Can Mitochondrial Cytochrome Oxidase Mediate Hypoxic Vasodilation Via Nitric Oxide Metabolism?

Analytical Expression for the NO Concentration Profile Following NONOate Decomposition in the Presence of Oxygen

We have derived an analytical expression for the time dependent NO concentration from NONOate donors in the presence of oxygen for the process of NO release from NO donors following autoxidation. This analytical solution incorporates the kinetics of the releases with the autoxidation and is used to fit the simulated NO concentration profile to the experimental data. This allows one to determine the NO release rate constant, k 1, the NO release stoichiometric coefficient, v NO, and the NO autoxidation reaction rate constant, k 2. This analytical solution also allows us to predict the real NO concentration released from NO donors under aerobic conditions, while v NO is reportedly two under aerobic conditions, it falls to lower values in the presence of oxygen.

Hemoglobin Effects on Nitric Oxide Mediated Hypoxic Vasodilation

The brain responds to hypoxia with an increase in cerebral blood flow (CBF). However, such an increase is generally believed to start only after the oxygen tension decreases to a certain threshold level. Although many mechanisms (different vasodilator and different generation and metabolism mechanisms of the vasodilator) have been proposed at the molecular level, none of them has gained universal acceptance. Nitric oxide (NO) has been proposed to play a central role in the regulation of oxygen supply since it is a vasodilator whose production and metabolism are both oxygen dependent. We have used a computational model that simulates blood flow and oxygen metabolism in the brain (BRAINSIGNALS) to test mechanism by which NO may elucidate hypoxic vasodilation. The first model proposed that NO was produced by the enzyme nitric oxide synthase (NOS) and metabolized by the mitochondrial enzyme cytochrome c oxidase (CCO). NO production declined with decreasing oxygen concentration given that oxygen is a substrate for nitric oxide synthase (NOS). However, this was balanced by NO metabolism by CCO, which also declined with decreasing oxygen concentration. However, the NOS effect was dominant; the resulting model profiles of hypoxic vasodilation only approximated the experimental curves when an unfeasibly low K m for oxygen for NOS was input into the model. We therefore modified the model such that NO generation was via the nitrite reductase activity of deoxyhemoglobin instead of NOS, whilst keeping the metabolism of NO by CCO the same. NO production increased with decreasing oxygen concentration, leading to an improved reproduction of the experimental CBF versus PaO2 curve. However, the threshold phenomenon was not perfectly reproduced. In this present work, we incorporated a wider variety of oxygen dependent and independent NO production and removal mechanisms. We found that the addition of NO removal via oxidation to nitrate mediated by oxyhemoglobin resulted in the optimum fit of the threshold phenomenon by the model. Our revised model suggests, but does not prove, that changes in NO concentration can be the primary cause of the relationship between pO2 and cerebral blood flow.

The key role of nitric oxide in hypoxia: hypoxic vasodilation and energy supply-demand matching

SIGNIFICANCE

A mismatch between energy supply and demand induces tissue hypoxia with the potential to cause cell death and organ failure. Whenever arterial oxygen concentration is reduced, increases in blood flow--hypoxic vasodilation--occur in an attempt to restore oxygen supply. Nitric oxide (NO) is a major signaling and effector molecule mediating the body's response to hypoxia, given its unique characteristics of vasodilation (improving blood flow and oxygen supply) and modulation of energetic metabolism (reducing oxygen consumption and promoting utilization of alternative pathways).

RECENT ADVANCES

This review covers the role of oxygen in metabolism and responses to hypoxia, the hemodynamic and metabolic effects of NO, and mechanisms underlying the involvement of NO in hypoxic vasodilation. Recent insights into NO metabolism will be discussed, including the role for dietary intake of nitrate, endogenous nitrite (NO₂⁻) reductases, and release of NO from storage pools. The processes through which NO levels are elevated during hypoxia are presented, namely, (i) increased synthesis from NO synthases, increased reduction of NO₂⁻ to NO by heme- or pterin-based enzymes and increased release from NO stores, and (ii) reduced deactivation by mitochondrial cytochrome c oxidase.

CRITICAL ISSUES

Several reviews covered modulation of energetic metabolism by NO, while here we highlight the crucial role NO plays in achieving cardiocirculatory homeostasis during acute hypoxia through both vasodilation and metabolic suppression.

FUTURE DIRECTIONS

We identify a key position for NO in the body's adaptation to an acute energy supply-demand mismatch.

Modeling hemoglobin nitrite reductase activity as a mechanism of hypoxic vasodilation?

The brain's response to hypoxia is to increase cerebral blood flow (CBF). However, the molecular mechanism underpinning this phenomenon is controversial. We have developed a model to simulate brain blood flow and oxygen metabolism called BRAINSIGNALS. This model is primarily designed to assist in the interpretation of multimodal noninvasive clinical measurements. However, we have recently used this model to test the feasibility of a range of molecular mechanisms proposed to explain hypoxic vasodilation. An increase in the concentration of the vasodilator nitric oxide (NO) at low pO2 is a feature of many such mechanisms. One model suggests that mitochondrial cytochrome c oxidase (CCO) catalyzes the metabolism of NO. This metabolism declines at low pO2, resulting in an increase in the steady-state levels of NO and a consequent increase in CBF. Using BRAINSIGNALS we were able to model this effect. However, the increases in NO and CBF occurred at far lower pO2 values than predicted from physiological data (Rong et al. 2013 Adv Exp Med Biol. 765, 231-238). The aim of the present study was to test an alternative mechanism, one that actively generates NO as pO2 drops, namely, the reduction of nitrite to NO by deoxyhemoglobin. In this mechanism, NO synthesis has a maximum of NO production near the hemoglobin p50. The addition of this mechanism resulted in a significantly better fit to the experimental data of the CBF(PaO2) curve.