Cyr D G, Idler D R, Audet C, McLeese J M, Eales J G
Maurice Lamontagne Institute, Department of Fisheries and Oceans, Mont-Joli, Quebec, Canada.
Gen Comp Endocrinol. 1998 Jan;109(1):24-36. doi: 10.1006/gcen.1997.6994.
The recent collapse of the Northwestern Atlantic cod fisheries has coincided with a cooling of water temperatures. During this time the condition factor of cod has been poor. The objective of the present study was to determine the effects of long-term temperature acclimation on growth reproduction and thyroid function in laboratory held Atlantic cod (Gadus morhua). One of the key parameters used to assess thyroid function is the peripheral metabolism of L-thyroxine (T4) by microsomal deiodinase enzymes. Deiodinase function has not been described for gadid fish. T4 outer-ring deiodinating activity (apparent K(m) 1-2 nM) was confined primarily to liver. Its properties resembled those for hepatic T4ORD activity of other teleosts and the mammalian type II deiodinase. The T4ORD activity of cod liver exceeded that of salmonids and could explain the high plasma T3 levels (10-18 ng/ml), which were 2-5 times greater than T4 levels. T4 and T3 inner-ring deiodination was confined mainly to brain. In order to determine the effects of long-term temperature acclimation on cod, somatic growth, reproduction, and thyroidal status were assessed monthly in 400-900-g satiation-fed male Atlantic cod captured in June from the St. Lawrence Estuary and then acclimated from August to the following June under a natural photoperiod at 2-4 degrees C (LT) or 6-10 degrees C (HT). Reproductive status was determined from the gonadosomatic index (GSI), plasma testosterone (T) and 11-ketotestosterone (11-KT) levels, and the appearance of milt; thyroidal status was determined from plasma T4 and 3,5,3'-triiodo-L-thyronine (T3) levels and hepatic T4ORD activity to produce biologically active T3. Testis maturation (high levels of 1 and 11-KT, and milt release) occurred in April and May and was uninfluenced by acclimation temperature. LT cod grew more slowly than HT cod. Differences in body weight were particularly evident from December to February. In conclusion, (i) cod possess outer- and inner-ring deiodinase activities, predominating respectively in liver and brain, and with properties resembling those of other teleosts, (ii) T4ORD activity of liver is unusually high and may account for the high plasma T3 levels in this species, (iii) T4ORD activity tends to increase during periods of increased somatic growth, and (iv) chronic acclimation of male cod to 2-4 degrees C, as opposed to 6-10 degrees C, decreases somatic growth but does alter circulating levels of thyroid hormones and androgens and it does not change the time of sexual maturation.
西北大西洋鳕鱼渔业近期的崩溃与水温下降同时发生。在此期间，鳕鱼的状况不佳。本研究的目的是确定长期温度驯化对实验室养殖的大西洋鳕鱼（Gadus morhua）生长、繁殖和甲状腺功能的影响。用于评估甲状腺功能的关键参数之一是微粒体脱碘酶对外周L-甲状腺素（T4）的代谢。尚未描述鳕科鱼类的脱碘酶功能。T4外环脱碘活性（表观K(m)为1 - 2 nM）主要局限于肝脏。其特性类似于其他硬骨鱼类的肝脏T4ORD活性和哺乳动物II型脱碘酶。鳕鱼肝脏的T4ORD活性超过鲑科鱼类，这可以解释血浆中高浓度的T3水平（10 - 18 ng/ml），其比T4水平高2 - 5倍。T4和T3内环脱碘主要局限于大脑。为了确定长期温度驯化对鳕鱼的影响，每月对400 - 900克饱食的雄性大西洋鳕鱼进行评估，这些鳕鱼于6月从圣劳伦斯河口捕获，然后在自然光照周期下于2 - 4摄氏度（低温组，LT）或6 - 10摄氏度（高温组，HT）从8月驯化至次年6月。通过性腺体指数（GSI）、血浆睾酮（T）和11 - 酮睾酮（11 - KT）水平以及精液外观来确定生殖状态；通过血浆T4和3,5,3'-三碘-L-甲状腺原氨酸（T3）水平以及肝脏T4ORD活性以产生生物活性T3来确定甲状腺状态。睾丸成熟（高水平的T和11 - KT以及精液排出）发生在4月和5月，不受驯化温度影响。低温组鳕鱼比高温组鳕鱼生长得更慢。体重差异在12月至2月尤为明显。总之，（i）鳕鱼具有外环和内环脱碘酶活性，分别在肝脏和大脑中占主导，且特性类似于其他硬骨鱼类；（ii）肝脏的T4ORD活性异常高，可能是该物种血浆T3水平高的原因；（iii）在体细胞生长增加期间，T4ORD活性趋于增加；（iv）雄性鳕鱼长期适应2 - 4摄氏度而非6 - 10摄氏度，会降低体细胞生长，但不会改变甲状腺激素和雄激素的循环水平，也不会改变性成熟时间。
