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  • Title: 3,5,3'-Triiodo-L-thyronine and L-thyroxine uptake into red blood cells of rainbow trout, Oncorhynchus mykiss.
    Author: McLeese JM, Eales JG.
    Journal: Gen Comp Endocrinol; 1996 Apr; 102(1):47-55. PubMed ID: 8860308.
    Abstract:
    Uptake of the thyroid hormones (TH) 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) by trout red blood cells (RBC) was studied by incubating washed RBC in a balanced salts medium containing glucose and [125I]TH at the fish acclimation temperature of 12 degrees. RBC were separated from the medium by centrifugation through silicone oil and glycine buffer (pH 10.5). Maximal [125I]T3 uptake occurred by 10-15 min, but not by 60 min for [125I]T4. First-order uptake was measured at 30 sec for T3 and at 90 sec for T4. Total T4 uptake was enhanced 15-fold from pH 8 to 6 and was affected most below pH 7.2; total T3 uptake was maximal between pH 6.4 and 7.0, but was relatively insensitive to pH. At 0.2 nM, nonsaturable uptake of T3 exceeded that of T4 3- to 6-fold, accounting for 3% (T3) and 50% (T4) of total uptake. Saturable TH uptake was described by Michaelis-Menten kinetics. The saturable transport system for T3 had an apparent K(t) (carrier affinity) of 70-119 nM and J(max) (maximal uptake velocity) of 540-1116 pmol . 10(6) cells(-1) . min(-1). A saturable system was also found for T4, with an apparent K(t) of 99 pM-1.1 nM and J(max) of 8-77 fmol . 10(6) cells(-1) . min(-1). Saturable uptake of both TH depended on temperature. Activation energies for the nonsaturable component were 48 (T4) and 64 (T3) KJ . mol(-1) over the range 0-21 degrees. Activation energies for the saturable components were 52 KJ . mol(-1) (T4, 0-21 degrees), 52 KJ . mol(-1) (T3, 0-10 degrees), and 3 KJ . mol(-1) (T3, 10-21 degrees). During a 16-month study saturable and nonsaturable uptake of both TH increased, probably due to fish age. We conclude that in trout RBC, rapid T3 uptake by a pH- and temperature-sensitive saturable carrier greatly exceeds T4 uptake. The rate of T3 uptake exceeds by 100- to 1000-fold that of mammals and amphibia, and in contrast to those taxa some saturable T4 uptake also occurs.
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