Nawata C Michele, Walsh Patrick J, Wood Chris M
Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC, Canada V0R 1B0 Department of Biology, McMaster University, 1280 Main Street, West, Hamilton, ON, Canada L8S 4K1 Department of Physiology, University of Arizona, 1501 N. Campbell Avenue Tucson, AZ 85724, USA
Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, BC, Canada V0R 1B0 Department of Biology and Centre for Advanced Research in Environmental Genomics, University of Ottawa, Ottawa, ON, Canada K1N 6N5.
J Exp Biol. 2015 Jan 15;218(Pt 2):238-48. doi: 10.1242/jeb.114967.
In teleosts, a branchial metabolon links ammonia excretion to Na(+) uptake via Rh glycoproteins and other transporters. Ureotelic elasmobranchs are thought to have low branchial ammonia permeability, and little is known about Rh function in this ancient group. We cloned Rh cDNAs (Rhag, Rhbg and Rhp2) and evaluated gill ammonia handling in Squalus acanthias. Control ammonia excretion was <5% of urea-N excretion. Sharks exposed to high environmental ammonia (HEA; 1 mmol(-1) NH4HCO3) for 48 h exhibited active ammonia uptake against partial pressure and electrochemical gradients for 36 h before net excretion was re-established. Plasma total ammonia rose to seawater levels by 2 h, but dropped significantly below them by 24-48 h. Control ΔP(NH3) (the partial pressure gradient of NH3) across the gills became even more negative (outwardly directed) during HEA. Transepithelial potential increased by 30 mV, negating a parallel rise in the Nernst potential, such that the outwardly directed NH4(+) electrochemical gradient remained unchanged. Urea-N excretion was enhanced by 90% from 12 to 48 h, more than compensating for ammonia-N uptake. Expression of Rhp2 (gills, kidney) and Rhbg (kidney) did not change, but branchial Rhbg and erythrocytic Rhag declined during HEA. mRNA expression of branchial Na(+)/K(+)-ATPase (NKA) increased at 24 h and that of H(+)-ATPase decreased at 48 h, while expression of the potential metabolon components Na(+)/H(+) exchanger2 (NHE2) and carbonic anhydrase IV (CA-IV) remained unchanged. We propose that the gill of this nitrogen-limited predator is poised not only to minimize nitrogen loss by low efflux permeability to urea and ammonia but also to scavenge ammonia-N from the environment during HEA to enhance urea-N synthesis.
在硬骨鱼中，一个鳃代谢体通过Rh糖蛋白和其他转运蛋白将氨的排泄与钠离子的摄取联系起来。排泄尿素的板鳃亚纲动物被认为鳃对氨的通透性较低，对于这个古老类群中Rh的功能了解甚少。我们克隆了Rh的cDNA（Rhag、Rhbg和Rhp2），并评估了棘鲨鳃对氨的处理情况。对照氨排泄量小于尿素氮排泄量的5%。暴露于高环境氨（HEA；1 mmol⁻¹ NH₄HCO₃）48小时的鲨鱼，在重新建立净排泄之前，有36小时表现出逆分压和电化学梯度的主动氨摄取。血浆总氨在2小时内升至海水水平，但在24至48小时显著降至海水水平以下。在HEA期间，对照鳃的ΔP(NH₃)（NH₃的分压梯度）变得更加负（向外）。跨上皮电位增加了30 mV，抵消了能斯特电位的平行上升，使得向外的NH₄⁺电化学梯度保持不变。从12小时到48小时，尿素氮排泄增加了90%，足以补偿氨氮的摄取。Rhp2（鳃、肾脏）和Rhbg（肾脏）的表达没有变化，但在HEA期间鳃Rhbg和红细胞Rhag下降。鳃Na⁺/K⁺-ATP酶（NKA）的mRNA表达在24小时增加，H⁺-ATP酶的mRNA表达在48小时下降，而潜在代谢体成分Na⁺/H⁺交换体2（NHE2）和碳酸酐酶IV（CA-IV）的表达保持不变。我们认为，这种氮限制型捕食者的鳃不仅准备通过对尿素和氨的低外排通透性来最小化氮损失，而且在HEA期间从环境中清除氨氮以增强尿素氮的合成。
