Energy expenditure associated with sodium/potassium transport and protein synthesis in skeletal muscle and isolated hepatocytes from hyperthyroid sheep

Abstract

The object of the present study was to determine the effect of thyroxine (T 4 ) treatment of sheep on protein synthesis and associated energy costs in skeletal muscle and hepatocytes. Protein synthesis, and ouabain-sensitive and cycloheximide-sensitive respiration in isolated intercostal muscle and hepatocytes were determined in sheep after 5 weeks of daily injections of either saline or T 4 . Plasma T 4 and total triiodothyronine (T 3 ) concentrations were doubled and free T 3 concentrations were quadrupled by T 4 injections. The fractional rates of protein synthesis increased in isolated external intercostal muscle and hepatocytes from hyperthyroid sheep. Fractional rates of protein synthesis in isolated external intercostal muscle and hepatocytes were linearly correlated with plasma free T 3 concentrations. Total oxygen consumption of muscle and hepatocytes was unaffected by T 4 injections. Ouabain-sensitive respiration increased in hepatocytes and muscle of T 4 -treated animals. Cycloheximide-sensitive respiration was elevated in hepatocytes from hyperthyroid sheep. Cycloheximide-sensitive respiration in muscle was unaffected by T 4 treatment. The present experiment demonstrates that T 4 increases protein synthesis in ruminants. The energy expenditure in support of Na + , K + -ATPase and protein synthesis in skeletal muscle and hepatocytes may account for 34–60% of total cellular energy expenditure.