The Effects and function of the Hypothalamic‐pituitary‐thyroid axis on fasting Northern elephant seals

• Published in: The FASEB Journal
• Main Authors: Martinez, Bridget, Sonanez, Jose Guadalupe, Rodriguez, Ruben, Viscarra, Jose, Vasquez-Medina, Jose Pablo, Ortiz, Rudy M
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2012
• Subjects: Elephant Seals
• Online Access: http://dx.doi.org/10.1096/fasebj.26.1_supplement.lb709

Northern elephant seals sustain long bouts of food deprivation while remaining physically active with relatively high metabolic rates, and unlike hibernating mammals, they also remain normothermic. In terrestrial mammals, food deprivation is associated with reduced thyroid hormone (TH) levels and metabolism; however, elephant seals pups exhibit sustained, if not, elevated TH. The functional relevance of this apparent paradox is not understood. To address the hypothesis that increased TH levels in prolong‐fasted seals are associated with elevated TH receptor expression, we measured the changes in fasting‐induced TH levels and the mRNA expression of the TH receptor, THrβ‐1, in muscle and blubber. To better ascertain the relevance of the changes in THrβ‐1, we also measured the protein content and mRNA expression of products associated with changes in THrβ‐1. Preliminary data shows almost a three‐fold increase in THrβ‐1 gene expression in both muscle and blubber and we observed an elevation in tT3, tT4, and fT4 over the course of the fast decrease in total T3:T4 ratio over the 49 days. Furthermore, both Deiodinase 1 and 2 increased in muscle tissue suggesting an increase in availability of t3 to meet the higher demands of an increase in the receptor. It would be pertinent to attempt to clarify the mechanism of the hypothalamic‐pituitary axis and its overall function and effect on metabolism.