仔稚魚鈣離子平衡策略之比較

Abstract

仔魚在發育時期器官尚未發育完全，但是為了提供成長發育所需，必須大量吸收鈣離子。仔魚生活在不同離子組成的水域環境，其所面臨的鈣離子濃度由0.002 mM(軟水)到10 mM(海水)。在不同環境因數中存活的仔稚魚究竟如何調控其鈣離子的吸收，至今機制仍不明確。本研究旨在探討不同種仔魚在面對不同鈣環境時，其鈣離子平衡的比較。 將金魚(Carassius auratus)、斑馬魚(Danio rerio)、香魚(Plecoglossus altivelis)的仔魚在孵化前以三種鈣離子濃度處理(高鈣[Ca2+]=2mM，低鈣[Ca2+]=0.2mM，低鈣[Ca2+]=0.02mM)，在孵化後不同時間點分別測量仔魚體內鈣離子含量，及測量鈣離子流入速率。此外，將馴養在正常鈣環境([Ca2+]＝0.2mM)的仔魚，在不同時間點置於低鈣環境，以決定其對低鈣環境之敏感性。最後，則是將馴養於高鈣及低鈣環境的金魚和斑馬魚，在孵化後第三天分別比較其對鈣離子吸收動力學的變化。 在魚體鈣離子含量方面，斑馬魚、香魚的魚體鈣離子含量在低鈣適應組均顯著低於在中高鈣適應組。而金魚在三個不同處理組的魚體鈣離子含量則無顯著差異。 在鈣離子流入速率方面，斑馬魚、香魚低鈣適應組的鈣離子流入速率均顯著低於中高鈣適應組。但在金魚上則顯示出相反的情況，在長期適應於低鈣水之後，其鈣離子流入速率反而顯著高於高鈣組。 在仔魚對於低鈣環境之敏感性方面，不論金魚，斑馬魚或香魚，在不同發育時期突然面臨低鈣環境，皆會顯著降低其鈣離子流入速率。金魚降低至正常鈣組的72-88%，斑馬魚和香魚則分別為49-72%以及35-65%，此結果顯示面臨低鈣環境時，金魚的抵抗性較另外兩種魚種為強。 在鈣離子流入的動力學方面，適應於低鈣環境的金魚其極限初速(J下標(max))較適應於高鈣環境組的金魚增加了13%，斑馬魚則增加了8%；另外適應於低鈣組的金魚其鈣離子吸收的米氏常數(Km)較適應於高鈣組的金魚降低84%，斑馬魚則降低了67%。此結果顯示金魚及斑馬魚可增加對鈣離子吸收的極限初速及親和力以適應於低鈣環境。 本研究結果顯示在不同魚種仔魚間的鈣離子調節效率也有不同，其中以金魚的鈣離子調節效率最高，這些差異可能與不同魚種發育過程及其棲息環境不同有關。

Larvae whose organs are under developing uptake plenty of Ca2+ from water for development. However, the environment fish inhabited varies in Ca2+ concentration from 0.002mM (softwater) to 10mM (seawater). The purpose of the present study is to compare the strategies of the larvae of different species, goldfish (Carassius auratus), zebrafish (Danio rerio) and ayu (Plecoglossus altivelis), in calcium balance mechanism. Pre-hatched embryos were incubated in low-, mid- and high-Ca2+ artificial freshwater to determine their Ca2+ balance capabilities, which were examined via the measurement of whole body Ca2+ content and Ca2+ influx. Moreover, larvae acclimated to mid-Ca2+ medium were directly transferred to low-Ca2+ medium to determine their sensitivities to low-Ca2+ environments. In addition, the Ca2+ uptake kinetics were deteremined in zebrafish and goldfish incubated in high- or low-Ca2+ artificial freshwater. The results showed that Ca2+ content of both zebrafish and ayu acclimated to low-Ca2+ media were significantly lower than those acclimated to mid- or high-Ca2+ media after long term acclimation. However, Ca2+ content of goldfish in low-, mid- and high-Ca2+ groups showed no significant difference. In the aspect of the Ca2+ influx rate, low-Ca2+ group was significantly higher than mid- and high-Ca 2+ groups in goldfish after long term acclimation. In contrast, the Ca2+ influx rate in low-Ca2+ group was significantly lower than that in mid- and high-Ca2+ groups in zebrafish and ayu. During low-Ca2+ challenges, goldfish, zebrafish and ayu reduced their Ca2+ influx with different degrees, i.e., 72-88%, 49-72% and 35-65%, respectively. These data indicates that goldfish are more resistant to low-Ca2+ environments among the three species. 3-day-old goldfish larvae acclimated to low-Ca2+ environment resulted in a 13% increase in J(subscript max) and an 84% decrease in K when compared to that acclimated to high-Ca2+ group. Similar changes were also found in zebrafish larvae (8% increase in Jmax and a 67% decrease in K(subscript m)). From the present study, it is concluded that Ca2+ uptake efficiency in fish larvae varies among the three species. Goldfish possesses a more effective Ca2+ regulatory capacity than do zebrafish and ayu. The differences in the strategies for Ca2+ balance may be associated with the differences in the development patterns and the inhabiting environments.