Effect of endotoxin challenge on hepatic 5'-deiodinase activity in cattle

Thyroid status is compromised in a variety of acute and chronic infections and toxin-mediated disease states. Conversion of thyroxine (T-4) into the m etabolically active hormone, triiodothyronine (T-3), is catalyzed by 5'-dei odinase (5'D). Our objective was to determine the effect of endotoxin (LPS) challenge with and without L-arginine (Arg) infusion on hepatic activity o f 5'D and plasma concentrations of T-4 and T-3. In a 2 x 2 factorial, beef heifers (275-310 kg b.wt.) were fed low (8% CP; 6.5 kg/d) or high (14% CP; 7.2 kg/d) isocaloric protein diets (1.96 Mcal/kg DM) for 10 d before LPS ch allenge. L-Arginine in saline (0.5 g/kg b.wt.) or saline alone was infused iv throughout an 8 hr period starting 2 kr before bolus LPS injection (Esch erichia coli, 055:B5; 0.2 mu g/kg; iv). Blood samples were collected at -2, 0, 3, 6, 12, and 24 hr relative to LPS injection. Liver samples were obtai ned 20 hr before, and then 6 and 24 hr after LPS challenge using a biopsy n eedle. Plasma T-4 and T-3 concentrations were not affected by dietary CP or Arg. Compared with levels at 0 hr, LPS challenge decreased plasma T-4 (P < 0.01) and T-3 (P < 0.001), respectively, 8.4% and 28.9% at 6 hr and 19.7% and 31.3% at 24 hr. Consistent with these changes, the T-3:T-4 ratio was lo wer than that at 0 hr (P < 0.001) 22.0% at 6 hr and 13.5% at 24 hr. Hepatic 5'D activities 20 hr before LPS injection were 2.80 +/- 0.11 nmol I-. hr(- 1). mg protein(-1) and decreased 24 hr after LPS, respectively, 45.4% (P < 0.01) and 17.6% (P < 0.05) in saline- and Arg-infused heifers. The results indicate that mild LPS challenge in cattle inhibits hepatic generation of T -3 and decreases plasma concentrations of thyroid hormones. The data also s uggest that the impact of LPS on 5'D activity in liver can be altered by Ar g supplementation. (C) 2000 Elsevier Science Inc. All rights reserved.