Exercising an option to prevent age related decline of vascular BH₄ and uncoupling of eNOS

It is well documented that skeletal muscle vascular conductance and endothelium-dependent vasodilatation are reduced with ageing. Studies utilizing isolated skeletal muscle arterioles reveal that age-related deficits involve, at least in part, alterations in the NO signalling pathway (Muller-Delp et al. 2002; Delp et al. 2008). Although manifold factors could contribute to age-related declines in NO-mediated function, recent studies highlight the potential importance of tetrahydrobiopterin (BH₄), a nitric oxide synthase (NOS) cofactor. When BH₄ is limited, eNOS can become uncoupled and produce superoxide instead of NO. Thus, BH₄ is potentially of junctional importance in the link between NO, reactive oxygen species (ROS) and vascular function.

In humans, a single oral dose of BH₄ restored endothelium-dependent dilatation in the conduit brachial artery of sedentary older men (Eskurza et al. 2005). Furthermore, Delp et al. (2008) demonstrated that age-related declines in flow-mediated dilatation of rat soleus muscle arterioles resulted, at least in part, from limitations in availability of the eNOS cofactor tetrahydrobiopterin (BH₄), and that administration of the BH₄ precursor sepiapterin improved flow-induced dilatation in vessels from aged rats. These results collectively support the argument that interventions to preserve vascular BH₄ may be a therapeutic option for age-related vascular dysfunction (Pierce & LaRocca, 2008).

In the current issue of The Journal of Physiology, Sindler et al. proposed to determine if age-related declines in vascular BH₄ were associated with eNOS uncoupling and ROS production, and whether these age-related changes could be reversed by a therapeutic exercise training intervention. Aerobic exercise training has previously been shown to blunt or eliminate age-dependent declines in NO-mediated endothelium-dependent dilatation and to improve skeletal muscle blood flow in a variety of models (Seals et al. 2008), and the study of Sindler et al. proposes to add novel understanding of the underlying mechanisms related to BH₄. The approach is integrative, combining functional assessment of vasodilatation with biochemical measurements of BH₄, NO and ROS production, all in isolated arterioles from the soleus muscle under a number of controlled pharmacological conditions. As expected, flow-mediated dilatation was impaired with ageing and restored by exercise training; novel observations point to potential underlying mechanisms: age-related reduction in BH₄ occurred in conjunction with decline in flow-induced NO signalling, and increase in superoxide production (eNOS uncoupling); and exercise training prevented the age-related loss of BH₄ and improved NO bio-availability by balancing accelerated NO and ROS production. These results extend the appreciation of oxidative stress impacting NO bio-availability and vasomotor function beyond the traditional, but insufficient, concept of NO destruction by ROS (Rush et al. 2005).

The findings of the study of Sindler et al. are timely, reported roughly contemporaneously with identification of the need for pharmacological therapies directed toward a similar result, the restoration of BH₄ in aged blood vessels. This creates a discovery environment in which potential pharmacological- and exercise-oriented approaches to the problem will undoubtedly co-evolve. The findings of Sindler et al. additionally serve as a progenitor for many hypotheses regarding BH₄ in vascular adaptations to age and exercise. Fundamental questions raised include those related to identifying the cellular mechanisms accounting for age-related declines and exercise-related restoration of vascular BH₄. In this regard, emerging data suggest that H₂O₂ is a regulatory factor controlling BH₄ synthesis in vascular cells via induction of the enzyme GTP-cyclohydrolase I (GTPCH; Shimizu et al. 2003). The data of Sindler et al. indicate that H₂O₂ contributes to flow-induced dilatation of skeletal muscle arterioles of young sedentary animals and that, like the flow-induced dilatation functional response itself, this H₂O₂ signalling declines with age but is restored by exercise training. Is it possible that this same pattern of H₂O₂ signalling could contribute to maintenance of BH₄ in young arterioles, its loss with age, and its restoration with exercise in aged arterioles, consistent with the indicated effect of H₂O₂ on GTPCH expression and BH₄ synthesis? If so, H₂O₂ takes on another important role in the acute and chronic regulation of vascular function.

A recurring theme in vascular biology is heterogeneity in phenotype and function depending on vessel type (conduit vs. resistance), location (vascular bed), and physiological circumstances (e.g. other conditions in vivo). While these considerations are in part cautionary regarding the global interpretation of the results of Sindler et al., they can more appropriately be seen as an invitation for additional studies designed to create a more integrative understanding of cellular mechanisms of vascular ageing and the therapeutic effects of exercise. The study of Sindler et al. creates a new paradigm justifying such an invitation with respect to BH₄, vasomotor function and the regulation of skeletal muscle blood flow.