To examine whether the low plasma levels of triiodothyronine (T3) in fasted rats might limit the recovery of muscle protein synthesis on refeeding, rats were fasted for either 3 or 4 days and refed with or without pretreatment with thyroid hormones. Fasting suppressed T3 levels, plasma insulin and the rate of the translational phase of muscle protein synthesis (K RNA; the rate per unit RNA), especially after the 4-day fast. On refeeding, plasma T3 levels remained low for more than 3 h after the 3-day fast and for more than 8 h after the 4-day fast. Insulin concentrations increased within the first hour of refeeding, eventually achieving supranormal concentrations after the 3-day fast. The K RNA increased within the first hour of refeeding, achieving well-fed control values by 3 h after the 3-day fast or 24 h after the 4-day fast. The increases in K RNA were significantly correlated with the increases in insulin at low insulin concentrations, achieving a plateau value at 150 pmol/l, so that further increases in insulin were not associated with any further increases in protein synthesis. Pretreatment with thyroid hormone induced increased T3 levels which were maintained for up to 8 h of refeeding. This had no effect on the responses of either insulin or protein synthesis to refeeding after the 3-day fast, but did result in an acceleration of the recovery in the K RNA and plasma insulin levels in the rats fasted for 4 days. Analysis of the insulin–K RNA relationship showed no evidence for any increase in the insulin sensitivity of muscle protein synthesis with thyroid pretreatment, the initial stimulation of protein synthesis on refeeding the rats fasted for 4 days reflecting increased insulin secretion. Since in the untreated animals, insulin secretion on refeeding was also correlated with T3 levels, these results are consistent with the previously reported thyroidal dependence of insulin secretion.
J. Endocr. (1988) 118, 417–422