The role of rumen epithelial urea transport proteins in urea nitrogen salvage: A review

The symbiotic relationship between the host and the rumen microbiome plays a crucial role in ruminant physiology. One of the most important processes enabling this relationship is urea nitrogen salvaging (UNS). This process is important for both maintaining ruminant nitrogen balance and supporting production of their major energy supply, bacterially-derived short chain fatty acids (SCFA). The key step in UNS is the trans-epithelial movement of urea across the ruminal wall and this is a highly regulated process. At the molecular level, the key transport route is via the facilitative urea transporter-B2, localized to ruminal papillae epithelial layers. Additional urea transport through aquaporins (AQP), such as AQP3, is now also viewed as important. Long-term regulation of these ruminal urea transport proteins appears to mainly involve dietary fermentable carbohydrates; whereas, transepithelial urea transport is finely regulated by local conditions, such as CO2 levels, pH and SCFA concentration. Although the key principles of ruminal urea transport physiology are now understood, there remains much that is unknown regarding the regulatory pathways. One reason for this is the limited number of techniques currently used in many studies in the field. Therefore, future research in this area that combines a greater range of techniques could facilitate improvements to livestock efficiency, and potentially, reductions in the levels of waste nitrogen entering the environment. (C) 2022 Chinese Association of Animal Science and Veterinary Medicine. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd.

Automated summary

The symbiotic relationship between the host and the rumen microbiome, particularly the process of urea nitrogen salvaging, plays a crucial role in ruminant physiology. The trans-epithelial movement of urea across the ruminal wall, regulated by transport proteins, is a key step in this process. Further research and the use of a wider range of techniques could lead to improvements in livestock efficiency and a reduction in waste nitrogen levels entering the environment.