Laboratório de Ecofisiologia, CIMAR/CIIMAR - Centro Interdisciplinar de Investigação Marinha e Ambiental - Universidade do Porto, Portugal.
J Exp Biol. 2013 Feb 15;216(Pt 4):623-32. doi: 10.1242/jeb.074401. Epub 2012 Oct 17.
The weatherloach, Misgurnus angulliacaudatus, is an intestinal air-breathing, freshwater fish that has the unique ability to excrete ammonia through gut volatilization when branchial and cutaneous routes are compromised during high environmental ammonia or air exposure. We hypothesized that transepithelial gut NH(4)(+) transport is facilitated by an apical Na(+)/H(+) (NH(4)(+)) exchanger (NHE) and a basolateral Na(+)/K(+)(NH(4)(+))-ATPase, and that gut boundary layer alkalinization (NH(4)(+) → NH(3) + H(+)) is facilitated by apical HCO(3)(-) secretion through a Cl(-)/HCO(3)(-) anion exchanger. This was tested using a pharmacological approach with anterior (digestive) and posterior (respiratory) intestine preparations mounted in pH-stat-equipped Ussing chambers. The anterior intestine had a markedly higher conductance, increased short-circuit current, and greater net base (J(base)) and ammonia excretion rates (J(amm)) than the posterior intestine. In the anterior intestine, HCO(3)(-) accounted for 70% of J(base). In the presence of an imposed serosal-mucosal ammonia gradient, inhibitors of both NHE (EIPA, 0.1 mmol l(-1)) and Na(+)/K(+)-ATPase (ouabain, 0.1 mmol l(-1)) significantly inhibited J(amm) in the anterior intestine, although only EIPA had an effect in the posterior intestine. In addition, the anion exchange inhibitor DIDS significantly reduced J(base) in the anterior intestine although only at a high dose (1 mmol l(-1)). Carbonic anhydrase does not appear to be associated with gut alkalinization under these conditions as ethoxzolamide was without effect on J(base). Membrane fluidity of the posterior intestine was low, suggesting low permeability, which was also reflected in a lower mucosal-serosal J(amm) in the presence of an imposed gradient, in contrast to that in the anterior intestine. To conclude, although the posterior intestine is highly modified for gas exchange, it is the anterior intestine that is the likely site of ammonia excretion and alkalinization leading to ammonia volatilization in the gut.
泥鳅，Misgurnus angulliacaudatus，是一种肠道呼吸的淡水鱼，具有独特的能力，当鳃和皮肤途径在高环境氨或空气暴露期间受损时，通过肠道挥发将氨从肠道中排出。我们假设上皮内肠道 NH(4)(+)转运是由顶端 Na(+)/H(+) (NH(4)(+))交换器 (NHE) 和基底外侧 Na(+)/K(+)(NH(4)(+))-ATP 酶促进的，并且肠道边界层碱化 (NH(4)(+) → NH(3) + H(+)) 是通过顶端 Cl(-)/HCO(3)(-)阴离子交换器促进 HCO(3)(-)分泌而促进的。这是通过在前肠 (消化) 和后肠 (呼吸) 准备物上安装 pH -stat 配备的 Ussing 室的药理学方法进行测试的。前肠的电导率明显更高，短路电流增加，净基础 (J(base)) 和氨排泄率 (J(amm)) 大于后肠。在前肠中，HCO(3)(-)占 J(base)的 70%。在存在强制的浆膜-黏膜氨梯度的情况下，NHE 的抑制剂 (EIPA，0.1 mmol l(-1)) 和 Na(+)/K(+)-ATP 酶 (ouabain，0.1 mmol l(-1)) 在前肠中显著抑制了 J(amm)，尽管只有 EIPA 在后肠中起作用。此外，阴离子交换抑制剂 DIDS 在前肠中显著降低了 J(base)，尽管仅在高剂量 (1 mmol l(-1)) 时才有效。在这些条件下，碳酸酐酶似乎与肠道碱化无关，因为 ethoxzolamide 对 J(base)没有影响。后肠的膜流动性低，表明通透性低，这也反映在存在强制梯度时黏膜-浆膜 J(amm)较低，与前肠相反。总之，尽管后肠高度适应气体交换，但前肠可能是氨排泄和碱化的部位，导致肠道氨挥发。
