Hepatic and muscle expression of thyroid hormone receptors in association with body and muscle growth in large yellow croaker, Pseudosciaena crocea (Richardson)

The role of thyroid hormone (TH) and its receptors (TRs) in the regulation of body growth and muscle accretion is well established in mammals and birds, whereas the involvement of THs and TRs in fish growth, especially during the muscle accretion period of juvenile-adult transition, is unknown. This study describes the cloning of the partial cDNA sequences of TRalpha and TRbeta in large yellow croaker, Pseudosciaena crocea (Richardson) and the patterns of TRalpha and TRbeta mRNA expression in liver and muscle of 1- and 2-year-old large yellow croaker, associated with changes in body mass and muscle characteristics. Two TRalpha isoforms (TRalpha1, TRalpha2) and TRbeta were identified in large yellow croaker. The deduced amino acid sequences showed high homology to the TRs of human and other teleosts. Hepatic TRbeta mRNA expression was markedly lower in 2-year-old large yellow croaker compared with the 1-year-old, while no significant age difference was observed for hepatic TRalpha mRNA expression. Muscle expression of TRalpha mRNA was significantly higher in 2-year-old large yellow croaker, whereas TRbeta exhibited no significant age difference. Meanwhile, serum concentration of T(4) was significantly decreased in 2-year-old large yellow croaker, but no change was observed for T(3). The body mass, fork length and body height of 2-year-old large yellow croaker were 4.7, 1.6 and 1.7 times greater, respectively compared with that of 1-year-old. Average diameters of skeletal muscle in 2-year-old large yellow croaker were remarkably larger than that in 1-year-old with no significant difference in muscle crude fat content. The down-regulation of hepatic TRbeta expression was associated with the decrease in general growth rate and the increase in muscle expression of TRalpha was accompanied with muscle accretion and myofiber hypertrophy, implicating the different roles of TRs in the regulation of growth in large yellow croaker.