial Protection Afforded by Either Ischemic Preconditioning or Diazoxide

Background—The sulfonylurea glibenclamide (Glib) abolishes the cardioprotective effect of ischemic preconditioning (IP), presumably by inhibiting mitochondrial KATP channel opening in myocytes. Glimepiride (Glim) is a new sulfonylurea reported to affect nonpancreatic KATP channels less than does Glib. We examined the effects of Glim on IP and on the protection afforded by diazoxide (Diaz), an opener of mitochondrial KATP channels. Methods and Results—Rat hearts were Langendorff-perfused, subjected to 35 minutes of regional ischemia and 120 minutes of reperfusion, and assigned to 1 of the following treatment groups: (1) control; (2) IP of 2× 5 minutes each of global ischemia before lethal ischemia; or pretreatment with (3) 30 &mgr;mol/L Diaz, (4) 10 &mgr;mol/L Glim, (5) 10 &mgr;mol/L Glib, (6) IP+Glim, (7) IP+Glib, (8) Diaz+Glim, or (9) Diaz+Glib. IP limited infarct size (18.5±1% vs 43.7±3% in control, P <0.01) as did Diaz (22.2±4.7%, P <0.01). The protective actions of IP or Diaz were not abolished by Glim (18.5±3% in IP+Glim, 22.3±3% in Diaz+Glim;P <0.01 vs control). However, Glib abolished the infarct-limiting effects of IP and Diaz. Patch-clamp studies in isolated rat ventricular myocytes confirmed that both Glim and Glib (each at 1 &mgr;mol/L) blocked sarcolemmal KATP currents. However, in isolated cardiac mitochondria, Glim (10 &mgr;mol/L) failed to block the effects of KATP opening by GTP, in contrast to the blockade caused by Glib. Conclusions—Although it blocks sarcolemmal currents in rat cardiac myocytes, Glim does not block the beneficial effects of mitochondrial KATP channel opening in the isolated rat heart. These data may have significant implications for the treatment of type 2 diabetes in patients with ongoing ischemic heart disease.

The cardioprotective and mitochondrial depolarising properties of exogenous nitric oxide in mouse heart

OBJECTIVE Nitric oxide (NO) is reported to be both protective and detrimental in models of myocardial ischaemia/reperfusion injury, which may be concentration dependent. Our objective was to characterise this dichotomy using the nitric oxide donor, S-nitroso N-acetyl penicillamine (SNAP) in isolated perfused mouse heart and isolated mouse cardiac mitochondria. METHODS To determine the effect of nitric oxide concentration on myocardial viability, isolated mouse hearts were subjected to 35 min global ischaemia and 30 min reperfusion in the presence of SNAP (0.02-20 microM). To determine whether NO mediated protection was via opening of the putative mitochondrial K(ATP) channel and/or free radical synthesis, SNAP perfused hearts were also treated with the mitochondrial K(ATP) channel blocker, 5-hydroxy decanoate (5-HD) and the free-radical scavenger, N-(2-mercaptopropionyl)-glycine (MPG). This data was correlated with mitochondrial membrane potential (Delta Psi(m)), measured with the potentiometric dye, tetra-methyl rhodium methyl ester (TMRM), in isolated mitochondria,by flow cytometry. RESULTS SNAP dose-dependently attenuated infarct size, with maximal protection observed at 2 microM (17+/-4% versus controls 32+/-3%, P<0.01). At greater concentrations however, protection was lost with infarct sizes tending towards control at 20 microM (29+/-3%). These results were paralleled by changes in Delta Psi(m) in the isolated mitochondria: Delta Psi(m) depolarisation peaking with 1 microM SNAP (26+/-4% shift in TMRM fluorescence, P<0.01); at greater concentrations, this relationship was lost. The mitochondrial K(ATP) channel blocker, 5-HD, resulted in both abrogation of SNAP infarct size reduction and concomitant loss of Delta Psi(m) depolarisation in the mitochondria. MPG however did not influence the cardioprotective properties of SNAP. CONCLUSION We demonstrate that nitric oxide can mediate cardioprotection in a dose-dependent fashion by an effect that may be related to Delta Psi(m). Both cardioprotection and Delta Psi(m) changes are sensitive to 5-HD and the cardioprotection appears independent of free-radical synthesis.

Myocardial Protection from Either Ischaemic Preconditioning or Nicorandil Is Not Blocked by Gliclazide

AbstractObjective: We compared the effects of two sulphonylureas, glibenclamide and gliclazide, on ischaemic preconditioning (IPC) and nicorandil-induced protection in the in-vivo rat. We also studied the effects of these agents on the membrane potential of isolated rat mitochondria. Methods: Anaesthetised male Sprague-Dawley rats were used in an open chest model of myocardial infarction. Animals were randomly assigned to receive one of the following drugs: (1) saline control, (2) glibenclamide, 0.3 mg/kg, or (3) gliclazide, 1 mg/kg i.v. bolus. Each was then further randomised to one of the following treatments: (a) control, (b) IPC (consisting of 2 × 5 mins of regional ischaemia and 5 minutes reperfusion) or (c) nicorandil (50 ug/kg/min i.v). infusion. Each group then underwent 25 mins regional ischaemia and 2 hrs reperfusion. Infarct to risk zone ratio (%) was calculated by computerised planimetry of tetrazolium stained heart slices. The membrane potential of mitochondria isolated from rat ventricles was measured using flow cytometry. Comparisons were made between groups in control medium, nicorandil alone, and nicorandil with either glibenclamide or gliclazide. Results: Infarct size was significantly reduced with IPC (15.0 ± 1.1%,) and nicorandil (25.5 ± 4.2%), versus control (44.1 ± 3.2%), p < 0.005. Glibenclamide abolished IPC (40.8 ± 4.6%) and nicorandil-induced protection completely (39.5 ± 5.1%). Gliclazide had no adverse effect on IPC (20.4 ± 1.9%) or nicorandil-induced protection (23.6 ± 2.2%), p < 0.005. Nicorandil caused a partial depolarisation of the mitochondrial membrane potential (−14.92 ± 2.34%), which was abolished by glibenclamide (+2.03 ± 0.53%), but not gliclazide (−16.47 ± 3.36%), p < 0.01. Conclusion: Both IPC and nicorandil-induced protection are abolished by glibenclamide but not gliclazide in-vivo. These results may have important clinical implications in type II diabetic patients at risk of acute coronary syndromes.

Attenuation of Doxorubicin-Induced Cardiotoxicity by mdivi-1: A Mitochondrial Division/Mitophagy Inhibitor

Doxorubicin is one of the most effective anti-cancer agents. However, its use is associated with adverse cardiac effects, including cardiomyopathy and progressive heart failure. Given the multiple beneficial effects of the mitochondrial division inhibitor (mdivi-1) in a variety of pathological conditions including heart failure and ischaemia and reperfusion injury, we investigated the effects of mdivi-1 on doxorubicin-induced cardiac dysfunction in naïve and stressed conditions using Langendorff perfused heart models and a model of oxidative stress was used to assess the effects of drug treatments on the mitochondrial depolarisation and hypercontracture of cardiac myocytes. Western blot analysis was used to measure the levels of p-Akt and p-Erk 1/2 and flow cytometry analysis was used to measure the levels p-Drp1 and p-p53 upon drug treatment. The HL60 leukaemia cell line was used to evaluate the effects of pharmacological inhibition of mitochondrial division on the cytotoxicity of doxorubicin in a cancer cell line. Doxorubicin caused a significant impairment of cardiac function and increased the infarct size to risk ratio in both naïve conditions and during ischaemia/reperfusion injury. Interestingly, co-treatment of doxorubicin with mdivi-1 attenuated these detrimental effects of doxorubicin. Doxorubicin also caused a reduction in the time taken to depolarisation and hypercontracture of cardiac myocytes, which were reversed with mdivi-1. Finally, doxorubicin caused a significant elevation in the levels of signalling proteins p-Akt, p-Erk 1/2, p-Drp1 and p-p53. Co-incubation of mdivi-1 with doxorubicin did not reduce the cytotoxicity of doxorubicin against HL-60 cells. These data suggest that the inhibition of mitochondrial fission protects the heart against doxorubicin-induced cardiac injury and identify mitochondrial fission as a new therapeutic target in ameliorating doxorubicin-induced cardiotoxicity without affecting its anti-cancer properties.

Doxorubicin induced myocardial injury is exacerbated following ischaemic stress via opening of the mitochondrial permeability transition pore.

Chemotherapeutic agents such as doxorubicin are known to cause or exacerbate cardiovascular cell death when an underlying heart condition is present. However, the mechanism of doxorubicin-induced cardiotoxicity is unclear. Here we assess the cardiotoxic effects of doxorubicin in conditions of myocardial ischaemia reperfusion and the mechanistic basis of protection, in particular the role of the mitochondrial permeability transition pore (mPTP) in such protection. The effects of doxorubicin (1μM)±cyclosporine A (CsA, 0.2μM; inhibits mPTP) were investigated in isolated male Sprague-Dawley rats using Langendorff heart and papillary muscle contraction models subjected to simulated ischaemia and reperfusion injury. Isolated rat cardiac myocytes were used in an oxidative stress model to study the effects of drug treatment on mPTP by confocal microscopy. Western blot analysis evaluated the effects of drug treatment on p-Akt and p-Erk 1/2 levels. Langendorff and the isometric contraction models showed a detrimental effect of doxorubicin throughout reperfusion/reoxygenation as well as increased p-Akt and p-Erk levels. Interestingly, CsA not only reversed the detrimental effects of doxorubicin, but also reduced p-Akt and p-Erk levels. In the sustained oxidative stress assay to study mPTP opening, doxorubicin decreased the time taken to depolarization and hypercontracture, but these effects were delayed in the presence of CsA. Collectively, our data suggest for the first that doxorubicin exacerbates myocardial injury in an ischaemia reperfusion model. If the inhibition of mPTP ameliorates the cardiotoxic effects of doxorubicin, then more selective inhibitors of mPTP should be further investigated for their utility in patients receiving doxorubicin.

Molecular basis of cancer-therapy-induced cardiotoxicity: introducing microRNA biomarkers for early assessment of subclinical myocardial injury.

Development of reliable biomarkers for early clinical assessment of drug-induced cardiotoxicity could allow the detection of subclinical cardiac injury risk in vulnerable patients before irreversible damage occurs. Currently, it is difficult to predict who will develop drug-induced cardiotoxicity owing to lack of sensitivity and/or specificity of currently used diagnostics. miRNAs are mRNA regulators and they are currently being extensively profiled for use as biomarkers due to their specific tissue and disease expression signature profiles. Identification of cardiotoxicity-specific miRNA biomarkers could provide clinicians with a valuable tool to allow prognosis of patients at risk of cardiovascular injury, alteration of a treatment regime or the introduction of an adjunct therapy in order to increase the long-term survival rate of patients treated with cardiotoxic drugs.

Effects of adenosine receptor agonists on guinea-pig isolated working hearts and the role of endothelium and NO

The hypothesis that the coronary vasodilator effects of adenosine receptor agonists are independent of the vascular endothelium or mediators derived therefrom was examined in guinea‐pig isolated working hearts. Adenosine receptor agonists, 5′‐(N‐ethylcarboxamido)‐adenosine (NECA; two‐fold selective for A2 over A1 receptors), 2‐[p‐(2‐carboxyethyl)phenylethylamino]‐5′‐N‐ethylcarboxamidoadenosine (CGS21680; A2A selective), N6‐cyclopentyl‐adenosine (CPA; A1 selective) and N6‐(3‐iodobenzyl)adenosine‐5′‐N‐methyluronamide (IB‐MECA; A3 selective), were infused (3 times 10−7 M) after endothelium removal by passing oxygen through the coronary circulation. In spontaneously beating hearts, CGS21680 and NECA increased, while CPA decreased, coronary flow. NECA and CPA reduced heart rate, left ventricular pressure and aortic output. The nitric oxide synthase (NOS) inhibitor, NG‐nitro‐L‐arginine (L‐NOARG; 3 times 10−5 M) abolished the vasodilatation by NECA but not CGS21680, indicating that nitric oxide (NO) of a non‐endothelial source mediated the NECA response. Coronary vasodilatation by CGS21680 was inhibited by the A2A receptor antagonist, 4‐(2‐[7‐amino‐2‐(2‐furyl)[1,2,4]triazolo [2,3‐a][1,3,5]triazin‐5‐ylamino]ethyl)phenol (ZM241385). Indometacin (10−6 M) attenuated the coronary vasodilatation to CGS21680, suggesting a partial role for cyclooxygenase products. IBMECA had no effect, indicating no A3 receptor involvement. In paced working hearts, the responses were similar except CPA had no effect on coronary flow or aortic output and CGS21680 increased left ventricular pressure and the maximum rate of ventricular pressure rise. This study has demonstrated functionally effective removal of the endothelium by a novel method of passing oxygen through the coronary vasculature. A coronary vasodilator action of adenosine receptor agonists mediated via A2A receptors is endothelium‐and NO‐independent, but partially involves cyclooxygenase products.

Interactions Between Gasotransmitters

It is well established that nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) have signaling roles in the body. There are important similarities among them in their actions and generation, but there are also intriguing differences. The mechanism of action of H2S still has not been fully elucidated. It is becoming increasingly clear that there are important interactions among the gasotransmitters. There is clear evidence of links between the NO- and CO-generating systems. So far, this is most apparent in the control of the cardiovascular system, and knowledge of the function of NO has led to new therapeutic interventions. There is also a suggestion of synergy between NO and H2S that is not yet fully understood. Interactions between CO and H2S have not yet been explored, and more research is required in this area. Interactions in the immune system also require more research, and increased understanding of this area could lead to novel therapies.

Predictivity of in vitro non-clinical cardiac contractility assays for inotropic effects in humans--A literature search.

Adverse drug effects on the cardiovascular system are a major cause of compound attrition throughout compound discovery and development. There are many ways by which drugs can affect the cardiovascular system, including effects on the electrocardiogram, vascular resistance, heart rate and the force of contraction of the heart (inotropy). Compounds that increase the force of contraction of the heart can be harmful in patients with ischemic heart disease, whilst negative inotropes can induce symptoms of heart failure. There is a range of non-clinical in vitro and in vivo assays used to detect inotropic effects of drugs. We have conducted a literature review of the in vitro assays and compared the findings from these with known effects on cardiac contractility in man. There was a wide variety of assays used, ranging from perfuse whole hearts to isolated regions of the heart (papillary muscle, ventricle and atria), which were removed from a number of species (cat, guinea pig, rabbit and rat). We conducted two analyses. The first was investigating the concordance of the findings from the in vitro assays at any concentration with those observed in man (an assessment of hazard identification) and the second was the concordance of the in vitro findings at concentrations tested up to 10-fold higher than those tested in the clinic. We found that when used as a hazard identification tool, the available assays had good sensitivity (88%), although the specificity was not so good (60%), but when used as a risk management tool the sensitivity was considerably reduced (sensitivity 58-70% and specificity 60%). These data would suggest that the available in vitro assays can be used as hazard identification tools for adverse drug effects on cardiac contractility, but there is a need for new assays to better predict the exposures in man that may cause a change in cardiac contractility and therefore better predict the likely therapeutic index of compounds prior to nomination of compounds for clinical development.