吳郭魚仔魚鈣離子吸收能力之調節

Abstract

本論文藉由環境鎘、鈣濃度的改變來探討吳郭魚（ Oreochromis mossambicus ）仔魚鈣離子吸收能力之調節。 （一）低鈣水（ 0.2mM ）有鎘存在時，抑制仔魚生長發育（全長、體重、卵黃囊直徑） , 150μgl-1鎘時仔魚死亡率高達 70－80％。仔魚體內鎘累積量，隨水中鎘增加而減少。鈣量除 25μgl-1處理組與控制組相同外，亦隨鎘濃度增加而減少。鈣離子吸收速率，則是以 50μgl-1鎘以下的較控制組高，其餘都較低。高鈣水（ 2.0mM ) ，控制組、50 、100 或 150μgl-1處理組都維持相同的成長、鈣量和鈣離子吸收速率。鎘含量則隨環境鎘增加而增加，仔魚死亡率皆低於 2％。顯示高鈣環境中仔魚能獲得足夠鈣，故仔魚的鈣離子吸收速率無需任何調節。 （二）為更進一步瞭解，鈣離子吸收速率與適應環境之關係，將低鈣鎘水的仔魚，轉移至低鈣無鎘環境中。轉移後的前 24 小時，鎘前處理組鈣離子吸收速率突增 2倍,24 小時後緩慢下降，直到 72 小時時與控制組相同。鈣量之變化最初 24 小時亦增加快速，鈣量約控制組的50％左右。根據以上結果推測：[l]鈣離子吸收速率的增加，可能是補償在有鎘環境鎘所造成的鈣不足。[2]鈣離子吸收速率的升高並不能完全以去除環境鎘所解釋。[3]吸收速率之變化可能受仔魚體內鈣量多寡影響。 （三）為進一步證明此推論，將生存高鈣鎘水的仔魚，轉移到高鈣無鎘水。鎘前處理的仔魚，在高鈣無鎘環境間鈣離子吸收速率未有任何變化，鈣量亦與控制組相同。此結果與上述假設符合。 綜合上述：鈣離子吸收能力之調節可能受到環境鈣濃度或體內鈣含量多寡所影響，而且這調節能力即可在數天內完成。相較於成魚，更顯示仔魚具有很強調適能力。

The present work is to study the modulation of calcium uptake in tilapia larvae (Oreochromis mossambicus ). Changes in the total length, body weight, yolk diameter, cadmium content, calcium content and calcium influx of tilapia larvae during cadmium treatment and detoxification were examined. (1) In low-calcium medium larvae treated with 150μg l-1 cadmium showed over 70% mortality and a significant inhibition in the development and growth at 96 h during the treatment, while those treated with 150μg l-1 cadmium in high-calcium water for 96 h had less than 2% mortality and developed and grew normally. Similarity, cadmium caused a significant decrease in the calcium content of the larvae in low-calcium medium but no significant effect in those in high-calcium medium. The larvae in high-calcium medium increased, while those in low-calcium medium maintained a constant amount of accumulated cadmium following the increase of environmental cadmium levels. In low- calcium medium, when compared with the control, the larvae treated with 25-50μg l-1 cadmium enhanced their calcium influx rates to obtain sufficient calcium from the environment. Those treated with150μg l-1 cadmium diminshed their calcium uptake capacity due to the serious abnormaly in development, growth and physiological state. On the other hand, high-calcium media may have supplied sufficient calcium for larvae so that 50-150μg l-1 cadmium did not develop significant change in the calcium uptake capacity of the larvae. (2) In order to examine regulation of calcium influx, newly-hatched tilapia larvae were exposed to cadmium freshwater ([Ca2+] =0.2 mM) and then transferred to cadmium-free freshwater. The cadmium-pretreated larvae revealed a dramatic change in the calcium influx, increasing within 24h after the transfer and then decreasing to the level of control group. The increase of calcium content was more rapid than that in the control right after the transfer, and then slowed down 24 h later while the calcium content reached 56% of that in the control. These results would suggest that: [1] The increased calcium uptake in the first 24 h is a compensation for calcium loss caused by cadmium exposure. [2] The higher calcium influx, however, could not be completely explained by the clearance of inhibitor (cadmium) from calcium uptake mechanism. [3]Cadmium-pretreated tilapia larvae seemed to regulate their calcium uptake capacity efficiently depending upon the calcium content in the body. (3) The hypothesis stated above was further testified by the following experiment. Newly hatched tilapia larvae were exposed to cadmium freshwater ([Ca2+] =2.0 mM) and then transferred to cadmium-free freshwater ([Ca2+] =2.0 mM). Compared with the control, the cadmium-pretreated larvae did not enhance significant change in both calcium influx and calcium content. Obviously, cadmium-pretreated larvae needed not do any compensation in calcium influx. The result would confirm that calcium content in the body, but not the ambient cadmium, is the main factor that affect calcium influx. It is concluded that calcium uptake capacity of tilapia larvae were modulated depending upon environmental calcium levels and/or larval calcium content. Moreover, the modulation will be achieved in only a few days, which is faster than adult fish, and this indicates that the tilapia larvae have efficient ability on adaptating to different environments.