Physiological and pathological
cardiac hypertrophy have directionally opposite changes in transcription of
thyroid hormone (TH)-responsive genes, including alpha- and
beta-myosin heavy chain (MyHC) and
sarcoplasmic reticulum Ca(2+)-ATPase (SERCA), and TH treatment can reverse molecular and functional abnormalities in pathological
hypertrophy, such as pressure overload. These findings suggest relative
hypothyroidism in pathological
hypertrophy, but serum levels of TH are usually normal. We studied the regulation of TH receptors (TRs) beta1, alpha1, and alpha2 in pathological and physiological rat
cardiac hypertrophy models with hypothyroid- and
hyperthyroid-like changes in the TH target genes, alpha- and beta-MyHC and SERCA. All 3 TR subtypes in myocytes were downregulated in 2
hypertrophy models with a hypothyroid-like
mRNA phenotype,
phenylephrine in culture and pressure overload in vivo. Myocyte TRbeta1 was upregulated in models with a
hyperthyroid-like phenotype, TH (
triiodothyronine, T3), in culture and exercise in vivo. In myocyte culture, TR overexpression, or excess T3, reversed the effects of
phenylephrine on TH-responsive mRNAs and promoters. In addition, TR cotransfection and treatment with the TRbeta1-selective agonist
GC-1 suggested different functional coupling of the TR
isoforms, TRbeta1 to transcription of beta-MyHC, SERCA, and TRbeta1, and TRalpha1 to alpha-MyHC transcription and increased myocyte size. We conclude that TR
isoforms have distinct regulation and function in rat cardiac myocytes. Changes in myocyte TR levels can explain in part the characteristic molecular phenotypes in physiological and pathological
cardiac hypertrophy.