McMaster University, 1280 Main St. West, Hamilton, ON, L8S4K1, Canada.
Wilfrid Laurier University, 75 University Avenue West, Waterloo, ON, N2L3C5, Canada.
J Comp Physiol B. 2019 Oct;189(5):549-566. doi: 10.1007/s00360-019-01231-x. Epub 2019 Sep 5.
We examined mechanisms of ammonia handling in the anterior, mid, and posterior intestine of unfed and fed freshwater rainbow trout (Oncorhynchus mykiss), with a focus on the Na:K:2Cl co-transporter (NKCC), Na:K -ATPase (NKA), and K channels. NKCC was localized by immunohistochemistry to the mucosal (apical) surface of enterocytes, and NKCC mRNA was upregulated after feeding in the anterior and posterior segments. NH was equally potent to K in supporting NKA activity in all intestinal sections. In vitro gut sac preparations were employed to examine mucosal ammonia flux rates (Jm, disappearance from the mucosal saline), serosal ammonia flux rates (Js, appearance in the serosal saline), and total tissue ammonia production rates (Jt = Js - Jm). Bumetanide (10 mol L), a blocker of NKCC, inhibited Js in most preparations, but this was largely due to reduction of Jt; Jm was significantly inhibited only in the anterior intestine of fed animals. Ouabain (10 mol L), a blocker of NKA, generally reduced both Jm and Js without effects on Jt in most preparations, though the anterior intestine was resistant after feeding. Barium (10 mol L), a blocker of K channels, inhibited Jm in most preparations, and Js in some, without effects on Jt. These pharmacological results, together with responses to manipulations of serosal and mucosal Na and K concentrations, suggest that NKCC is not as important in ammonia absorption as previously believed. NH appears to be taken up through barium-sensitive K channels on the mucosal surface. Mucosal NH uptake via both NKCC and K channels is energized by basolateral NKA, which plays an additional role in scavenging NH on the serosal surface to possibly minimize blood toxicity or enhance ion uptake and amino acid synthesis following feeding. Together with recent findings from other studies, we have provided an updated model to describe the current understanding of intestinal ammonia transport in teleost fish.
我们研究了未进食和进食的淡水虹鳟（Oncorhynchus mykiss）前、中、后肠中氨处理的机制，重点研究了 Na:K:2Cl 协同转运蛋白（NKCC）、Na:K -ATP 酶（NKA）和 K 通道。免疫组织化学将 NKCC 定位于肠上皮细胞的黏膜（顶端）表面，并且 NKCC mRNA 在前段和后段进食后上调。NH 在所有肠段中与 K 一样能够支持 NKA 活性。在体外肠囊制剂中，我们研究了黏膜氨通量速率（Jm，从黏膜盐水中消失）、浆膜氨通量速率（Js，在浆膜盐水中出现）和总组织氨产生速率（Jt=Js - Jm）。布美他尼（10 摩尔 L），一种 NKCC 阻断剂，抑制了大多数制剂中的 Js，但这主要是由于 Jt 的减少；仅在前肠的进食动物中，Jm 受到显著抑制。哇巴因（10 摩尔 L），一种 NKA 阻断剂，通常会降低 Jm 和 Js，但在大多数制剂中对 Jt 没有影响，尽管进食后前肠具有抗性。钡（10 摩尔 L），一种 K 通道阻断剂，抑制了大多数制剂中的 Jm，并在某些制剂中抑制了 Js，但对 Jt 没有影响。这些药理学结果，以及对浆膜和黏膜 Na 和 K 浓度操作的反应，表明 NKCC 在氨吸收中的重要性不如以前认为的那么重要。NH 似乎通过黏膜表面的钡敏感 K 通道被吸收。NKCC 和 K 通道的黏膜 NH 摄取由基底外侧 NKA 提供能量，NKA 在浆膜表面清除 NH 方面发挥额外作用，可能最大限度地减少血液毒性或增强进食后离子摄取和氨基酸合成。结合其他研究的最新发现，我们提供了一个更新的模型来描述鱼类肠道氨转运的当前理解。
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