Department of Biology, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4K1, Canada.
J Exp Biol. 2011 Nov 1;214(Pt 21):3557-69. doi: 10.1242/jeb.057802.
Steelhead rainbow trout acclimated to either freshwater (FW) or seawater (SW) were exposed to high environmental ammonia (HEA, 1000 μmol l(-1) NH(4)HCO(3), pH 7.8-8.0) for 24 h. SW trout restored ammonia excretion more rapidly (3-6 h versus 9-12 h in FW), despite higher production rates and lower plasma pH. Plasma total ammonia levels stabilized at comparable levels below the external HEA concentration, and blood acid-base disturbances were small at both salinities. The electrochemical gradients for NH(4)(+) entry (F(NH(4))(+)) were the same in the two salinities, but only because FW trout allowed their transepithelial potential to rise by ∼15 mV during HEA exposure. Elevation of plasma [cortisol] during HEA exposure was more prolonged in SW fish. Plasma [glucose] increased in SW, but decreased in FW trout. Plasma [urea-N] also decreased in FW, in concert with elevated urea transporter (UT) mRNA expression in the gills. Of 13 branchial transporters, baseline mRNA expression levels were higher for Rhcg1, NHE2, NKCC1a and UT, and lower for NBC1 and NKA-α1a in SW trout, whereas NKA-α1b, NHE3, CA2, H(+)-ATPase, Rhag, Rhbg and Rhcg2 did not differ. Of the Rh glycoprotein mRNAs responding to HEA, Rhcg2 was greatly upregulated in both FW and SW, Rhag decreased only in SW and Rhcg1 decreased only in FW. H(+)-ATPase mRNA increased in FW whereas NHE2 mRNA increased in SW; NHE3 did not respond, and V-type H(+)-ATPase activity declined in SW during HEA exposure. Branchial Na(+),K(+)-ATPase activity was much higher in SW gills, but could not be activated by NH(4)(+). Overall, the more effective response of SW trout was explained by differences in physical chemistry between SW and FW, which greatly reduced the plasma NH(3) tension gradient for NH(3) entry, as well as by the higher [Na(+)] in SW, which favoured Na(+)-coupled excretion mechanisms. At a molecular level, responses in SW trout showed subtle differences from those in FW trout, but were very different than in the SW pufferfish. Upregulation of Rhcg2 appears to play a key role in the response to HEA in both FW and SW trout, and NH(4)(+) does not appear to move through Na(+),K(+)-ATPase.
适应淡水 (FW) 或海水 (SW) 的虹鳟鱼被暴露于高环境氨 (HEA，1000 μmol l(-1) NH(4)HCO(3)，pH 值 7.8-8.0) 24 小时。尽管 SW 鳟鱼的氨排泄率更高 (3-6 小时与 FW 中的 9-12 小时)，但它们更快地恢复了氨排泄。尽管在两种盐度下，SW 鳟鱼的血浆总氨水平稳定在低于外部 HEA 浓度的可比水平，但血液酸碱平衡紊乱很小。NH(4)(+)进入的电化学梯度 (F(NH(4))(+)) 在两种盐度下相同，但这仅仅是因为 FW 鳟鱼在 HEA 暴露期间允许其跨上皮电位升高约 15 mV。在 SW 鱼类中，HEA 暴露期间血浆 [皮质醇] 的升高更为持久。SW 鱼类的血浆 [葡萄糖] 增加，但 FW 鳟鱼的血浆 [葡萄糖] 减少。FW 鳟鱼的血浆 [尿素氮] 也减少，与鳃中尿素转运蛋白 (UT) mRNA 表达升高一致。在 13 种鳃转运蛋白中，Rhcg1、NHE2、NKCC1a 和 UT 的基线 mRNA 表达水平在 SW 鳟鱼中较高，而 NBC1 和 NKA-α1a 的基线 mRNA 表达水平较低，而 NKA-α1b、NHE3、CA2、H(+)-ATPase、Rhag、Rhbg 和 Rhcg2 则没有差异。在对 HEA 有反应的 Rh 糖蛋白 mRNAs 中，Rhcg2 在 FW 和 SW 中均被极大地上调，而 Rhag 仅在 SW 中下调，Rhcg1 仅在 FW 中下调。FW 中的 H(+)-ATPase mRNA 增加，而 SW 中的 NHE2 mRNA 增加；NHE3 没有反应，并且 SW 中的 V 型 H(+)-ATPase 活性在 HEA 暴露期间下降。SW 鳃中的 Na(+),K(+)-ATPase 活性高得多，但不能被 NH(4)(+) 激活。总的来说，SW 鳟鱼更有效的反应是由 SW 和 FW 之间的物理化学差异解释的，这大大降低了 NH(3)进入的血浆 NH(3)张力梯度，以及 SW 中的高 [Na(+)]，这有利于 Na(+)-偶联的排泄机制。在分子水平上，SW 鳟鱼的反应与 FW 鳟鱼的反应略有不同，但与 SW 河豚鱼的反应非常不同。Rhcg2 的上调似乎在 FW 和 SW 鳟鱼对 HEA 的反应中都发挥了关键作用，并且 NH(4)(+) 似乎不通过 Na(+),K(+)-ATPase 移动。
