Short-term exposure to high relative humidity increases blood urea and influences colonic urea-nitrogen metabolism by altering the gut microbiota

Graphical abstract
Highlights • Plasma urea was increased along with erythrocyte Na+/K+ -ATPase activity reduced and abnormal erythrocyte morphologies appeared during 14-day high relative humidity (90 ± 2%) exposure. • Shortly after 12-h and 24-h exposures, urea influx and ammonia level were increased in the colon of mice, respectively. • Colonic urea-nitrogen metabolism was influenced by the increased levels of ammonia, amino acids and short-chain fatty acids during 14-day exposure. • Gut bacteria related to urease production, amino acids metabolism and SCFAs production was enriched during the exposure.
Introduction Colonic urea-nitrogen metabolites have been implicated in the pathogenesis of certain diseases which can be affected by environmental factors. Objectives We aimed to explore the influence of ambient humidity on colonic urea-nitrogen metabolism. Methods Blood biochemical indexes, metabolites of intestinal tract, and gut microbiota composition of mice (n = 10/group) exposed to high relative humidity (RH, 90 ± 2%) were analyzed during the 14-day exposure. Results After 12-h exposure, plasma blood urea nitrogen (BUN) level increased along with a decrease in the activity of erythrocyte Na+/K+ -ATPase. Moreover, abnormal erythrocyte morphologies appeared after 3 days of exposure. The colonic BUN and ammonia levels increased significantly after the 12-h and 24-h exposure, respectively. The colonic level of amino acids, partly synthesized by gut microbiota using ammonia as the nitrogen source, was significantly higher on the 7th day. Furthermore, the level of fecal short-chain fatty acids was significantly higher after 3-day exposure and the level of branched-chain fatty acids increased on the 14th day. Overall, gut microbiota composition was continuously altered during exposure, facilitating the preferential proliferation of urea-nitrogen metabolism bacteria. Conclusion Our findings suggest that short-term high RH exposure influences colonic urea-nitrogen metabolism by increasing the influx of colonic urea and altering gut microbiota, which might further impact the host health outcomes.