Wilson R W, Taylor E W
School of Biological Sciences, University of Birmingham, UK.
J Exp Biol. 1992 May;166:95-112. doi: 10.1242/jeb.166.1.95.
Transbranchial ammonia gradients and blood acid-base status have been examined in rainbow trout acclimated to fresh water (FW), 33% sea water (33% SW) and sea water (SW) and exposed to 1.0 mmol l-1 total ammonia (TAmm) at pH 7.9 for 24 h. At all three salinities trout maintained large negative (inwardly directed) NH3 and NH4+ gradients throughout the exposure, presumably by active excretion of NH4+ to counteract the passive inward diffusion of ammonia. Analysis of blood non-respiratory acid-base status (delta H+m) revealed an acid load in FW trout and a base load in SW trout following 24 h of exposure. This indicates that active NH4+/H+ exchange predominates in FW whereas NH4+/Na+ is the principal exchange utilised in SW under these experimental conditions. The plasma TAmm load incurred during ammonia exposure increased with salinity. Compared to FW trout, plasma TAmm values were 34 and 73% higher in the 33% SW and SW trout, respectively, after 24 h. This cannot be explained by differences in the prevailing transbranchial PNH3 gradient because ambient PNH3 was substantially lower at the higher salinities (due to higher pK' and solubility values). We interpret the difference between FW and SW trout as an increased permeability to NH4+ in fish acclimated to the higher-salinity environments. Transbranchial diffusion of NH4+ is, therefore, probably more important as a route for ammonia excretion in SW than in FW trout, especially considering the favourable transepithelial potentials normally found in SW teleosts. In addition, increased NH4+ permeability implies that the toxicity of ammonia will be greater in seawater than in freshwater teleosts and should not simply be measured as a function of the unionised ammonia concentration when considering seawater-adapted species.
研究了虹鳟鱼在适应淡水（FW）、33%海水（33%SW）和海水（SW）环境下，于pH 7.9条件下暴露于1.0 mmol/L总氨（TAmm）24小时后的跨鳃氨梯度和血液酸碱状态。在所有三种盐度下，虹鳟鱼在整个暴露过程中都保持着较大的负（向内）NH₃和NH₄⁺梯度，推测是通过主动排泄NH₄⁺来抵消氨的被动向内扩散。对血液非呼吸酸碱状态（δH⁺m）的分析表明，暴露24小时后，FW虹鳟鱼有酸负荷，而SW虹鳟鱼有碱负荷。这表明在这些实验条件下，FW中主动的NH₄⁺/H⁺交换占主导，而SW中NH₄⁺/Na⁺是主要的交换方式。氨暴露期间产生的血浆TAmm负荷随盐度增加而增加。与FW虹鳟鱼相比，24小时后，33%SW和SW虹鳟鱼的血浆TAmm值分别高出34%和73%。这不能用当时的跨鳃PNH₃梯度差异来解释，因为在较高盐度下环境PNH₃显著较低（由于较高的pK'和溶解度值）。我们将FW和SW虹鳟鱼之间的差异解释为适应高盐度环境的鱼类对NH₄⁺的通透性增加。因此，跨鳃NH₄⁺扩散作为氨排泄途径在SW虹鳟鱼中可能比在FW虹鳟鱼中更重要，特别是考虑到SW硬骨鱼通常存在的有利跨上皮电位。此外，NH₄⁺通透性增加意味着海水硬骨鱼中氨的毒性将比淡水硬骨鱼更大，在考虑适应海水的物种时，不应简单地将其毒性测量为非离子氨浓度的函数。
