Mechanisms of nitrite reduction to nitric oxide in the heart and vessel wall

Nitric oxide (NO) is an important regulator of a variety of biological functions, and also has a role in the pathogenesis of cellular injury. It had been generally accepted that NO is solely generated in biological tissues by specific nitric oxide synthases (NOS) which metabolize arginine to citrulline with the formation of NO. However, over the last 15 years, nitrite-mediated NO production has been shown to be an important mechanism of NO formation in the heart and cardiovascular system. Now numerous studies have demonstrated that nitrite can be an important source rather than simply a product of NO in mammalian cells and tissues and can be a potential vasodilator drug for cardiovascular diseases. There are a variety of mechanisms of nitrite reduction to NO and it is now appreciated that this process, while enhanced under hypoxic conditions, also occurs under normoxia. Several methods, including electron paramagnetic resonance, chemiluminescence NO analyzer, and NO electrode have been utilized to measure, quantitate, and image nitrite-mediated NO formation. Results reveal that nitrite-dependent NO generation plays critical physiological and pathological roles, and is controlled by oxygen tension, pH, reducing substrates and nitrite levels. In this manuscript, we review the mechanisms of nitrite-mediated NO formation and the effects of oxygen on this process with a focus on how this occurs in the heart and vessels.