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Primary cultured adult rat hepatocytes were used to study regulation of thyroid hormone deiodination. Control studies showed that these cells survived for at leas 4 d, during which time they actively deiodinated both the phenolic (5'-) and non-phenolic (5-) rings of L-thyroxine (T4),3,5,3'-triiodo-L-thyronine, and 3,3',5'-triiodothyronine. Increasing the substate concentration caused a decrease in fractional iodide release and a corresponding increase in conjugation with sulfate and glucuronide. Propylthiouracil strongly inhibited the 5'-deiodinase activity and caused only a slight decrease in 5-deiodinase activity. Thus, these monolayer-cultured cells preserved many of the properties of normal hepatocytes. Incubation with combinations of insulin, glucagon, and/or glucose for 5 h showed that insulin stimulated T4 5'-deiodination, whereas glucagon inhibited the insulin stimulation but had no effect in the absence of insulin. Glucose had no effect and did not alter the effect of the hormones. The insulin-enhanced deiodination increased between 1 and 5 h, which suggests that the previous inability to demonstrate an insulin effect was due to the short survival of the in vitro liver systems used in those studies. The present data suggest that the inhibition of T4 5'-deiodination observed during fasting, and its restoration by refeeding, may be related to the effects of feeding on insulin and glucagon release rather than on glucose per se.

Thyroid hormone metabolism in primary cultured rat hepatocytes. Effects of glucose, glucagon, and insulin.