Nocturnal Increase of Type II Thyroxine 5′-Deiodinase Activity in the Syrian Hamster Harderian Gland is Abolished by Light Exposure and Induced by Isoproterenol

The presence of type II 5′-deiodinase activity in the Syrian hamster Harderian gland was investigated. This enzyme exhibited an increase of its activity after animals entered the normal dark phase, with maximal activity occurring at 04.00 hr (8 hr after lights off). The nocturnal increase was prevented by maintaining the animals in light during the night. Isoproterenol subcutaneously injected every 2 hr (1.0 mg/kg body wt) from 20.00 hr through 0.400 hr to animals exposed to light during the normal dark period mimicked the effect of darkness, i.e., with this treatment an increase in 5′-deiodinase activity with maximal peak values at 02.00 hr was observed. The results show that 5′-deiodinase activity in the Syrian hamster Harderian gland exhibits a nyctohemeral profile dependent on β-adrenergic activation of the gland.