Characterization of 3,5,3'-triiodo-L-thyronine transport into hepatocytes isolated from juvenile rainbow trout (Oncorhynchus mykiss), and comparison with L-thyroxine transport.

Uptake of 3,5,3'-triiodo-L-thyronine (T3) by isolated trout hepatocytes was characterized after 40 sec incubation of cells in a balanced salts medium containing [125]T3, and compared to L-thyroxine (T4) uptake (Riley and Eales, Gen. Comp. Endocrinol. 90, 31-42, 1993). T3 uptake resembled T4 uptake in several ways. There was a small (< 10%) diffusion component. The balance of the uptake was temperature- and energy-dependent and involved a protein carrier, but did not depend on the presence of Na+ in the medium or Na+ transport. Tyrosine and phenylalanine were ineffective competitors. Inhibition by colchicine and chloroquine indicated an endocytotic process. T3 uptake differed from T4 uptake in having a higher pH optimum (6-8) than T4 (5-6), and in having a much lower Kt (0.074 microM) than T4 (0.52 microM). T3 uptake was far more strongly inhibited than T4 uptake by TRIPROP (8% of control uptake), reverse T3 (9%), and 3,3'-diiodo-L-thyronine (9%). T4 inhibited T3 transport (Ki = 0.18 microM), but kinetic analyses indicated that the mutual inhibitions were noncompetitive, suggesting separate T3 and T4 binding sites. In conclusion, T3 uptake into isolated trout hepatocytes resembles that for T4 uptake in being an energy-dependent, carrier-mediated endocytotic process, but differs from T4 uptake in having a lower Kt, a higher pH optimum, and a greater sensitivity to inhibition by related iodothyronines. T3 and T4 uptakes may involve separate carrier systems, providing scope for individual control of T3 and T4 uptake by trout hepatocytes.