3,5,3'-TRIIODO-L-THYRONINE AND L-THYROXINE UPTAKE INTO RED-BLOOD-CELLS OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Uptake of the thyroid hormones (TH) 3,5,3'-triiodo-L-thyronine (T-3) a nd L-thyroxine (T-4) by trout red blood cells (RBC) was studied by inc ubating washed RBC in a balanced salts medium containing glucose and [ I-125]TH at the fish acclimation temperature of 12 degrees. RBC were s eparated from the medium by centrifugation through silicone oil and gl ycine buffer (pH 10.5). Maximal [I-125]T-3 uptake occurred by 10-15 mi n, but not by 60 min for [I-125]T-4. First-order uptake was measured a t 30 sec for T-3 and at 90 sec for T-4 Total. T-4 uptake was enhanced approximate to 15-fold from pH 8 to 6 and was affected most below pH 7 .2; total T-3 uptake was maximal between pH 6.4 and 7.0, but was relat ively insensitive to pH. At 0.2 nM, nonsaturable uptake of T-3 exceede d that of T-4 3- to 6-fold, accounting for 3% (T-3) and 50% (T-4) of t otal uptake. Saturable TH uptake was described by Michaelis-Menten kin etics. The saturable transport system for T-3 had an apparent K-t (car rier affinity) of 70-119 nM and J(max) (maximal uptake velocity) of 54 0-1116 pmol . 10(6) cells(-1) . min(-1). A saturable system was also f ound for T-4, 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 nonsaturabIe component we re 48 (T-4) and 64 (T-3) KJ . mol(-1) over the range 0-21 degrees. Act ivation energies for the saturable components were 52 KJ . mol(-1) (T- 4, 0-21 degrees), 52 KJ . mol(-1) (T-3, 0-10 degrees), and 3 KJ . mol( -1) (T-3, 10-21 degrees). During a 16-month study saturable and nonsat urable uptake of both TH increased, probably due to fish age. We concl ude that in trout RBC, rapid T-3 uptake by a pH- and temperature-sensi tive saturable carrier greatly exceeds T-4 uptake. The rate of T-3 upt ake exceeds by 100- to 1000-fold that of mammals and amphibia, and in contrast to those taxa some saturable T-4 uptake also occurs. (C) 1996 Academic Press, Inc.