Large quantities of protein are degraded in the fermentative parts of the gut to ammonia, which is absorbed, detoxified to urea, and excreted, leading to formation of nitrogenous compounds such as N2O that are associated with global warming. In ruminants, channel-mediated uptake of NH4+ from the rumen predominates. The molecular identity of these channels remains to be clarified. Ruminal cells and epithelia from cows and sheep were investigated using patch clamp, Ussing chamber, microelectrode techniques, and qPCR. In patch clamp experiments, bovine ruminal epithelial cells expressed a conductance for NH4+ that could be blocked in a voltage-dependent manner by divalent cations. In the native epithelium, NH4+ depolarized the apical potential, acidified the cytosol and induced a rise in short-circuit current (Isc) that persisted after the removal of Na+, was blocked by verapamil, enhanced by the removal of divalent cations, and was sensitive to certain transient receptor potential (TRP) channel modulators. Menthol or thymol stimulated the Isc in Na+ or NH4+ containing solutions in a dose-dependent manner and modulated transepithelial Ca2+ fluxes. On the level of messenger RNA (mRNA), ovine and bovine ruminal epithelium expressed TRPA1, TRPV3, TRPV4, TRPM6, and TRPM7, with any expression of TRPV6 marginal. No bands were detected for TRPV1, TRPV5, or TRPM8. Functional and molecular biological data suggest that the transport of NH4+, Na+, and Ca2+ across the rumen involves TRP channels, with TRPV3 and TRPA1 emerging as prime candidate genes. TRP channels may also contribute to the transport of NH4+ across other epithelia.
Ammonium TRP channel Rumen Climate gas Menthol Thymol
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Financial support for this study came from the Forschungskommission of the Freie Universität Berlin and from the European Social Fund (ESF) and the German Ministry of Economics and Technology based on a decision of the German Parliament (Projektträger Jülich 03EFABE057). In the latter part of the study, Katharina Schrapers (née Hille) was supported by the “Akademie für Tiergesundheit.” The expert technical help of Gisela Manz, Susanne Trappe, Martin Grunau, Uwe Tietjen, and last but certainly not least, Katharina Wolf is gratefully acknowledged.
Compliance with ethical standards
This study was performed for purely scientific reasons within a conventional academic framework but has recently led to a patent that is pending (involving the authors Friederike Stumpff and Julia Rosendahl, both as employees of the Freie Universität) and a start-up company supported by “Exist” and the Freie Universität Berlin. The company is owned by Julia Rosendahl, Hannah Braun, Katharina Schrapers, and Friederike Stumpff. This publication will have no impact on the commercial success or failure of that enterprise, and there is no conflict of interest.
Some preliminary results were published in the form of abstracts and a thesis (Julia Rosendahl).
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