Abstract: | These experiments were done to clarify that the differential effects of thyroxine (T4) and triiodothyronine (T3) on skeletal muscle protein turnover are caused by their roles on ATP production. Primary cultured chick muscle cells were treated with a physiological level of T4 (60 ng/ml), T3 (12 ng/ml), or ATP (0.5 mM) for 6 days and the protein content, ATP production, proteasome activity, and myofibrillar protein breakdown were measured. The protein content measured as an index of cell growth was not affected by T4, T3, or ATP. The cellular ATP level was increased by T3 and ATP, but not by T4. Proteasome activity and N τ-methylhistidine (MeHis) release measured as an index of myofiblillar protein breakdown was also increased by T3 and ATP, but not by T4. These results indicate that T3 but not T4 increases ATP production followed by an increase in proteasome activity, and thus stimulates myofibrillar proteolysis. |