ERYTHROCYTE-ASSOCIATED TRIIODOTHYRONINE IN THE RAT - A SOURCE OF HORMONE FOR TARGET-CELLS

The accumulation of T3 by rat erythrocytes and its transfer to cultured rat hepatocytes were investigated. The amount of erythrocyte-associated T3 in whole rat blood was determined at 37°C. The ration of erythrocyte-associated T3 to plasma total T'3 was 0.235 over a wide rang of T3 concentrations, i.e. there is 25 times as much T3 in the erythrocyte compartment as free in the plasma. Influx and efflux of T3, which were shown previously to be carrier-mediated, proceeded rapidly (t 1/2 ~ 12-15 sec at 25°C). These results suggest that erythrocytes are involved in the supply of T3 to target cells. This was checked by studying [125I]T3 uptake by cultured rat hepatocytes incubated either with erythrocyte suspensions pre-equilibrated with labelled T3 or with extracellular medium from the same erythrocyte suspensions. In protein-free medium, the initial velocity ofT3 uptake was 1.5-fold faster in the presence of the erythrocyte suspension. Uptake was saturable, the apparent K(m) of T3 uptake (nmol/l) was 163±13 in the absence of erythrocytes and 102±6 in their presence. V(max) (fmol·min-1·well-1) was similar in both cases (477 ± 26 and 511 ± 20, respectively). In the presence of diluted plasma (1:16 dilution) and in the presence of the erythrocyte suspension, a 2-fold increase of initial velocity was obtained. Plasma by itself increased (4-5 times) the initial velocity of uptake. The time-courses and kinetic constants of T3 binding to hepatocyte nuclear receptors were similar whether hepatocytes were incubated with or without erythrocytes. These results show that part of the erythrocyte-associated T3 adds to the free pool of hormone and can thus be considered as a biologically active compartment, suggesting that rat erythrocytes may serve as a blood-carrier for T3.