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, 5T2), 3, 3T2, and 3-monoiodothyronine (3T1). 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, 3T2 concentrations were increased by rT3 and T3 treatment, both singly and in combination. Serum 35T2 levels were undetectable (<0.055 nM), except after rT3 administration. 3T1 levels were unchanged by treatments, except that high T3 administration slightly, but significantly, decreased serum 3T1 concentrations. These results indicate that in the sheep 1) T3 markedly increases plasma cAMP and decreases T4 and rT3 levels, whereas rT3 administration is inactive in both of these systems; 2) neither T3 nor rT3 alters plasma cGMP; 3) degradation of rT3 is directed relatively more to 3, 3T2 than to 35T2; 4) metabolism of both T3 and rT3 contribute comparably to 3, 3T2 levels; and 5) T3 may enhance the conversion of rT3 to both 3, 3T2 and 3, 5T2.
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