Effect of different thyroid states on rat liver glucokinase synthesis and degradation in vivo.

The role of different thyroid states on the rate of rat liver glucokinase synthesis and degradation was studied in vivo by the radioimmunochemical technique. In eu- and hyperthyroid starved rats, glucose refeeding induced a rapid and similar increase in glucokiase synthesis and activity, whereas in hypothyroid rats, only minor alterations in synthesis and activity was observed. 3,3',5'-Triiodo-L-thyronine substitution in hypothyroid animals restored the response of the enzyme within 24 h. The thyroid states per se had only a minor effect on glucokinase synthesis during the starvation period. In addition, in hypo-, eu-, and hyperthyroid rats adapted to a glucose diet, glucokinase degradation was estimated by double-pulse-labeling experiments, applying [14C]- and [3H]leucine. From the 3H/14C ratios, similar apparent half-lives were calculated: 17-19 h. It is concluded that thyroid hormones in their physiological range are an essential factor in the induction of hepatic glucokinase in vivo, exerting their action probably via a "permissive" effect, yet the degradation rate is unaffected by the thyroid state.