Hypometabolic effect of 3,3',5'-triiodothyronine in chickens: interaction with hypermetabolic effect of 3,5,3'-triiodothyronine

The effect of 3,3',5'-triiodothyronine (rT3) and 3,5,3'-triiodothyronine (T3) on O2 consumption in 1-day-old chickens was studied. The birds were divided into five groups, each of six chickens: (1) control--without injection; (2) control--injected with 100 microliters of solvent (0.01 N NaOH in saline); (3) injected with 10 micrograms rT3/chicken; (4) injected with 0.5 micrograms T3/chicken; and (5) injected with 10 micrograms rT3 + 0.5 microgram T3/chicken. O2 consumption was measured using a Kipp & Zonen diaferometer at neutral temperature (30 degrees) 0, 1, 2, 3, and 4 hr after injection of hormones. Corresponding groups of other chickens served only for blood collection. rT3 and T3 were measured by radioimmunoassay. Reverse T3 decreased O2 consumption by 10.87%. Contrary to this, T3 increased O2 consumption by 29.41%. Reverse T3, injected together with T3, interacted with the hypermetabolic effect of T3 up to 2 hr after injection; then, O2 consumption started to increase, and was about 16.7% higher compared with the basal level 3 hr after injection. The blood plasma level of rT3 increased about 29-fold at the first hour after injection, without changes in the basal level of T3. Administration of T3 increased its level 6-fold 2 hr after injection, which was accompanied by a gradual decrease in the basal level of rT3 (3.7-fold) 4 hr after injection. Administration of rT3 + T3 increased the rT3 level 30-fold at 2 hr and the T3 level 1.7-fold at the first hour after injection. Thus, rT3 acts hypometabolically and interacts with the hypermetabolic effect of T3; administration of T3 lowered the basal level of rT3; and the plasma level of T3 did not change after administration of rT3.