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. NH4+ 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 (Jmamm, disappearance from the mucosal saline), serosal ammonia flux rates (Jsamm, appearance in the serosal saline), and total tissue ammonia production rates (Jtamm = Jsamm − Jmamm). Bumetanide (10−4 mol L−1), a blocker of NKCC, inhibited Jsamm in most preparations, but this was largely due to reduction of Jtamm; Jmamm was significantly inhibited only in the anterior intestine of fed animals. Ouabain (10−4 mol L−1), a blocker of NKA, generally reduced both Jmamm and Jsamm without effects on Jtamm in most preparations, though the anterior intestine was resistant after feeding. Barium (10−2 mol L−1), a blocker of K+ channels, inhibited Jmamm in most preparations, and Jsamm in some, without effects on Jtamm. 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. NH4+ appears to be taken up through barium-sensitive K+ channels on the mucosal surface. Mucosal NH4+ uptake via both NKCC and K+ channels is energized by basolateral NKA, which plays an additional role in scavenging NH4+ 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.

中文翻译：

---

禁食和饲喂淡水虹鳟鱼肠道氨转运的体外分析：NKCC、K+ 通道和 Na+、K+ ATP 酶的作用

我们检查了未喂食和喂食淡水虹鳟 (Oncorhynchus mykiss) 前、中和后肠的氨处理机制，重点是 Na+:K+:2Cl- 协同转运蛋白 (NKCC)、Na+:K +- ATP 酶 (NKA) 和 K+ 通道。NKCC 通过免疫组织化学定位于肠细胞的黏膜（顶端）表面，在前段和后段进食后 NKCC mRNA 上调。NH4+ 与 K+ 在支持所有肠道切片中的 NKA 活性方面​​同样有效。体外肠囊制剂用于检查粘膜氨通量率（Jmamm，从粘膜盐水中消失）、浆膜氨通量率（Jsamm，出现在浆膜盐水中）和总组织氨生成率（Jtamm = Jsamm - Jmamm） . 布美他尼 (10−4 mol L−1) 是 NKCC 的阻断剂，在大多数制剂中抑制 Jsamm，但这主要是由于 Jtamm 的减少；Jmamm 仅在喂食动物的前肠中受到显着抑制。Ouabain (10-4 mol L-1) 是 NKA 的一种阻滞剂，在大多数制剂中通常会降低 Jmamm 和 Jsamm 而对 Jtamm 没有影响，尽管前肠在喂食后有抵抗力。钡 (10−2 mol L−1) 是 K+ 通道的阻断剂，在大多数制剂中抑制 Jmamm，在一些制剂中抑制 Jsamm，但对 Jtamm 没有影响。这些药理学结果，连同对浆膜和黏膜 Na+ 和 K+ 浓度操作的反应，表明 NKCC 在氨吸收中并不像以前认为的那样重要。NH4+ 似乎通过粘膜表面上的钡敏感 K+ 通道被吸收。通过 NKCC 和 K+ 通道的粘膜 NH4+ 摄取由基底外侧 NKA 提供能量，它在清除浆膜表面的 NH4+ 方面发挥着额外的作用，以尽可能减少血液毒性或增强喂养后的离子摄取和氨基酸合成。连同其他研究的最新发现，我们提供了一个更新的模型来描述目前对硬骨鱼肠道氨转运的理解。