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 (T-3) by isolated trout hepatocyt es was characterized after 40 sec incubation of cells in a balanced sa lts medium containing [(125)]T-3, and compared to L-thyroxine (T-4) up take (Riley and Eales, Gen. Comp. Endocrinol. 90, 31-42, 1993). T-3 up take resembled T-4 uptake in several ways. There was a small (<10%) di ffusion component. The balance of the uptake was temperature- and ener gy-dependent and involved a protein carrier, but did not depend on the presence of Na+ in the medium or Na+ transport. Tyrosine and phenylal anine were ineffective competitors. Inhibition by cholchicine and chlo roquine indicated an endocytotic process. T-3 uptake differed from T-4 uptake in having a higher pH optimum (6-8) than T-4 (5-6), and in hav ing a much lower K-t (0.074 mu M) than T, (0.52 mu M). T-3 uptake was far more strongly inhibited than T-4 uptake by TRIPROP (8% of control uptake), reverse T-3 (9%), and 3,3'-diiodo-L-thyronine (9%). T-4 inhib ited T-3 transport (K-i = 0.18 mu M), but kinetic analyses indicated t hat the mutual inhibitions were noncompetitive, suggesting separate T- 3 and T-4 binding sites. In conclusion, T-3 uptake into isolated trout hepatocytes resembles that for T-4 uptake in being an energy-dependen t, carrier-mediated endocytotic process, but differs from T-4 uptake i n having a lower K-t, a higher pH optimum, and a greater sensitivity t o inhibition by related iodothyronines. T-3 and T-4 uptakes may involv e separate carrier systems, providing scope for individual control of T-3 and T-4 uptake by trout hepatocytes. (C) 1994 Academic Press, Inc.