Factors influencing the steady-state distribution and exchange of thyroid hormones between red blood cells and plasma of rainbow trout, Oncorhynchus mykiss.

We studied effects of in vitro conditions on the steady-state distribution and exchange of thyroid hormones (TH) between red blood cells (RBC) and plasma of rainbow trout. At steady state at 12 degrees C the RBC contained 5-11% of L-thyroxine (T4), 14-23% of 3,5,3'-triiodo-L-thyronine (T3), and 23-24% of 3,3',5'-triiodo-L-thyronine (rT3) present in whole blood. The steady-state distribution was (i) higher in immature than in mature trout for T3, (ii) increased by incubation temperature from 0 to 22 degrees C for both T4 and T3, (iii) unaltered by blood T4 or T3 concentration (0-40 ng/ml), and (iv) increased by O2 gassing and decreased by N2 gassing for T4 and rT3, but negligibly for T3. The exchange between RBC and plasma was more rapid for T3 (50% of maximal influx or efflux at 30-40 s) than rT3 (14 min) than T4 (30 min). Efflux of T4 and T3 was greatly reduced in the absence of plasma protein. Incubation with 10% bovine serum albumin extracted > 98% of labeled T4 and T3 from RBC. We conclude that for trout (i) steady-state distribution and exchange kinetics between RBC and plasma differ greatly for T4, T3, and rT3 and vary with the in vitro conditions, (ii) almost all TH in RBC are reversibly bound to intracellular sites, (iii) efflux is strongly influenced by plasma binding sites, (iv) T3 exchange is rapid and may allow T3 access to RBC TH receptors, buffer plasma T3 levels, or aid T3 delivery to tissues, (v) T4 exchange is slow and this may prevent oxygenation state from altering T4 uptake into RBC, and (vi) rT3 uptake into RBC may contribute to low rT3 levels in trout plasma.