Effect of triiodothyronine on cultured neonatal rat heart cells: beating rate, myosin subunits and CK-isozymes.

The effect of thyroid hormone on cell contractility, myosin subunit composition and creatine kinase activity was explored in cultured rat myocytes. Triiodothyronine (5 nM) was administered to neonatal rat heart myocytes grown in chemically defined medium. The hormone induced a 30% enhancement in the rate of cell beating and a complete transition from beta- to alpha-myosin heavy chain synthesis. Myosin light chains as well as creatine kinase activity and isozymic distribution were unaffected by the hormone. The arrest of spontaneous contraction by either membrane depolarization or Ca2+ channel blockage did not interfere with the shift towards alpha-myosin heavy chain predominance. We conclude that thyroid regulation of myosin subunits is confined to the molecule heavy subunits and occurs irrespective of cell contraction. Furthermore, the genomic expression of creatine kinase is not regulated by thyroid hormone.