Characterization of the thyroid hormone transport system of cerebrocortical rat neurons in primary culture.

The uptake of 3',3,5-triiodo-L-thyronine (T3) and L-thyroxine (T4) by primary cultures derived from rat brain hemispheres was studied under initial velocity conditions, at 25 degrees C. Uptake of both hormones was carrier mediated and obeyed simple Michaelis-Menten kinetics. The Km of T3 uptake was very similar to that of T4, and did not vary significantly from day 1 to 4 in culture (310-400 nM). The maximal velocity (Vmax) of T3 uptake nearly doubled between day 1 and 4 of culture (41 +/- 3 vs. 70 +/- 5 pmol/min/mg of DNA, respectively). The Vmax of T4 uptake did not change (28 +/- 8 and 31 +/- 4 pmol/min/mg of DNA on days 1 and 4, respectively). The rank order of unlabeled thyroid hormone analogues to compete with labeled T3 or T4 uptakes were the same (T3 > T4 > 3',5',3-triodo-L-thyronine > 3',3,5-triiodo-D-thyronine > triiodothyroacetic acid), indicating that the transport system is stereospecific. Unlabeled T4 was a stronger competitor of labeled T4 uptake than of labeled T3 uptake, whereas unlabeled T3 had the same potency for both processes. These results suggest that T3 and T4 are transported either by two distinct carriers or by the same carrier bearing separate binding sites for each hormone. They also indicate that the efficiency of T3 uptake increases during neuronal maturation.