Department of Biology, McMaster University, 1280 Main Street West, Hamilton, ON, Canada, L8S 4K1.
J Exp Biol. 2011 Aug 15;214(Pt 16):2678-89. doi: 10.1242/jeb.055541.
Ammonia, the third respiratory gas in teleost fish, acts as an acute stimulant to ventilation in ammoniotelic rainbow trout. We investigated whether this sensitivity is maintained in trout chronically exposed (1+ months) to high environmental ammonia [HEA, 250 μmol l(-1) (NH(4))(2)SO(4)] in the water, and whether gill neuroepithelial cells (NECs) are involved in ammonia sensing. Hyperventilation was induced both by acute external (NH(4))(2)SO(4) exposure [250 or 500 μmol l(-1) (NH(4))(2)SO(4)] and by intra-arterial (NH(4))(2)SO(4) injection (580 μmol kg(-1) of ammonia) in control trout, but these responses were abolished in chronic HEA animals. Hyperventilation in response to acute ammonia exposure persisted after bilateral removal of each of the four gill arch pairs separately or after combined removal of arches III and IV, but was delayed by removal of gill arch I, and eliminated by combined removal of arches I and II. NECs, identified by immunolabeling against 5-HT, were mainly organized in two lines along the filament epithelium in all four gill arches. In control trout, NECs were slightly smaller but more abundant on arches I and II than on arches III and IV. Chronic HEA exposure reduced the density of the NECs on all four arches, and their size on arches I and II only. Fura-2 fluorescence imaging was used to measure intracellular free calcium ion concentration (Ca(2+)) responses in single NECs in short-term (24-48 h) culture in vitro. Ca(2+) was elevated to a comparable extent by perfusion of 30 mmol l(-1) KCl and 1 mmol l(-1) NH(4)Cl, and these Ca(2+) responses presented in two different forms, suggesting that ammonia may be sensed by multiple mechanisms. The Ca(2+) responses to high ammonia were attenuated in NECs isolated from trout chronically exposed to HEA, especially in ones from gill arch I, but responses to high K(+) were unchanged. We conclude that the hyperventilatory response to ammonia is lost after chronic waterborne HEA exposure, and that NECs, especially the ones located in gill arches I and II, are probably ammonia chemoreceptors that participate in ventilatory modulation in trout.
氨是硬骨鱼类的第三呼吸气体，作为一种急性刺激物，可刺激具氨氮代谢能力的虹鳟鱼的通气。我们研究了虹鳟鱼在慢性暴露于高环境氨[HEA，250 μmol l(-1)（NH(4))(2)SO(4)]的水中时，是否仍能保持这种敏感性，以及鳃神经上皮细胞（NEC）是否参与氨感觉。急性外部（NH(4))(2)SO(4)暴露[250 或 500 μmol l(-1)（NH(4))(2)SO(4)]和动脉内（NH(4))(2)SO(4)注射（580 μmol kg(-1)的氨）都能引起对照鳟鱼的过度通气，但这些反应在慢性 HEA 动物中被消除。急性氨暴露引起的过度通气在分别切除每对四个鳃弓或同时切除第三和第四对鳃弓后仍然存在，但切除第一对鳃弓会延迟，而同时切除第一和第二对鳃弓则会消除。用 5-HT 免疫标记鉴定的 NEC 主要沿所有四个鳃弓的鳃丝上皮组织排列成两排。在对照鳟鱼中，与第三和第四对鳃弓相比，NEC 在第一和第二对鳃弓上稍小但更丰富。慢性 HEA 暴露减少了所有四个鳃弓上 NEC 的密度，仅减少了第一和第二对鳃弓上 NEC 的大小。体外短期（24-48 小时）培养中，使用 Fura-2 荧光成像测量单个 NEC 中的细胞内游离钙离子浓度（Ca(2+)）反应。用 30 mmol l(-1) KCl 和 1 mmol l(-1) NH(4)Cl 灌注可使 Ca(2+)升高到相当程度，这些 Ca(2+)反应表现为两种不同的形式，表明氨可能通过多种机制被感知。与慢性暴露于 HEA 的鳟鱼分离的 NEC 对高氨的Ca(2+)反应减弱，特别是来自第一对鳃弓的 NEC，但对高 K(+)的反应不变。我们得出的结论是，慢性水 HEA 暴露后，对氨的过度通气反应消失，NEC，特别是位于第一和第二对鳃弓上的 NEC，可能是参与鳟鱼通气调节的氨化学感受器。
