Transcellular and transnuclear transport of 3,5,3'-triiodothyronine in isolated hepatocytes

Abstract

We have recently reported evidence for the presence of stereospecific energy-dependent transport processes for T3 in rat tissues. These processes were responsible for maintenance of concentration differences of free L- and D-T3 across the cellular plasma and nuclear membranes. In rat liver, the free L-T3 concentration in cytosol was almost 3 times higher than that in plasma, and nuclear free L-T3 was 58-fold that in cytosol. In the present studies, freshly isolated hepatocytes were used to study these processes in vitro. Kinetic experiments demonstrated that equilibrium of [125I]T3 between cells and medium was rapid and complete within 5 min. Neither the rate of cellular accumulation nor the equilibrium distribution of T3 between cells and medium was influenced by the addition of up to 2 X 10(-7) M T3. Equilibrium of T3 between the nuclear and extranuclear fractions of the hepatocytes was reached more slowly, only after 45-60 min of incubation. The nuclear free T3 concentration was calculated from mass action principles with knowledge of the association constant (Ka) of the nuclear T3-binding sites under in vitro conditions and the fractional occupancy of the sites. Cytosolic free T3 was determined from measurements of the fraction of cellular [125I]T3 associated with cytosol (pc), and the binding power of cytosol was determined by equilibrium dialysis (bc). The cytosol to plasma free T3 ratio in these cells was near unity, suggesting an absence of the concentration difference previously observed in liver in situ. The nuclear to cytosol free T3 ratio was 7.9, approximately 7 times less than that in vivo. The addition of 2 mM KCN caused a further 23% reduction in the nuclear to cytosol ratio. As previously reported for liver in situ, uptake of T3 by hepatocytes is stereospecific. Cellular uptake of D-T3 was greater than that for L-T3. However, nuclear transport favored L-T3. The nuclear to cell ratio for L-T3 was almost 4 times greater than that for D-T3 (mean +/- SEM, 0.020 +/- 0.0005 vs. 0.0085 +/- 0.0005; P less than 0.001). Our studies indicate the presence in the isolated hepatocyte of a nuclear transport process for T3 similar to that observed in vivo, but operating with a markedly reduced efficiency.