The occurrence and regulation by thyroid hormone of four protein kinases (cyclic AMP independent and dependent, calcium/calmodulin stimulated, and calcium/phosphatidyl serine stimulated protein kinases) was studied in primary cultures of cells dissociated from embryonic mouse brain. Serum from a thyroidectomized calf, which contained low levels of L-3,5,3′-triiodothyronine, T3 (<25 ng/100 ml), and thyroxine, T4 (<1 μg/100 ml) was used in the culture medium in place of normal calf-serum (T3, 130 ng/100 ml; T4 5.9 μg/100 ml) to render the cultures responsive to exogenously added T3. Cultures grown in hypothyroid calf-serum containing medium had less cAMP dependent and independent protein kinase activity than control cultures grown in normal calf-serum containing medium. However, this activity was restorable to a considerable degree if the cultures grown in hypothyroid calf serum containing medium were supplemented with L-3,5,3′-triiodothyronine (T3). The presence of calcium/calmodulin stimulated protein kinase was also distinctly observed. In comparison, the activity of calcium/phosphatidyl serine stimulated protein kinase was less than the other protein kinases.

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