Thyroid hormone metabolism by glial cells in primary culture

The metabolism of thyroxine (T4) and triiodothyronine (T3) in cultured glial cells was studied in situ. Cultures were prepared from fetal rat brain and grown for the last 4 days in a chemically defined medium (CDM). They contained astrocytes and oligodendrocytes as shown by the enzyme markers, glutamine synthetase and 2',3'-cyclic nucleotide phosphohydrolase. These cells contained high affinity (22-33 pM), limited capacity (120-230 fmol/mg DNA) nuclear receptors for T3. Cells incubated in situ with 50 pM [125I]T4 actively metabolized the hormone. The major iodothyronine produced was T3 (220-570 fmol/4 h/mg DNA). About 70% accumulated in the cells, the remainder was released into the medium. Within the cells, T3 was partly bound to the nuclear receptors (16.5-20 fmol/mg DNA). Reverse T3 (rT3) was a minor metabolite (30-45 fmol/4 h/mg DNA); it was almost completely released into the medium. The half-life of [125I]T3 (50 pM) was found to be about 15 h. These results show that, in situ, glial cell cultures containing astrocytes and oligodendrocytes grown in CDM actively deiodinate T4 to T3 and degrade T3 rather slowly.