Oki C, Atkinson S
CTAHR, University of Hawaii at Manoa, Honolulu, HI 96822, USA.
Gen Comp Endocrinol. 2004 Apr;136(2):289-97. doi: 10.1016/j.ygcen.2004.01.007.
A leading hypothesis for the dramatic population decline of the Alaskan harbor seal over the last 2-3 decades have related environmental changes to a decrease in the quality or quantity of available prey. If harbor seals are experiencing some sort of nutritional stress, it is important to understand the physiological and endocrine mechanisms controlling metabolic homeostasis in this species. The purpose of this research project was to determine whether circadian patterns of cortisol and total and free thyroxine (T(4)) and triiodothyronine (T(3)) are present in the harbor seal during the summer and winter seasons, and identify how they might alter metabolic rate and maintenance of body reserves. This study was carried out at the Alaska SeaLife Center in Seward, Alaska in June 2000 and January 2001. Blood samples were obtained every 2-3 h over a 24-h period through in-dwelling catheters inserted the day before sampling. The presence of a diurnal rhythm was determined by comparing levels between ante meridian (12 am to noon) and post meridian (noon to 12 midnight) time periods. Mean daily cortisol concentrations were not significantly different between seasons, but cortisol displayed a circadian rhythm only during the summer. Mean concentrations of cortisol (51.5 ng/ml+/-20.3) in the summer hours of ante meridian differed significantly from levels in the hours of post meridian (28.5 ng/ml+/-17.4). Neither total and free T(4), nor T(3), displayed a diurnal rhythm in either season. However, tT(4), tT(3), and fT(3) levels were significantly higher in the winter than in the summer. There was no seasonal effect on fT(4) levels. The absence of a circadian rhythm of cortisol during the winter may have been a result of the limited amount of daylight as well as the continual need to produce metabolic heat as a by-product of gluconeogenesis. Higher levels of thyroid hormones in the winter indicate an adaptive mechanism to cope with the low temperatures of winter.
过去二三十年阿拉斯加港海豹种群数量急剧下降的一个主要假设是，环境变化导致了可获得猎物的质量或数量下降。如果港海豹正经历某种营养压力，那么了解控制该物种代谢稳态的生理和内分泌机制就很重要。本研究项目的目的是确定在夏季和冬季，港海豹体内皮质醇、总甲状腺素（T4）、游离甲状腺素以及三碘甲状腺原氨酸（T3）是否存在昼夜节律模式，并确定它们如何改变代谢率和维持身体储备。本研究于2000年6月和2001年1月在阿拉斯加苏厄德的阿拉斯加海洋生物中心进行。在采样前一天通过留置导管在24小时内每隔2 - 3小时采集一次血样。通过比较午前（上午12点至中午）和午后（中午至午夜12点）时间段的水平来确定昼夜节律的存在。不同季节间皮质醇的日均浓度无显著差异，但皮质醇仅在夏季呈现昼夜节律。夏季午前时段的皮质醇平均浓度（51.5纳克/毫升±20.3）与午后时段的水平（28.5纳克/毫升±17.4）有显著差异。在两个季节中，总甲状腺素、游离甲状腺素以及三碘甲状腺原氨酸均未呈现昼夜节律。然而，冬季的总甲状腺素、总三碘甲状腺原氨酸和游离三碘甲状腺原氨酸水平显著高于夏季。游离甲状腺素水平没有季节效应。冬季皮质醇缺乏昼夜节律可能是由于日照时间有限以及持续需要通过糖异生产生代谢热作为副产品。冬季甲状腺激素水平较高表明这是一种应对冬季低温的适应性机制。
