Nitrate and Nitrite Metabolism in Aging Rats: A Comparative Study.

Nitric oxide (NO) (co)regulates many physiological processes in the body. Its short-lived free radicals force synthesis in situ and on-demand, without storage possibility. Local oxygen availability determines the origin of NO-either by synthesis by nitric oxide synthases (NOS) or by the reduction of nitrate to nitrite to NO by nitrate/nitrite reductases. The existence of nitrate reservoirs, mainly in skeletal muscle, assures the local and systemic availability of NO. Aging is accompanied by changes in metabolic pathways, leading to a decrease in NO availability. We explored age-related changes in various rat organs and tissues. We found differences in nitrate and nitrite contents in tissues of old and young rats at baseline levels, with nitrate levels being generally higher and nitrite levels being generally lower in old rats. However, there were no differences in the levels of nitrate-transporting proteins and nitrate reductase between old and young rats, with the exception of in the eye. Increased dietary nitrate led to significantly higher nitrate enrichment in the majority of old rat organs compared to young rats, suggesting that the nitrate reduction pathway is not affected by aging. We hypothesize that age-related NO accessibility changes originate either from the NOS pathway or from changes in NO downstream signaling (sGC/PDE5). Both possibilities need further investigation.