Effects of Triiodothyronine on Adult Rat Cardiomyocytes

Effects of triiodothyronine (T3) on the expression of cytoskeletal and myofibrillar proteins in adult rat cardiomyocytes (ARC) were followed during two weeks of culture in the presence of 20% T3-depleted (stripped) FCS. Control cultures expressed mainly b -myosin heavy chain (MHC) mRNA. T3 caused a switch to a -MHC expression and a dose-dependent increase of a -smooth muscle ( a -sm) actin mRNA and protein. In parallel, the number of a -sm actin immunoreactive cells increased from 1% in controls to 29 and 62% in ARC treated with 5 and 100 nM T3. In the presence of T3, cells exhibited a higher beating rate than controls. The distribution of myofibrils in T3-treated cells was restricted to the perinuclear area with a sharp boundary. Only 5% of the control cells but 30 and 62% of the T3-treated (5 and 100 nM) ARC showed this restricted myofibrillar phenotype. Basic fibroblast growth factor (bFGF) which restricts myofibrillar growth and upregulates a -sm actin in ARC cultured with normal FCS had no effect on a -sm actin in ARC cultured in stripped FCS, but potentiated the effect of T3. In contrast, insulin-like growth factor I (IGF I), which suppresses a -sm actin and stimulates myofibrillogenesis in the presence of normal FCS suppressed T3-induced a -sm actin expression in stripped FCS. Thus, T3 appears to be permissive for the action of bFGF and IGF I on a -sm actin expression. ( J. Clin. Invest. 1996. 98:1737–1744.)