Biology Faculty, University of British Columbia, Kelowna, BC V1V 1V7, Canada.
Aquat Toxicol. 2013 Jul 15;136-137:1-12. doi: 10.1016/j.aquatox.2013.03.002. Epub 2013 Mar 29.
Ammonia is a highly toxic molecule and often introduced in considerable amounts into aquatic environments due to anthropogenic activities. Many aquatic and semi-aquatic amphibians utilize, in addition to their kidneys, the skin for osmoregulation and nitrogen excretion. In the present study the effects of prolonged (7-21 days) exposure to high environmental ammonia (HEA, 1 mmol l(-1) NH4Cl) on cutaneous nitrogen excretion and gene expression of key-transporters involved in nitrogen excretion and acid-base regulation were investigated in the fully aquatic African clawed frog, Xenopus laevis. The study revealed that X. laevis excretes predominately ammonia of which approximately 50% is excreted via the skin. Both the ventral and dorsal skin were capable to generate a net ammonia efflux, which was significantly activated by 10 mmol l(-1) of the phosphodiesterase blocker theophylline. The obtained data further suggest that the ammonia efflux was promoted by an acidification of the unstirred boundary layer, likely generated by an apical localized V-ATPase, with NH3 being transported via cutaneous expressed ammonia transporters, Rhbg and Rhcg. Prolonged HEA exposure did significantly reduce the net-flux rates over the ventral skin with Vmax changing from 256 nmol cm(-2) h(-1) in control frogs to 196 nmol cm(-2) h(-1) in HEA exposed animals. Further, prolonged HEA exposure caused a decrease in mRNA expression levels of the ammonia transporter Rhbg, Na(+)/K(+)-ATPase (α-subunit) and V-ATPase (subunit H) in the ventral and dorsal skin and the kidney. In contrast, Rhcg expression levels were unaffected by HEA in skin tissues.
氨是一种毒性很强的分子，由于人类活动，经常被大量引入水生环境。许多水生和半水生两栖动物除了肾脏外，还利用皮肤进行渗透调节和氮排泄。在本研究中，我们研究了长时间（7-21 天）暴露于高环境氨（HEA，1 mmol l（-1）NH4Cl）对完全水生非洲爪蟾（Xenopus laevis）皮肤氮排泄和参与氮排泄和酸碱调节的关键转运体基因表达的影响。研究表明，X. laevis 主要排泄氨，其中约 50%通过皮肤排泄。腹侧和背侧皮肤都能够产生净氨流出，这一过程可被磷酸二酯酶抑制剂茶碱（10 mmol l（-1））显著激活。获得的数据进一步表明，氨的流出是通过未搅动边界层的酸化来促进的，这种酸化可能是由顶端局部化的 V-ATPase 产生的，NH3 通过皮肤表达的氨转运体 Rhbg 和 Rhcg 进行转运。长时间的 HEA 暴露显著降低了腹侧皮肤的净通量率，Vmax 从对照组青蛙的 256 nmol cm（-2）h（-1）降至 HEA 暴露动物的 196 nmol cm（-2）h（-1）。此外，长时间的 HEA 暴露导致腹侧和背侧皮肤以及肾脏中氨转运体 Rhbg、Na（+）/K（+）-ATPase（α 亚基）和 V-ATPase（亚基 H）的 mRNA 表达水平下降。相比之下，HEA 对皮肤组织中的 Rhcg 表达水平没有影响。
