Laberge Tammy, Walsh Patrick J, McDonald M Danielle
Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA.
J Exp Biol. 2009 Aug;212(Pt 15):2394-402. doi: 10.1242/jeb.030411.
The gulf toadfish (Opsanus beta) is a facultatively ureotelic fish that excretes primarily urea under conditions of crowding or confinement. To examine the relationship between ammonia production, urea production and the ornithine-urea cycle (O-UC) enzyme activity and mRNA expression, we subjected toadfish to two-day and seven-day crowding regimes. Plasma cortisol levels were measured and liver tissue was assayed for ammonia and urea concentrations. Liver glutamine synthetase (GS), carbamoyl phosphate synthetase III (CPS), ornithine carbamoyl transferase (OCT) and arginase (ARG) activities were also measured. Quantitative PCR was utilized to determine liver GS, CPS, OCT, ARG, argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) mRNA expression. Hepatic ammonia concentrations decreased with increased duration of crowding whereas liver urea and circulating cortisol levels increased. An elevation in enzyme activity with increased duration of crowding was observed for all four O-UC enzymes examined. By contrast, mRNA expression was variable for the O-UC enzymes and only CPS and ASS had mRNA expression levels that were elevated in crowded fish. These results suggest that the activities of O-UC enzymes are better predictors for urea production than O-UC enzyme mRNA expression levels.
海湾蟾鱼（Opsanus beta）是一种兼性排尿素鱼类，在拥挤或受限条件下主要排泄尿素。为了研究氨生成、尿素生成与鸟氨酸-尿素循环（O-UC）酶活性及mRNA表达之间的关系，我们将蟾鱼置于为期两天和七天的拥挤环境中。测量血浆皮质醇水平，并检测肝脏组织中的氨和尿素浓度。还测量了肝脏谷氨酰胺合成酶（GS）、氨甲酰磷酸合成酶III（CPS）、鸟氨酸氨甲酰转移酶（OCT）和精氨酸酶（ARG）的活性。利用定量PCR来测定肝脏中GS、CPS、OCT、ARG、精氨琥珀酸合成酶（ASS）和精氨琥珀酸裂解酶（ASL）的mRNA表达。随着拥挤时间的延长，肝脏氨浓度降低，而肝脏尿素和循环皮质醇水平升高。在所检测的所有四种O-UC酶中，均观察到随着拥挤时间延长酶活性升高。相比之下，O-UC酶的mRNA表达存在差异，只有CPS和ASS在拥挤的鱼类中有升高的mRNA表达水平。这些结果表明，与O-UC酶的mRNA表达水平相比，O-UC酶的活性是尿素生成更好的预测指标。
