Nakajima Takuro, Shimura Haruka, Yamazaki Miyuki, Fujioka Yasuhiro, Ura Kazuhiro, Hara Akihiko, Shimizu Munetaka
Faculty of Fisheries Sciences, Hokkaido University, Hokkaido, Japan;
Shiga Prefecture Fishery Experiment Station, Shiga, Japan.
Am J Physiol Regul Integr Comp Physiol. 2014 Aug 15;307(4):R414-25. doi: 10.1152/ajpregu.00474.2013. Epub 2014 Jun 18.
Landlocking of salmon relaxes selective pressures on hypoosmoregulatory ability (seawater adaptability) and may lead to the abandonment of its physiological system. However, little is known about the mechanism and consequence of the process. Biwa salmon is a strain/subspecies of Oncorhynchus masou that has been landlocked in Lake Biwa for an exceptionally long period (about 500,000 years) and has low ability to adapt to seawater. We compared activity of gill Na(+),K(+)-ATPase (NKA) of Biwa salmon with those of anadromous strains of the same species (masu and amago salmon) during downstream migration periods and after exogenous hormone treatment. Gill NKA activity in anadromous strains increased during their migration periods, while that in Biwa salmon remained low. However, treatments of Biwa salmon with growth hormone (GH) and cortisol increased gill NKA activity. Cortisol treatment also improved the whole body seawater adaptability of Biwa salmon. Receptors for GH and cortisol responded to hormonal treatments, whereas their mRNA levels during downstream migration period were essentially unchanged in Biwa salmon. Circulating levels of cortisol in masu salmon showed a peak during downstream migration period, while no such increase was seen in Biwa salmon. The present results indicate that Biwa salmon can improve its seawater adaptability by exogenous hormonal treatment, and hormone receptors are capable of responding to the signals. However, secretion of the endogenous hormone (cortisol) was not activated during the downstream migration period, which explains, at least in part, their low ability to adapt to seawater.
鲑鱼的陆封现象减轻了对低渗调节能力（海水适应性）的选择压力，并可能导致其生理系统的退化。然而，对于这一过程的机制和后果却知之甚少。琵琶湖鲑是马苏大麻哈鱼的一个品系/亚种，已经在琵琶湖陆封了异常长的时间（约50万年），并且适应海水的能力较低。我们比较了琵琶湖鲑与同物种溯河洄游品系（马苏大麻哈鱼和樱鳟）在向下游洄游期间以及外源激素处理后鳃中钠钾ATP酶（NKA）的活性。溯河洄游品系在洄游期间鳃NKA活性增加，而琵琶湖鲑的鳃NKA活性保持较低水平。然而，用生长激素（GH）和皮质醇处理琵琶湖鲑可增加鳃NKA活性。皮质醇处理还提高了琵琶湖鲑的整体海水适应性。GH和皮质醇的受体对激素处理有反应，而在向下游洄游期间，它们在琵琶湖鲑中的mRNA水平基本不变。马苏大麻哈鱼在向下游洄游期间循环皮质醇水平出现峰值，而琵琶湖鲑中未观察到这种增加。目前的结果表明，琵琶湖鲑可通过外源激素处理提高其海水适应性，并且激素受体能够对信号作出反应。然而，在向下游洄游期间内源性激素（皮质醇）的分泌未被激活，这至少部分解释了它们适应海水能力较低的原因。
