Effect of 3,5,3'-triiodothyronine and 3,3',5'-triiodothyronine administration on serum tri-, di-, and monoiodothyronines and plasma cyclic nucleotides in sheep.

Abstract

T3 and/or rT3 were administered to sheep in order 1) to determine their influence on plasma cyclic nucleotides and 2) to examine their effects on serum T4, T3, rT3, 3′,5′-diiodothyronine (3′,5′T2), 3,3′T2, and 3′-monoiodothyronine (3′T1). The animals were divided into five groups, and each group received one of the following treatments: 1) placebo, 2) low T3 (1.5 μg/kg), 3) high T3 (4.5 μg/kg), 4) high rT3 (4 μg/kg), or 5) low rT3 (2.5 μ/kg) plus low T3 (1.5 μ/kg) in combination. Treatments were administered every 8 h for 6 days, and the animals were studied throughout this period. Cyclic nucleotides and iodothyronines were measured by RIA. High dose T3 treatment increased plasma cAMP by 52% (P < 0.02), whereas plasma cGMP was unaffected. Other treatments did not alter plasma cyclic nucleotides. Both low and high dose T3 treatment decreased T4 and rT3 levels, whereas rT3 treatment did not alter levels of serum T4 or T3. Serum 3,3′T2 concentrations were increased by rT3 and T3 treatment, both singly and in...