Thyroxine 5'‐deiodinase type II activity in chick pineal and Harderian gland: Nyctohemeral rhythmicity and its regulation by noradrenergic input

摘要

Abstract:Circadian rhythmicity of type II thyroxine 5′‐deiodinase (5′‐D) activity was studied in the pineal gland and Harderian glands of chicks. Only Harderian 5′‐D activity showed a nyctohemeral rhythmicity with a maximal peak during the day time (1300), while no rhythm of enzyme activity was found in the pineal gland. Besides type II 5′‐D activity, we found high basal levels of the type I isoenzyme in both glands; this activity was specifically suppressed by the addition of 6‐n‐propyl‐thiouracil (PTU). However, day‐night differences in Harderian 5′‐D activity were maintained even after the addition of PTU. This activity was not affected for either continuous light exposure or darkness during the day. 5′‐D activity seems to be regulated by the noradrenergic input, since the enzymatic activity was stimulated by a β‐adrenergic agonist, isoproterenol, and by the α‐adrenergic agonist, phenylephrine, in both pineal and Harderian glands. Both drugs affected 5′‐D activity in the Harderian gland by stimulat