Wright Patricia A, Wood Chris M
Department of Integrative Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada.
J Exp Biol. 2009 Aug;212(Pt 15):2303-12. doi: 10.1242/jeb.023085.
Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells, whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H(+) excretion and Na(+) uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na(+) uptake and acid excretion. In this model, Rhag facilitates NH(3) flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical "Na(+)/NH (+)(4) exchange complex" consisting of several membrane transporters (Rhcg, V-type H(+)-ATPase, Na(+)/H(+) exchanger NHE-2 and/or NHE-3, Na(+) channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase (CA-2) and basolateral Na(+)/HCO (-)(3) cotransporter (NBC-1) and Na(+)/K(+)-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH(3) by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H(+) ions created by the catalysed or non-catalysed hydration of expired metabolic CO(2). Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO(2) transporters, their responses to feeding, and their roles in early life stages prior to the full development of gills.
鱼类鳃部氨排泄的研究已有80年之久，但其中涉及的机制仍存在争议。相对较新发现的恒河猴（Rh）蛋白具有氨转运功能，该蛋白家族与细菌、酵母和植物中甲基氨和氨转运体的Mep/Amt家族相关，并且这些基因和糖基化蛋白在鱼类鳃部的存在开启了一个新的范例。我们提供了Rh蛋白进化和功能的背景信息，并综述了最近利用分子生理学进行的研究，这些研究证明了它们对鳃部氨外流的重要贡献。Rhag存在于红细胞中，而Rhbg和Rhcg的几种同工型存在于许多组织中。在鳃上皮中，Rhcg似乎定位于顶端膜，而Rhbg定位于基底外侧膜。在暴露于高环境氨或内部氨注入期间，它们的基因表达上调，并且可能对氨和皮质醇的协同刺激敏感。特别是Rhcg似乎与H(+)排泄和Na(+)摄取机制相关联。我们提出了一种淡水鱼氨排泄的新模型及其与Na(+)摄取和酸排泄的可变联系。在这个模型中，Rhag促进NH(3)从红细胞中流出，Rhbg将其转运穿过鳃离子细胞的基底外侧膜，并且由几种膜转运体（Rhcg、V型H(+)-ATP酶、Na(+)/H(+)交换体NHE-2和/或NHE-3、Na(+)通道）共同作为一个代谢体组成的顶端“Na(+)/NH(+)(4)交换复合体”提供了一种用于顶端排泄的酸捕获机制。细胞内碳酸酐酶（CA-2）、基底外侧Na(+)/HCO(-)(3)协同转运体（NBC-1）和Na(+)/K(+)-ATP酶发挥间接作用。这些机制通常叠加在通过简单扩散的大量NH(3)外向移动之上，这可能取决于由呼出的代谢CO(2)的催化或非催化水合作用产生的H(+)离子在边界层水中的酸捕获作用。文中突出了鱼类Rh蛋白未来研究的有益领域：它们在海水鱼鳃部氨排泄机制中的作用、它们在皮肤氨排泄中的可能重要性、它们作为CO(2)转运体的潜在双重作用、它们对摄食的反应以及它们在鳃完全发育之前的早期生命阶段中的作用。
