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6 cells have been investigated as a potential tissue culture model for the study of thyroid hormone effects on skeletal muscle metabolism. Differentiated L6 myotubes contained high-affinity triiodothyronine (T3) receptors with a Kd of 3 X 10(-10) M and a maximal binding capacity of 24 fmol T3/100 micrograms DNA. Undifferentiated cells contained receptors with the same Kd, but the binding capacity was reduced by at least a factor of three. Sarcoplasmic reticulum vesicle calcium pumping was demonstrated in L6 cell homogenates. The Vmax for calcium pumping was increased 2.5-fold when T3 was present in the culture medium, but the Kd was unchanged. L6 cells contained high affinity thyroid hormone receptors and were thyroid hormone responsive. These cells may be useful as a tissue culture model for studying the effects of thyroid hormones on skeletal muscle metabolism. In addition, the increase in T3 receptor number with the differentiated state suggests this as a model system for studying regulation of T3 receptor number and the role of T3 in the induction or maintenance of the differentiated state.
