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性。广盐鱼和罗非鱼都能适应淡水(FW)和海水(SW)环境，而Osorezan湖中的耐酸鱼类可以在pH 3.6-3.7的酸性湖水中生存。氯细胞的功能多样性在鱼类适应不同离子组成和浓度的不同环境中起着重要作用。特别参照氯离子细胞，研究了适应于SW和FW的剑鱼之间的形态差异。在整装制备法中，检测到了丰富的氯细胞。在两种组织中，FW组的细胞体积均大于SW组。西南鱼氯细胞顶膜凹陷形成凹坑，而西南鱼氯细胞顶膜扁平或突起，有微绒毛。我们的发现表明，SW型和FW型氯离子细胞在两种环境中的活性相同，但发挥不同的离子转运功能。我们成功地鉴定了适应酸性环境的鱼鳃中的Na~+/H~~-交换器(NHE3)。NHE3是一种离子转运蛋白，它通过交换H^分泌来吸收Na^。在酸性环境中，鳃中NHE3的表达高于中性水中。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。当罗非鱼暴露在酸性条件下(pH 4.5)时，部分鳃氯细胞的顶膜扩张。在中性水中保存的鱼没有观察到这种形态变化。与DACE的情况一样，我们推测氯细胞在罗非鱼对酸性环境的适应中起着核心作用。

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.