Biliary Excretion of Iodothyronines in Rats as Determined by High Pressure Liquid Chromatography:Effect of Starvation*

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Abstract
Iodothyronines were measured in bile by reverse phase ion pairing high pressure liquid chromatography. After the iv injection into rats of 50–100 μCi (0.3 μg) [125T]T4, bile was collected for up to 6 h. Five radiolabeled peaks were observed: 1) free iodide, 2) T3 conjugates, 3) T4 conjugates, 4) T4, and 5) tetraiodothyroacetic acid (tetrac). Tetrac only appeared in bile during the first hour after T4 injection. Injection of T4 of a higher specific activity did not lead to the appearance of free tetrac in bile. Equilibrium, in terms of the percentage distribution of the labeled products, was reached 2 h after T4 injection. As determined by enzymatic hydrolysis with sulfatase and beta;-glucuronidase, iodothyronine conjugates were almost entirely in the glucuronide form, with minimal quantities of sulfate. After deconjugation, complete separation of radiolabeled T3, rT3, and T4 was achieved; 3,3′-diiodothyronine (3,3′-T2) and 3′5′-T2 eluted together, but separately from all other iodothyronines. Analysis of deconjugated l25I-labeled iodothyronines in bile collected between 2–.5 h after [125I]T4 injection into fed animals revealed a biliary T3 to rT3 ratio of 2.59 ± 0.34 (mean ± SE). In rats starved for 3 days, the ratio declined to 1.21 ± 0.19 (P < 0.001). When the rats were injected with T4 (2 μ/00 g BWday) for 3 days before as well as during the 3-day fast, the biliary T3 to rT3 ratio was 2.57 ± 0.18. In fed animals similarly injected with T4, the T3 to rT3 ratio (2.37 ± 0.54) was not significantly different. The biliary T2 to rT3 ratio (3,3′-T2 and 3′,5′-T2) was lower in the fasted (1.91 ± 0.32) than in the fed (3.38 ± 0.33; P1< 0.005) rats; however, T4 administration did not abolish this difference (fasted, 1.40 plusmn; 0.39; fed, 3.30 ± 0.34; P < 0.001). These data provide support for a decrease in 5′-deiodinase activity in starvation and indicate that T4 treatment of starved animals does not completely reverse this alteration.