CHARACTERIZATION OF THE THYROID-HORMONE TRANSPORT-SYSTEM OF CEREBROCORTICAL RAT NEURONS IN PRIMARY CULTURE

The uptake of 3',3,5-triiodo-L-thyronine (T-3) and L-thyroxine (T-4) 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 K-m of T-3 uptake was very similar to that of T-4, and did not vary significantly from day 1 to 4 in culture (310-400 nM). The maximal velocity (V-max) of T-3 uptake nearly doubled between day 1 and 4 of culture (41 +/- 3 vs. 70 +/- 5 pmol/min/mg of DNA, respectively). The V-max of T-4 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 T-3 or T-4 uptakes were the same (T-3 > T-4 > 3',5',3-triiodo-L-thyronine > 3',3,5-triiodo-D-thyronine > triiodothyroacetic acid), indicating that the transport system is stereospecific. Unlabeled T-3 was a stronger competitor of labeled T-4 uptake than of labeled TB uptake, whereas unlabeled T-3 had the same potency for both processes, These results suggest that T-3 and T-4 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 T-3 uptake increases during neuronal maturation.