Dietary Nitrate Metabolism in Porcine Ocular Tissues Determined Using ¹⁵N-Labeled Sodium Nitrate Supplementation

Nitrate (NO₃^-) obtained from the diet is converted to nitrite (NO₂^-) and subsequently to nitric oxide (NO) within the body. Previously, we showed that porcine eye components contain substantial amounts of nitrate and nitrite that are similar to those in blood. Notably, cornea and sclera exhibited the capability to reduce nitrate to nitrite. To gain deeper insights into nitrate metabolism in porcine eyes, our current study involved feeding pigs either NaCl or Na¹⁵NO₃ and assessing the levels of total and ¹⁵N-labeled NO₃^-/NO₂^- in various ocular tissues. Three hours after Na¹⁵NO₃ ingestion, a marked increase in ¹⁵NO₃^- and ¹⁵NO₂^- was observed in all parts of the eye; in particular, the aqueous and vitreous humor showed a high ¹⁵NO₃^- enrichment (77.5 and 74.5%, respectively), similar to that of plasma (77.1%) and showed an even higher ¹⁵NO₂^- enrichment (39.9 and 35.3%, respectively) than that of plasma (19.8%). The total amounts of NO₃^- and NO₂^- exhibited patterns consistent with those observed in ¹⁵N analysis. Next, to investigate whether nitrate or nitrite accumulate proportionally after multiple nitrate treatments, we measured nitrate and nitrite contents after supplementing pigs with Na¹⁵NO₃ for five consecutive days. In both ¹⁵N-labeled and total nitrate and nitrite analysis, we did not observe further accumulation of these ions after multiple treatments, compared to a single treatment. These findings suggest that dietary nitrate supplementation exerts a significant influence on nitrate and nitrite levels and potentially NO levels in the eye and opens up the possibility for the therapeutic use of dietary nitrate/nitrite to enhance or restore NO levels in ocular tissues.