Nitrate and Nitrite Metabolism in Aging Rats: A Comparative Study

Abstract

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.

The Skeletal Muscle, the Heart, and the Liver Are the Major Organs of the Accumulation of Nitric Oxide Metabolites after Oral Nitrite Treatment

“…This ability may underlie the extensively regulated blood flow in the brain, ensuring a constant supply of oxygen and nutrients. Consistent with this suggestion, previous studies [ 33 , 34 ] suggested that nitrite present in cerebral circulation serves as a source of NO, capable of compensating for compromised NOS function.…”

High nitrate levels in skeletal muscle contribute to nitric oxide generation via a nitrate/nitrite reductive pathway in mice that lack the nNOS enzyme

“…nNOS−/− mice treated with high nitrate showed a trend to increase XOR protein ( p = 0.0583, unpaired t -test), while nNOS−/− mice treated with low nitrate revealed a trend to increase sialin protein ( p = 0.0594, unpaired t -test). Since skeletal muscle protein expression of sialin and XOR is highly variable between individuals, as shown in our previous study in rats and humans ( Wylie et al, 2019 ; Piknova et al, 2023 ), a significant difference in protein expression in nitrate treatments could not be notably observed. Additionally, limitations of antibody suitability and availability for Western blotting result in changes more readily detectable by mRNA expression.…”

“…Nitrate is abundant in vegetables (such as beetroot, spinach, and lettuce) and has been used as nutritional supplementation in healthy and diseased humans ( Lidder and Webb, 2013 ; Ranchal-Sanchez et al, 2020 ; Babateen et al, 2021 ; Esen et al, 2022 ). We previously demonstrated in rats and humans that nitrate supplementation increases tissue nitrate/nitrite levels, that skeletal muscle contains a nitrate transporter (sialin) and nitrate/nitrite reductase [xanthine oxidoreductase (XOR)], and that skeletal muscle nitrate acts as a regulator to maintain a systemic NO homeostasis ( Piknova et al, 2016a ; Gilliard et al, 2018 ; Wylie et al, 2019 ; Piknova et al, 2022b ; Kadach et al, 2022 ; Piknova et al, 2023 ). In a recent study, in rats administered with 15 N-labeled nitrate, we found that nitrate treatment directly increases 15 N-labeled nitrate and nitrite accumulation in skeletal muscle and upregulates skeletal muscle gene expression of nitrate transporters [ sialin and chloride channel 1 ( CLC1 )] and XOR ( Park et al, 2023 ).…”

Botanical Mixture Containing Nitric Oxide Metabolite Enhances Neural Plasticity to Improve Cognitive Impairment in a Vascular Dementia Rat Model

“…Recent studies showed that various reductase can reduce the nitrite ion to NO, increasing NO bioavailability in the body. Thus, nitrite is considered as a NO metabolite [6,7].…”