Adaptation to various aquatic environments and functional diversity of chloride cells in Fish

在本研究中，研究了三种硬骨鱼对不同水生环境的适应。鳉鱼和罗非鱼等广盐类鱼类既能适应淡水（FW）环境，也能适应海水（SW）环境，而Osorezan湖的耐酸鲮鱼可以在pH为3.6-3.7的酸性湖水中生存。氯离子细胞的功能多样性是鱼类适应不同离子组成和浓度环境的重要因素。以氯细胞为特别参考，研究了SW -和- FW -适应鳉鱼的形态学差异。在SW和FW鱼的鳃和眼膜的全挂载制剂中检测到丰富的氯细胞群体。在两种组织中，FW细胞均大于SW细胞。SW鱼氯细胞顶膜内陷形成凹坑，FW鱼氯细胞顶膜扁平或凸出，并有微绒毛。我们的研究结果表明，SW -型和FW -型氯离子细胞在两种环境下都具有相同的活性，但发挥不同的离子传输功能。我们成功地鉴定了适应酸性环境的鱼鳃中Na^+/H^+ -交换剂（NHE3）。NHE3是一种离子转运蛋白，吸收Na^+以交换H^+的分泌。NHE3在酸性环境中的表达量大于中性环境。免疫细胞化学染色显示NHE3定位于氯细胞的顶膜。因此，NHE3被认为是鱼类耐酸机制的关键分子。当罗非鱼暴露于酸性条件（pH为4.5）时，一些鳃氯细胞的顶膜扩大。这种形态变化在中性水中没有观察到。与鲮鱼的情况一样，我们推测氯细胞在罗非鱼适应酸性环境中起着核心作用。

In the present study, adaptation to various aquatic environments was examined in three teleost species. Euryhaline species such as killifish and tilapia are adaptable to both freshwater (FW) and seawater (SW) environments, whereas acid-tolerant dace in Lake Osorezan can survive in the acidic lake water of pH 3.6-3.7. The functional diversity of chloride cells is importantly involved in fish adaptation to various environments with different ion composition and concentration.1. Morphological differences between SW - and - FW - adapted killifish were examined with special reference to chloride cells. A rich population of chloride cells was detected in the whole-mount preparations of the gills and opercular membrane in SW and FW fish. In both tissues, the cells were larger in FW than in SW. The apical membrane of chloride cells was invaginated to form a pit in SW fish, whereas it was flat or showed projections and equipped with microvilli in FW. Our findings suggest that SW - and FW - type chloride cells are equally active in both environments, but exerting different ion-transporting functions.2. We succeeded in identifying Na^+/H^+ -exchanger (NHE3) in the gills of Osorezan dace adapted to acid environment. NHE3 is an ion-transporting protein that uptakes Na^+ in exchange of H^+ secretion. The expression of NHE3 in the gills was greater in acidic environment than in neutral water. Immunocytochemical staining showed that NHE3 was localized in the apical membrane of chloride cells. Thus, NHE3 is considered to function as a key molecule for acid-tolerant mechanism in fish.3. When tilapia was exposed to acide condition (pH 4.5), the apical membranes was expanded in some population of gill chloride cells. Such morphological alteration was not observed in the fish maintained in neutral water. As is the case in dace, we speculate that chloride cells play a central role in adaptation to acidic environments in tilapia.