Brix Kevin V, Esbaugh Andrew J, Mager Edward M, Grosell Martin
University of British Columbia, Department of Zoology, 6270 University Drive, Vancouver, British Columbia V6T 1Z4, Canada.
University of Texas, Austin, Marine Science Institute, 750 Channel View Dr., Port Aransas, TX 78373, USA.
Comp Biochem Physiol A Mol Integr Physiol. 2015 Jul;185:115-24. doi: 10.1016/j.cbpa.2015.04.002. Epub 2015 Apr 11.
The euryhaline pupfish, Cyprinodon variegatus variegatus (Cvv), can successfully osmoregulate in ≥2 mM Na(+) and a freshwater population (Cyprinodon variegatus hubbsi; Cvh) osmoregulates at ≥0.1mM Na(+). We previously demonstrated that Cvv relies on an apical NKCC and NHE in the gill for Na(+) uptake in high (7mM) and intermediate (2 mM) Na(+) concentrations, while Cvh relies only on NHE for Na(+) uptake. This study investigated whether differential NHE isoform use explains differences in Na(+) uptake kinetics between these two populations. We further studied whether Cvh uses a NHE-Rh metabolon or carbonic anhydrase (CA) to overcome thermodynamic challenges of NHE function in dilute freshwater. Transfer to more dilute freshwater resulted in upregulation of nhe-2 (Cvv only) and nhe-3 (Cvv and Cvh). Relative expression of nhe-3 compared to nhe-2 was 2-fold higher in Cvv, but 200-fold higher in Cvh suggesting that nhe-3 expression is an important freshwater adaptation for Cvh. Simultaneous measurement of Na(+) and Tamm flux under various conditions provided no support for a NHE-Rh metabolon in either population. Carbonic anhydrase activity in Cvv was comparable in 7 and 2 mM Na(+) acclimated fish. In Cvh, CA activity increased by 75% in 0.1 mM Na(+) acclimated fish compared to 7 mM Na(+) fish. Ethoxzolamide had variable effects, stimulating and reducing Na(+) uptake in Cvv acclimated to 7 and 2 mM Na(+), while reducing Na(+) uptake in 7 and 0.1mM Na(+) acclimated Cvh. This suggests that CA plays important, but different roles in regulating Na(+) uptake in Cvv and Cvh.
广盐性的鳉鱼，即Cyprinodon variegatus variegatus（Cvv），能够在≥2 mM的Na⁺环境中成功进行渗透调节，而淡水种群（Cyprinodon variegatus hubbsi；Cvh）则在≥0.1 mM的Na⁺环境中进行渗透调节。我们之前证明，在高（7 mM）和中等（2 mM）Na⁺浓度下，Cvv依靠鳃中的顶端NKCC和NHE来摄取Na⁺，而Cvh仅依靠NHE来摄取Na⁺。本研究调查了不同的NHE亚型使用是否解释了这两个种群之间Na⁺摄取动力学的差异。我们还进一步研究了Cvh是否使用NHE-Rh代谢体或碳酸酐酶（CA）来克服在稀淡水环境中NHE功能的热力学挑战。转移到更稀的淡水环境中导致nhe-2（仅Cvv）和nhe-3（Cvv和Cvh）上调。与nhe-2相比，nhe-3在Cvv中的相对表达高2倍，但在Cvh中高200倍，这表明nhe-3表达是Cvh适应淡水的重要因素。在各种条件下同时测量Na⁺和Tamm通量，没有为任何一个种群中存在NHE-Rh代谢体提供支持。在适应7 mM和2 mM Na⁺的Cvv鱼中，碳酸酐酶活性相当。在Cvh中，与适应7 mM Na⁺的鱼相比，适应0.1 mM Na⁺的鱼的CA活性增加了75%。乙氧唑胺有不同的作用，刺激和减少适应7 mM和2 mM Na⁺的Cvv的Na⁺摄取，同时减少适应7 mM和0.1 mM Na⁺的Cvh的Na⁺摄取。这表明CA在调节Cvv和Cvh的Na⁺摄取中发挥着重要但不同的作用。
