Thyroxine induces transition of red towards white muscle in cultured heart cells.

Thyroid hormones (TH) have previously been shown to alter the force and velocity of cardiac muscle contractions. To investigate the mechanism responsible for these alterations, excess amounts of thyroxine (T4, 1 microM) were applied on rat heart cells grown in cell culture. We found the following biochemical alterations: a) 40% decrease in the myoglobin content within 2 days; b) 25% increase in the rate of Ca-uptake into sacroplasmic reticulum (SR) in myocytes following chemical skinning; and c) a two-fold increase in Na-K-ATPase activity measured by 86Rb-uptake. These changes support our hypothesis that TH induce the transition of slow-twitch ("red") muscles towards the fast-twitch ("white") muscle type. This may explain the changes in contractile activity known to occur under TH influence.