Rhythmicity of Triiodothyronine Generation by Type II Thyroxine 5′-Deiodinase in Rat Pineal Is Mediated by a β-Adrenergic Mechanism*

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Abstract
Type II T45''-deiodinase plays an essential role in converting T4 to T3 in etrathyroidal tissues, thus allowing the full development of a celllular 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 .alpha.-adrenergic blocker, propranolol. The .beta.-agonist isoproterenol, caused 3-fold increase in diurnal enzyme activity. There is thus a marked nyctohemeral variation in T4 activation in the pineal probably mediated by .alpha.-adrenergic mechanism. Since the temporal peak of pineal 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.
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