DIETARY FATTY-ACIDS MODULATE HORMONE RESPONSES IN LACTATING COWS - MECHANISTIC ROLE FOR 5'-DEIODINASE ACTIVITY IN TISSUE

Supplemental dietary fat provides excess fatty acids (FA), which can a lter circulating concentrations of several hormones. To test the effec ts of fatty acid isomer type and possible sites of regulation, we abom asally infused fat mixtures high in cis-C-18:1 FA (iCIS), high in tran s-C-18:1 FA (iTRS) or no infusion (NI) and performed intravenous argin ine (ARG) and intramuscular thyrotropin-releasing hormone (TRH) challe nges. The experimental design was a replicated 3 X 3 Latin square. Cha llenges were conducted on Days 10 (ARG) and 12 (TRH) after initiation of fat infusion on each of three 4-wk experimental periods. Plasma con centrations of IGF-I were lower (P < 0.01) when cows received iCIS or iTRS compared with NI. Plasma insulin concentrations increased with AR G but responses were not affected by FA. Plasma growth hormone (GH) wa s unchanged after ARG. Peak plasma GH and thyroid-stimulating hormone (TSH) responses to TRH were blunted (P < 0.05 and P < 0.1, respectivel y), whereas thyroxine (T-4) and triiodothyronine (T-3) responses were augmented post-TRH (P < 0.01) when cows received either FA isomer. Pro lactin responses to TRH were not different between infusion treatments , although basal plasma concentrations before TRH were higher in cows infused with iTRS (P < 0.05). To focus on fat regulation of the thyroi d axis, we tested directly in vitro the ability of fatty acids dissolv ed with sodium taurocholate to affect Type-I 5'-deiodinase (5'D) activ ity in bovine liver homogenates. Homogenate 5'D was not affected by C- 2:0-C-10:0 fatty acids, but decreased linearly (P < 0.01) with increas ing concentrations of C-12:0-C-16:0 and C-18:1 isomers. Cis C-18:1 dec reased 5'D more than the trans-isomer (P < 0.01), but the difference w as only apparent at concentrations greater than 0.25 mM. The data sugg est that various aspects of pituitary hormone regulation are different ially affected by FA composition. Fatty acid infusion may accentuate e nd organ responses in the thyroid axis and decrease IGF-I in the somat otropic axis. The data also suggest that FA isomer may alter patterns of extrathyroidal generation of thyroid hormones via direct influences on 5'D. (C) Elsevier Science Inc. 1997.