Short-chain fatty acids and acidic pH upregulate UT-B, GPR41, and GPR4 in rumen epithelial cells of goats

Abstract

Currently, the mechanism(s) responsible for the regulation of urea transporter B (UT-B) expression levels in the epithelium of the rumen remain unclear. We hypothesized that rumen fermentation products affect ruminal UT-B expression. Therefore, the effects of short-chain fatty acids (SCFA), pH, ammonia, and urea on mRNA and protein levels of UT-B were assayed in primary rumen epithelial cell cultures and in rumen epithelium obtained from intact goats. In vitro, SCFA and acidic pH were found to synergetically stimulate both mRNA and protein expression of UT-B, whereas NH 4Cl decreased mRNA and protein levels of UT-B at pH 6.8. Treatment with urea increased both levels at pH 7.4. When goats received a diet rich in nitrogen (N) and nonfiber carbohydrates (NFC), their rumen epithelium had higher levels of UT-B, and the rumen contained higher concentrations of SCFA and NH 3-N with a lower pH. An increase in plasma urea-N concentration was also observed compared with the plasma of the goats that received a diet low in N and NFC. In a second feeding trial, goats that received a NFC-rich, but isonitrogenous, diet had higher mRNA and protein levels of UT-B, and higher levels of G proteincoupled receptor (GPR) 41 and GPR4, in their rumen epithelium. The ruminal concentrations of SCFA and NH 3-N also increased, while a lower pH was detected. In contrast, the serum urea-N concentrations remained unchanged. These data indicate that ruminal SCFA and pH are key factors, via GPR4 and GPR41, in the dietary regulation of UT-B expression, and they have priority over changes in plasma urea.SCFA and pH; UT-B expression; GPR41; GPR4; rumen epithelial cells; diet RECYCLING OF UREA TO THE MAMMALIAN gut accounts for 20 -30% of liver-synthesized urea that is present in humans (17). In contrast, these levels vary from 10 to 90% in ruminants (21) with feeding strategy (21) and intake of fermentable carbohydrates (22). These characteristics make the ruminant forestomach, especially the rumen, an excellent model for studies of urea entry and regulation in the gut. Urea transport across the rumen epithelium is generally accepted to occur down a concentration gradient from blood to lumen by diffusion through transport proteins, such as urea transporter B (UT-B), or possibly aquaporins. In our laboratory's recent studies, rumen fermentation products, including short-chain fatty acids (SCFA) and ammonia, as well as pH, were found to rapidly modulate UT-B activity (1, 34). It is possible that multiple mechanisms mediate regulation of urea transport, although these mechanisms are not fully characterized. UT-B has been found in many tissues and species, including human and rodent colon tissues, in erythrocytes and kidney tissue, as well as in ovine and bovine rumen (52). In bovine tissues, the UT-B protein exists as a multimer (30). Previous studies (21, 39, 41) have shown that changes in dietary nitrogen (N) content causes significant changes in serum urea-N (BUN) concentrations and the entry of urea into the gut. However, a correlation between N ...

UT-B Urea Transporter Localization in the Bovine Gastrointestinal Tract

“…concentration specifically regulate urea transport (Lu et al 2014). Most recently, studies in goat rumen have confirmed that SCFAs and pH are the key factors in dietary regulation of UT-B expression (Lu et al 2015).…”

Urea transport and clinical potential of urearetics

“…Sheep rumen expresses both UT-B1 and UT-B2 transcripts by PCR, and urea transport was regulated by changes in pH and ammonia concentrations (50). In primary cultures of goat rumen epithelial cells, short-chain fatty acids and acid pH increased UT-B mRNA and protein abundance (51). Ruminal UT-B protein was also increased when the goats were fed a diet with large amounts of nitrogen and non-fiber carbohydrates (51).…”

“…In primary cultures of goat rumen epithelial cells, short-chain fatty acids and acid pH increased UT-B mRNA and protein abundance (51). Ruminal UT-B protein was also increased when the goats were fed a diet with large amounts of nitrogen and non-fiber carbohydrates (51). …”

Antibiotic pretreatment minimizes dietary effects on reconstructure of rumen fluid and mucosal microbiota in goats

“…Our previous studies have shown that dietary concentrate promotes microbial fermentation and energetic absorption in the goat rumen (Lu et al, 2015;Yan, Zhang, & Shen, 2014). In the present study, we first investigated the compositional recovery of rumen fluid and mucosal microbiota in goats undergoing a 10-day streptomycin (ST) treatment.…”