發育中魚類氯離子吸收之機制

Abstract

發育中仔魚的滲透壓調節器官雖然尚未分化發育完備，仍能維持體內滲透壓和離子恆定，但是在面對環境因數改變時，氯離子的平衡機制仍有不明之處。本研究旨在探討仔魚面對低氯環境時調控氯離子平衡的機制。將吳郭魚（Oreochromis mossambicus）、香魚（Plecoglossus altivelis）和金魚（Carassius auratus）仔魚適應在低氯（0.04 mM）和正常氯（0.4 mM）環境中，在孵化後第0、1、3、5天測量仔魚身體氯離子含量和氯離子吸收速率，並比較此三種魚孵化後第3天的仔魚氯離子吸收動力學的變化，更進一步以吳郭魚仔魚為例，探討仔魚在急速轉移至低氯環境時，調節氯離子吸收的能力。 結果顯示，此三種仔魚身體氯離子含量都隨著發育逐漸增加；經過低氯處理的吳郭魚仔魚氯離子含量在孵化後第5天顯著低於正常氯處理組，低氯處理的香魚孵化後第3天和第5天仔魚的身體氯離子含量都顯著低於正常氯處理組，而金魚經過低氯處理之後，從孵化後第0天到第5天身體氯離子的含量和控制組都沒有顯著差異。在氯離子流入速率方面，三種仔魚經過低氯處理7天後氯離子流入速率顯著低於控制組。 在氯離子流入的動力學方面，適應於低氯環境的吳郭魚、香魚和金魚極限初速度（Jmax）並沒有顯著改變，但是米氏常數（Km）比適應正常氯組分別降低72.7％、43.3％及52.3％，此結果顯示三種低氯環境適應的仔魚對氯離子親和力（1/Km）的都提高了，藉以增加氯離子的吸收。孵化後第0天吳郭魚仔魚突然面對低氯環境時，會降低氯離子的流入速率，在轉移24小時之後氯離子的流入速率增加至與控制組沒有顯著差異，但是在轉移72小時之後則顯著低於控制組。 本研究結果顯示適應於低氯環境的仔魚身體內氯離子的含量隨著發育而增加，仔魚可能藉由調節運輸蛋白的親和力來促進氯離子的吸收，但是氯離子流入速率的調節尚未達到與控制組相同的程度，因此可能還有其他的機制參與調控仔魚氯離子的平衡。而不同魚種仔魚面對低氯環境時氯離子調節策略也有所不同，這些差異可能是因為棲息環境不同所造成。

Developing fish larvae, whose organs are poorly or under developed, have to maintain ionic homeostasis in their body fluids. However, little is known about how fish larvae regulate their Cl- balance in hypotonic freshwater environments. The purpose of present study was to investigate the mechanism how developing larvae regulate their Cl- balance upon challenges with low-Cl- environments. Moreover, euryhaline tilapia (Oreochromis mossambicus) and ayu (Plecoglossus altivelis), and stenohaline goldfish (Carassius auratus) are different in habitats and life histories. The present paper was also to test whether these species develop different strategies for acclimation to diluted external media. Embryos were incubated in artificial fresh water with normal (0.4 mM) or low (0.04 mM) Cl- from eyed-egg stage to 5 d post-hatching (H5). Whole-body Cl- content and Cl- influx were measured to determine the Cl--balance capacities. Moreover, the Cl- uptake kinetics was measured in 3-d-old (H3) larvae acclimated to normal- and low-Cl- water. In addition, 0-d-old (H0) tilapia larvae were transferred directly to low-Cl- water to determine their sensitivities to low-Cl- environment. The results showed that whole-body Cl- content of three species increased significantly following development. Cl- content in low-Cl- group was not significantly lower than that in normal-Cl- group until H3 in ayu and H5 in tilapia, respectively. However, Cl- content in low-Cl- goldfish was not significantly different from that in normal-Cl- ones from HO to H5. In the aspect of Cl- influx, larvae of the 3 species acclimated to low-Cl- water had lower Cl- influx rates than did those acclimated to normal-Cl- water after 7 d acclimation. H3 larvae of tilapia, ayu, and goldfish acclimated to low-Cl- media showed a decrease in Km and no significant change in Jmax compared to those acclimated to normal-Cl- water. During acute low-Cl- challenges, HO tilapia reacted to decrease Cl- influx initially, and increased to the level in the control group at 24 h after exposure. It is concluded that whole-body Cl- content of larvae increased significantly following development. The strategies for low-Cl- acclimated larvae to regulate Cl- influx rates may involve in the modulation of transporters to operate their Cl- active uptake. However, the regulation in Cl- influx was not sufficient for recovering the ion loss, indicating that other mechanisms may also involve in the Cl- balance of larvae. Moreover, the present data showed that Cl- uptake efficiency varies among 3 species. Goldfish possessed a more effective Cl- regulatory capacity than did tilapia and ayu. The differences in the Cl- uptake strategies upon low-Cl- challenge among species may be associated with their habitats.