Effect of propylthiouracil and thyroxine on the inactivation of somatostatin by rat hypothalamus

The inactivation of somatostatin by hypothalamic and brain extracts were studied in rats treated with propylthiouracil (PTU) alone or with PTU and thyroxine. 90 days after the onset of treatment with PTU, the potency of hypothalamic extract to inactivate somatostatin was increased almost 2-fold; while no change was observed for brain extract. Thyroxine reversed the enhancing effect of PTU on the activity of the enzyme(s) degrading somatostatin. A pH study shows that both brain and hypothalamic enzymic systems are different. These data suggest that the inactivation process of somatostatin by hypothalamic extract could be an important factor in the regulation of the hypothalamic-hypophyseal thyroid axis in rat.