Characteristics of the uptake of 3,5,3'-triiodo-L-thyronine and L-thyroxine into red blood cells of rainbow trout (Oncorhynchus mykiss).
The mechanisms of uptake of thyroid hormones (TH) 3,5,3'-triiodo-L-thyronine (T3) and L-thyroxine (T4) by trout red blood cells (RBC) were characterized by coincubating washed RBC with [125I]T3 or [125I]T4 and a variety of inhibitors and potentially competing amino acids and TH analogues. Nonsaturable uptake of both TH was unaffected by almost all agents tested, consistent with a diffusion process. Saturable T3 uptake was unaffected by a major (80%) cyanide- and iodoacetate-induced reduction in RBC nucleotide triphosphate content or by inhibitors of Na+ transport, but was depressed by (i) inhibitors of protein binding (bromosulfophthalein, 8-anilino-1-naphthalene sulfonic acid, phloretin, and 5,5'-diphenylhydantoin, (ii) sulfhydryl blockers (p-hydroxy-mercuribenzoate and N-ethylmaleimide), (iii) endocytotic inhibitors (chloroquine, colchicine, and monodansylcadaverine), (iv) tryptophan and phenylalanine, and (v) certain TH analogues, particularly 3,3',5'-triiodo-L-thyronine (reverse T3). In contrast, saturable T4 uptake was depressed only by inhibitors or protein binding. Cross-competition between T3 and T4 for transport occurred only at highly pharmacologic TH levels. We conclude that T3 and T4 are transported into trout RBC by two separate systems. The relatively slow T4 uptake depends on binding to certain proteins. The much more rapid T3 uptake depends on protein binding, but is Na(+)-independent, and involves a system comparable to the T-system of aromatic amino acid transport. However, participation od receptor-mediated endocytosis cannot be excluded.