Heart rate reduction after genetic ablation of L-type Cav1.3 channels induces cardioprotection against ischemia-reperfusion injury

Background

Acute myocardial infarction (AMI) is the major cause of cardiovascular mortality worldwide. Most ischemic episodes are triggered by an increase in heart rate, which induces an imbalance between myocardial oxygen delivery and consumption. Developing drugs that selectively reduce heart rate by inhibiting ion channels involved in heart rate control could provide more clinical benefits. The Cav1.3-mediated L-type Ca2+ current (ICav1.3) play important roles in the generation of heart rate. Therefore, they can constitute relevant targets for selective control of heart rate and cardioprotection during AMI.

Objective

We aimed to investigate the relationship between heart rate and infarct size using mouse strains knockout for Cav1.3 (Cav1.3-/-) L-type calcium channel and of the cardiac G protein gated potassium channel (Girk4-/-) in association with the funny (f)-channel inhibitor ivabradine.

Methods

Wild-type (WT), Cav1.3+/-, Cav1.3-/- and Girk4-/- mice were used as models of respectively normal heart rate, moderate heart rate reduction, bradycardia, and mild tachycardia, respectively. Mice underwent a surgical protocol of myocardial IR (40 min ischemia and 60 min reperfusion). Heart rate was recorded by one-lead surface ECG recording, and infarct size measured by triphenyl tetrazolium chloride staining. In addition, Cav1.3-/- and WT hearts perfused on a Langendorff system were subjected to the same ischemia-reperfusion protocol ex vivo, without or with atrial pacing, and the coronary flow was recorded.

Results

Cav1.3-/- mice presented reduced infarct size (-29%), while Girk4-/- displayed increased infarct size (+30%) compared to WT mice. Consistently, heart rate reduction in Cav1.3+/- or by the f-channel blocker ivabradine was associated with significant decrease in infarct size (-27% and -32%, respectively) in comparison to WT mice.

Conclusion

Our results show that decreasing heart rate allows to protect the myocardium against IR injury in vivo and reveal a close relationship between basal heart rate and IR injury. In addition, this study suggests that targeting Cav1.3 channels could constitute a relevant target for reducing infarct size, since maximal heart rate dependent cardioprotective effect is already observed in Cav1.3+/- mice.

Cardiac mGluR1 metabotropic receptors in cardioprotection

AIMS

In a previous study using a genome-wide microarray strategy, we identified metabotropic glutamate receptor 1 (mGluR1) as a putative cardioprotective candidate in ischaemic postconditioning (PostC). In the present study, we investigated the role of cardiac mGluR1 receptors during cardioprotection against myocardial ischaemia-reperfusion injury in the mouse myocardium.

METHODS AND RESULTS

mGluR1 activation by glutamate administered 5 min before reperfusion in C57Bl/6 mice subjected to a myocardial ischaemia protocol strongly decreased both infarct size and DNA fragmentation measured at 24 h reperfusion. This cardioprotective effect was mimicked by the mGluR1 agonist, DHPG (10 μM), and abolished when glutamate was coinjected with the mGluR1 antagonist YM298198 (100 nM). Wortmannin (100 nM), an inhibitor of PI3-kinase, was able to prevent glutamate-induced cardioprotection. A glutamate bolus at the onset of reperfusion failed to protect the heart of mGluR1 knockout mice subjected to a myocardial ischaemia-reperfusion protocol, although PostC still protected the mGluR1 KO mice. Glutamate-treatment improved post-infarction functional recovery as evidenced by an echocardiographic study performed 15 days after treatment and by a histological evaluation of fibrosis 21 days post-treatment. Interestingly, restoration of functional mGluR1s by a PostC stimulus was evidenced at the transcriptional level. Since mGluR1s were localized at the surface membrane of cardiomyocytes, they might contribute to the cardioprotective effect of ischaemic PostC as other Gq-coupled receptors.

CONCLUSION

This study provides the first demonstration that mGluR1 activation at the onset of reperfusion induces cardioprotection and might represent a putative strategy to prevent ischaemia-reperfusion injury.

Counteracting neuronal nitric oxide synthase proteasomal degradation improves glucose transport in insulin-resistant skeletal muscle from Zucker fa/fa rats

AIMS/HYPOTHESIS

Insulin-mediated glucose transport and utilisation are decreased in skeletal muscle from type 2 diabetic and glucose-intolerant individuals because of alterations in insulin receptor signalling, GLUT4 translocation to the plasma membrane and microvascular blood flow. Catalytic activity of the muscle-specific isoform of neuronal nitric oxide synthase (nNOS) also participates in the regulation of glucose transport and appears to be decreased in a relevant animal model of drastic insulin resistance, the obese Zucker fa/fa rat. Our objective was to determine the molecular mechanisms involved in this defect.

METHODS

Isolated rat muscles and primary cultures of myocytes were used for western blot analysis of protein expression, immunohistochemistry, glucose uptake measurements and GLUT4 translocation assays.

RESULTS

nNOS expression was reduced in skeletal muscle from fa/fa rats. This was caused by increased ubiquitination of the enzyme and subsequent degradation by the ubiquitin proteasome pathway. The degradation occurred through a greater interaction of nNOS with the chaperone heat-shock protein 70 and the co-chaperone, carboxyl terminus of Hsc70-interacting protein (CHIP). In addition, an alteration in nNOS sarcolemmal localisation was observed. We confirmed the implication of nNOS breakdown in defective insulin-induced glucose transport by demonstrating that blockade of proteasomal degradation or overexpression of nNOS improved basal and/or insulin-stimulated glucose uptake and GLUT4 translocation in primary cultures of insulin-resistant myocytes.

CONCLUSIONS/INTERPRETATION

Recovery of nNOS in insulin-resistant muscles should be considered a potential new approach to address insulin resistance.