Rhythmicity of triiodothyronine generation by type II thyroxine 5'-deiodinase in rat pineal is mediated by a beta-adrenergic mechanism.

Type II T4 5'-deiodinase plays an essential role in converting T4 to T3 in extrathyroidal tissues, thus allowing the full development of a cellular effect of thyroid hormone. This enzyme is one of the most important factors in regulating local action of thyroid hormone. Its activity in the rat pineal was 20-30 times higher at midnight than at noon; this nocturnal rise was abolished by the beta-adrenergic blocker, propranolol. The beta-agonist isoproterenol, caused a 3-fold increase in diurnal enzyme activity. There is thus a marked nyctohemeral variation in T4 activation in the pineal, probably mediated by a beta-adrenergic mechanism. Since the temporal peak of pineal T4 activation corresponds to that of pineal function, our data suggest a previously unrecognized role of thyroid hormone in pineal regulation.