THYROXINE 5'-DEIODINASE TYPE-II ACTIVITY IN CHICK PINEAL AND HARDERIAN-GLAND - NYCTOHEMERAL RHYTHMICITY AND ITS REGULATION BY NORADRENERGICINPUT

Circadian rhythmicity of type II thyroxine 5'-deiodinase (5'-D) activi ty was studied in the pineal gland and Harderian glands of chicks. Onl y Harderian 5'-D activity showed a nyctohemeral rhythmicity with a max imal peak during the day time (1300), while no rhythm of enzyme activi ty was found in the pineal gland. Besides type II 5'-D activity, we fo und high basal levels of the type I isoenzyme in both glands; this act ivity was specifically suppressed by the addition of 6-n-propyl-thiour acil (PTU). However, day-night differences in Harderian 5'-D activity were maintained even after the addition of PTU. This activity was not affected tor either continuous light exposure or darkness during the d ay. 5'-D activity seems to be regulated by the noradrenergic input, si nce the enzymatic activity was stimulated by a beta-adrenergic agonist , isoproterenol, and by the alpha-adrenergic agonist, phenylephrine, i n both pineal and Harderian glands. Both drugs affected 5'-D activity in the Harderian gland by stimulating the enzyme activity over basal l evels.