Nitric oxide synthase-2 induction optimizes cardiac mitochondrial biogenesis after endotoxemia.

Mitochondrial biogenesis protects metabolism from mitochondrial dysfunction produced by activation of innate immunity by lipopolysaccharide (LPS) or other bacterial products. Here we tested the hypothesis in mouse heart that activation of toll-like receptor-4 (TLR4), which induces early-phase genes that damage mitochondria, also activates mitochondrial biogenesis through induction of nitric oxide synthase (NOS2). We compared three strains of mice: wild type (Wt) C57BL/6J, TLR4(-/-), and NOS2(-/-)for cardiac mitochondrial damage and mitochondrial biogenesis by real-time RT-PCR, Western analysis, immunochemistry, and isoform analysis of myosin heavy chain (MHC) after sublethal heat-killed Escherichia coli (HkEC). After HkEC, Wt mice displayed significant myocardial mtDNA depletion along with enhanced TLR4 and NOS2 gene and protein expression that normalized in 72 h. HkEC generated less cytokine stress in TLR4(-/-)and NOS2(-/-)than Wt mice, NOS2(-/-)mice had mtDNA damage comparable to Wt, and both knockout strains failed to restore mtDNA copy number because of mitochondrial transcriptosome dysfunction. Wt mice also showed the largest beta-MHC isoform switch, but MHC recovery lagged in the NOS2(-/-)and TLR4(-/-)strains. The NOS2(-/-)mice also unexpectedly revealed the codependency of TLR4 expression on NOS2. These findings demonstrate the decisive participation of NOS2 induction by TLR4 in optimization of mitochondrial biogenesis and MHC expression after gram-negative challenge.

The conduction and cardiac sympathetic systems: metabolic aspects.

Compared with the myocardium, glycolytic enzymes are reduced by 50% and mitochondrial enzymes and space by 70% in the conduction system of the calf heart. In addition, on the basis of adenosine triphosphate activities energy demands are reduced by more than 50%; this is in parallel with the reduction in myofibrillar space. The increased tolerance of the conduction system against ischemia can be explained by a reduction of energy demands and a higher proportion of (anaerobic) glycolytic as opposed to aerobic mitochondrial energy production. Among the structures of the conduction system, the sinoatrial and atrioventricular nodes are markedly susceptible to hypoxia in contrast to atrial conduction and ventricular conduction by way of the His-Purkinje system. In the isolated perfused rat heart, an increased net release of noradrenaline during the first 10 minutes of ischemia is only noted after sympathetic stimulation. During this phase, catecholamine overflow is limited by the activity of the neuronal reuptake. At a later second phase, from 15 to 40 minutes after the onset of ischemia, the mechanism of noradrenaline net release is carrier-mediated efflux inhibited by neuronal uptake blocking agents. During the third phase of ischemia, after about 40 minutes, spontaneous noradrenaline release is greatly augmented, probably as a result of leakage caused by membrane damage.