title: Modulation of sheep ruminal urea transport by ammonia and pH.
abstract: Ruminal fermentation products such as short-chain fatty acids (SCFA) and CO2 acutely stimulate urea transport across the ruminal epithelium in vivo, whereas ammonia has inhibitory effects. Uptake and signaling pathways remain obscure. The ruminal expression of SLC14a1 (UT-B) was studied using polymerase chain reaction (PCR). The functional short-term effects of ammonia on cytosolic pH (pHi) and ruminal urea transport across native epithelia were investigated using pH-sensitive microelectrodes and via flux measurements in Ussing chambers. Two variants (UT-B1 and UT-B2) could be fully sequenced from ovine ruminal cDNA. Functionally, transport was passive and modulated by luminal pH in the presence of SCFA and CO2, rising in response to luminal acidification to a peak value at pH 5.8 and dropping with further acidification, resulting in a bell-shaped curve. Presence of ammonia reduced the amplitude, but not the shape of the relationship between urea flux and pH, so that urea flux remained maximal at pH 5.8. Effects of ammonia were concentration dependent, with saturation at 5 mmol/l. Clamping the transepithelial potential altered the inhibitory potential of ammonia on urea flux. Ammonia depolarized the apical membrane and acidified pHi, suggesting that, at physiological pH (< 7), uptake of NH4 (+) into the cytosol may be a key signaling event regulating ruminal urea transport. We conclude that transport of urea across the ruminal epithelium involves proteins subject to rapid modulation by manipulations that alter pHi and the cytosolic concentration of NH4 (+). Implications for epithelial and ruminal homeostasis are discussed.
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
authors.forename: Zhongyan
authors.initials: Z
authors.lastname: Lu
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
authors.forename: Friederike
authors.initials: F
authors.lastname: Stumpff
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
authors.forename: Carolin
authors.initials: C
authors.lastname: Deiner
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
authors.forename: Julia
authors.initials: J
authors.lastname: Rosendahl
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
authors.forename: Hannah
authors.initials: H
authors.lastname: Braun
authors.affiliation: College of Food and Agriculture Science, King Saud University, Riyadh, Saudi Arabia.
authors.forename: Khalid
authors.initials: K
authors.lastname: Abdoun
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
authors.forename: Jörg R
authors.initials: JR
authors.lastname: Aschenbach
authors.affiliation: Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and Holger.Martens@fu-berlin.de.
authors.forename: Holger
authors.initials: H
authors.lastname: Martens
mesh_terms: D000641:Ammonia / Q000494:pharmacology*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002245:Carbon Dioxide / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005232:Fatty Acids, Volatile / Q000378:metabolism; D005260:Female; D006863:Hydrogen-Ion Concentration; D026901:Membrane Transport Proteins / Q000378:metabolism; D011522:Protons; D012417:Rumen / Q000187:drug effects / Q000378:metabolism*; D012756:Sheep / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D014508:Urea / Q000378:metabolism*; D000096982:Urea Transporters
chemical_list: D000641:Ammonia; D002245:Carbon Dioxide; D005232:Fatty Acids, Volatile; D026901:Membrane Transport Proteins; D011522:Protons; D014508:Urea; D000096982:Urea Transporters
keywords: ammonia; ion-selective microelectrode; recycling; rumen; urea; urea transporter B