Acute effect of salicylate on T4 and T3 levels in plasma and their excretion by bile in rats.

Male Wistar rats weighing about 400 g were anaesthetized with pentobarbiturate (40 mg kg-1) i.p. and thin polyethylene tubings were inserted in left femoral artery and vein as well as in a bile duct. Blood samples were taken at 30-60 min intervals for 6 h. The total bile was collected to pre-weighed glass vials. With the aid of radioimmunoassay the level of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) was estimated in plasma and the total excretion of these compounds by bile was measured after the incubation of bile aliquots with beta-glucuronidase-arylsulphatase. After i.v. injection of sodium salicylate (S; 80 mg kg-1) a significant decrease of T4 and T3 in plasma was found compared either to the initial level found in the same animals or to that found in controls not injected with S. Total excretion of bile was increased within 2 h after the administration of S. Under these conditions, the excretion of T4 was increased, while that of T3 was decreased. It is suggested that the effect of salicylate under the experimental conditions used consists of: 1. displacement of iodothyronines from plasma protein binding; 2. increased bile excretion (during the initial period after the administration of S) presumably due to the increased blood flow through the liver.