Cardiac responses to β-adrenoceptor stimulation is partly dependent on mitochondrial calcium uniporter activity

BACKGROUND AND PURPOSE

Despite the importance of mitochondrial Ca(2+) to metabolic regulation and cell physiology, little is known about the mechanisms that regulate Ca(2+) entry into the mitochondria. Accordingly, we established a system to determine the role of the mitochondrial Ca(2+) uniporter in an isolated heart model, at baseline and during increased workload following β-adrenoceptor stimulation.

EXPERIMENTAL APPROACH

Cardiac contractility, oxygen consumption and intracellular Ca(2+) transients were measured in ex vivo perfused murine hearts. Ru360 and spermine were used to modify mitochondrial Ca(2+) uniporter activity. Changes in mitochondrial Ca(2+) content and energetic phosphate metabolite levels were determined.

KEY RESULTS

The addition of Ru360 , a selective inhibitor of the mitochondrial Ca(2+) uniporter, induced progressively and sustained negative inotropic effects that were dose-dependent with an EC50 of 7 μM. Treatment with spermine, a uniporter agonist, showed a positive inotropic effect that was blocked by Ru360 . Inotropic stimulation with isoprenaline elevated oxygen consumption (2.7-fold), Ca(2+) -dependent activation of pyruvate dehydrogenase (5-fold) and mitochondrial Ca(2+) content (2.5-fold). However, in Ru360 -treated hearts, this parameter was attenuated. In addition, β-adrenoceptor stimulation in the presence of Ru360 did not affect intracellular Ca(2+) handling, PKA or Ca(2+) /calmodulin-dependent PK signalling.

CONCLUSIONS AND IMPLICATIONS

Inhibition of the mitochondrial Ca(2+) uniporter decreases β-adrenoceptor response, uncoupling between workload and production of energetic metabolites. Our results support the hypothesis that the coupling of workload and energy supply is partly dependent on mitochondrial Ca(2+) uniporter activity.

The failing Fontan: what’s NEXT…?

The Fontan procedure represents the final stage of the transition to single ventricle physiology. Conversion of very complex congenital heart anatomy, such as hypoplastic left heart syndrome, double-outlet right ventricle or double-inlet left ventricle, to a single ventricle has grown in popularity as morbidity and mortality have improved. As these patients grow, survivors are at risk for impaired ventricular dysfunction, plastic bronchitis, protein-losing enteropathy and late failure. Late failing Fontan patients represent a particularly vexing scenario for clinicians, as the only durable treatment option is cardiac transplantation. However, in the short-term, some of these patients require support beyond medical management, with mechanical circulatory support via extracorporeal life support or a ventricular assist device. We report the successful bridge of an adolescent female post-Fontan conversion with late severe cardiac failure. The patient was initially resuscitated with extracorporeal life support, transitioned to a single Berlin Heart EXCOR® ventricular assist device and, subsequently, underwent successful cardiac transplantation.

N-acetylcysteine inhibits alveolar epithelial-mesenchymal transition

The ability of transforming growth factor-beta1 (TGF-beta1) to induce epithelial-mesenchymal transition (EMT) in alveolar epithelial cells (AEC) in vitro and in vivo, together with the demonstration of EMT in biopsies of idiopathic pulmonary fibrosis (IPF) patients, suggests a role for TGF-beta1-induced EMT in disease pathogenesis. We investigated the effects of N-acetylcysteine (NAC) on TGF-beta1-induced EMT in a rat epithelial cell line (RLE-6TN) and in primary rat alveolar epithelial cells (AEC). RLE-6TN cells exposed to TGF-beta1 for 5 days underwent EMT as evidenced by acquisition of a fibroblast-like morphology, downregulation of the epithelial-specific protein zonula occludens-1, and induction of the mesenchymal-specific proteins alpha-smooth muscle actin (alpha-SMA) and vimentin. These changes were inhibited by NAC, which also prevented Smad3 phosphorylation. Similarly, primary alveolar epithelial type II cells exposed to TGF-beta1 also underwent EMT that was prevented by NAC. TGF-beta1 decreased cellular GSH levels by 50-80%, whereas NAC restored them to approximately 150% of those found in TGF-beta1-treated cells. Treatment with glutathione monoethyl ester similarly prevented an increase in mesenchymal marker expression. Consistent with its role as an antioxidant and cellular redox stabilizer, NAC dramatically reduced intracellular reactive oxygen species production in the presence of TGF-beta1. Finally, inhibition of intracellular ROS generation during TGF-beta1 treatment prevented alveolar EMT, but treatment with H2O2 alone did not induce EMT. We conclude that NAC prevents EMT in AEC in vitro, at least in part through replenishment of intracellular GSH stores and limitation of TGF-beta1-induced intracellular ROS generation. We speculate that beneficial effects of NAC on pulmonary function in IPF may be mediated by inhibitory effects on alveolar EMT.

CONGENITAL CYSTIC DILATATION OF THE COMMON BILE-DUCT

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