Biliary excretion of iodothyronines (T4, T3 and rT3) after administration of exogenous thyroxine: effect of glucose and insulin.

Thin polyethylene tubings were inserted into the bile duct and femoral vein to 5 groups of 6-12 rats each under pentobarbital anesthesia. The bile was collected into glass vials which were changed every 2 h for a total of 8 h. The individual groups were infused for 8 h with following solutions in a rate of 1.2 ml/h: 1. saline; 2. saline after fasting for 48 h before the experiment which was also the case in all following groups; 3.30% glucose in saline; 4. the same glucose with 1 U insulin per h; 5. 1 U insulin only in saline. After the first control interval each animal was injected 2 micrograms L-thyroxine in 0.5 ml alkaline saline into a venous cannula. In each bile sample total thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) were measured with the aid of specific radioimmunoassay and the results were expressed as ng/2 h. After the injection of exogenous T4 the excretion of this substance in bile significantly increased. In the case of T3 a significant decrease of biliary excretion was found in all fasted groups as compared with fed control (group 1). However, during the prolonged infusion of insulin resulting in severe hypoglycemia (group 5) the excretion of T3 stepwise significantly decreased, while at the same time that of rT3 significantly increased as compared with the other fasted groups including control, that infused with glucose or that infused glucose with insulin. It was suggested that this might result from the effect of counterregulatory hormones.