Lu Zhongyan, Stumpff Friederike, Deiner Carolin, Rosendahl Julia, Braun Hannah, Abdoun Khalid, Aschenbach Jörg R, Martens Holger
Institute of Veterinary Physiology, Freie Universität Berlin, Berlin, Germany; and.
College of Food and Agriculture Science, King Saud University, Riyadh, Saudi Arabia.
Am J Physiol Regul Integr Comp Physiol. 2014 Sep 1;307(5):R558-70. doi: 10.1152/ajpregu.00107.2014. Epub 2014 Jun 11.
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.
瘤胃发酵产物，如短链脂肪酸（SCFA）和二氧化碳，在体内可急性刺激尿素跨瘤胃上皮的转运，而氨则具有抑制作用。其摄取和信号通路仍不清楚。使用聚合酶链反应（PCR）研究了SLC14a1（UT-B）在瘤胃中的表达。使用pH敏感微电极并通过Ussing室中的通量测量，研究了氨对胞质pH（pHi）和瘤胃尿素跨天然上皮转运的功能性短期影响。从绵羊瘤胃cDNA中可以完全测序出两个变体（UT-B1和UT-B2）。在功能上，转运是被动的，并在存在SCFA和二氧化碳的情况下受到管腔pH的调节，随着管腔酸化而升高，在pH 5.8时达到峰值，随着进一步酸化而下降，形成钟形曲线。氨的存在降低了尿素通量与pH之间关系的幅度，但不改变其形状，因此尿素通量在pH 5.8时仍保持最大。氨的作用具有浓度依赖性，在5 mmol/l时达到饱和。钳制跨上皮电位改变了氨对尿素通量的抑制电位。氨使顶端膜去极化并使pHi酸化，这表明在生理pH（<7）下，NH4（+）进入细胞质可能是调节瘤胃尿素转运的关键信号事件。我们得出结论，尿素跨瘤胃上皮的转运涉及蛋白质，这些蛋白质会受到改变pHi和细胞质中NH4（+）浓度的操作的快速调节。讨论了其对上皮和瘤胃稳态的影响。
