Reply from G. D. Wadley, J. Choate and G. K. McConell

Impact of nitric oxide on metabolism in health and age-related disease

Nitric oxide (NO) serves as a messenger molecule in a variety of physiological systems and also converts into toxic radical species that can damage cells through a process known as nitrosative stress. While the physiological roles of NO in blood vessel dilation, the nervous system and the immune system are well established, recent studies have begun to investigate the role of NO in metabolism and energy expenditure through modulation of mitochondria. NO appears to stimulate mitochondrial biogenesis in certain situations through activation of proteins such as peroxisome proliferator-activated receptor γ (PPARγ) co-activator 1α (PGC1-α). Because of this link between NO and mitochondrial biogenesis, the role of NO in certain aspects of metabolism, including exercise response, obesity, fat cell differentiation and caloric restriction, are the subject of increasing investigation. In addition to its role in mitochondrial biogenesis, NO also stimulates mitochondrial fragmentation, which can be caused by too much mitochondrial fission or inhibition of mitochondrial fusion and can result in bioenergetic failure. While the contribution of NO-mediated mitochondrial fragmentation to neurodegenerative diseases seems clear, the mechanisms by which NO causes fragmentation are uncertain and controversial. In this review, we discuss the role of NO in manipulation of mitochondrial biogenesis and dynamics and how these events contribute to human health- and age-related disease.

Endothelial nitric oxide synthase is central to skeletal muscle metabolic regulation and enzymatic signaling during exercise in vivo

Endothelial nitric oxide synthase (eNOS) is associated with a number of physiological functions involved in the regulation of metabolism; however, the functional role of eNOS is poorly understood. We tested the hypothesis that eNOS is critical to muscle cell signaling and fuel usage during exercise in vivo, using 16-wk-old catheterized (carotid artery and jugular vein) C57BL/6J mice with wild-type (WT), partial (+/-), or no expression (-/-) of eNOS. Quantitative reductions in eNOS expression ( approximately 40%) elicited many of the phenotypic effects observed in enos(-/-) mice under fasted, sedentary conditions, with expression of oxidative phosphorylation complexes I to V and ATP levels being decreased, and total NOS activity and Ca(2+)/CaM kinase II Thr(286) phosphorylation being increased in skeletal muscle. Despite these alterations, exercise tolerance was markedly impaired in enos(-/-) mice during an acute 30-min bout of exercise. An eNOS-dependent effect was observed with regard to AMP-activated protein kinase signaling and muscle perfusion. Muscle glucose and long-chain fatty acid uptake, and hepatic and skeletal muscle glycogenolysis during the exercise bout was markedly accelerated in enos(-/-) mice compared with enos(+/-) and WT mice. Correspondingly, enos(-/-) mice exhibited hypoglycemia during exercise. Thus, the ablation of eNOS alters a number of physiological processes that result in impaired exercise capacity in vivo. The finding that a partial reduction in eNOS expression is sufficient to induce many of the changes associated with ablation of eNOS has implications for chronic metabolic diseases, such as obesity and insulin resistance, which are associated with reduced eNOS expression.