Differential responses of rat pineal thyroxine type II 5′-deiodinase and N-acetyltransferase activities to either light exposure, isoproterenol, phenylephrine, or propranolol

1. Compared to pineal N-acetyl transferase (NAT) activity, which exhibited a dramatic drop following acute light exposure at night, nocturnal rat pineal thyroxine type II 5′-deiodinase (5′-D) activity was minimally influenced by the same light exposure. The injection of cycloheximide, a potent inhibitor of protein synthesis, although it did curtail the rise in NAT activity for at least 2 hr, did not elicit decreases in the activities of either 5′-D or NAT enzymes. Propranolol, a β-adrenergic blocker, either delayed the continued nocturnal rise in 5′-D activity when injected at 0000 hr or slightly enhanced the fall in 5′-D activity when injected at 0200 hr. These results suggest that interruption of the synthesis of proteins is responsible for the slow deterioration of 5′-D activity induced by either light or propranolol. 2. The slight fall in 5′-D activity induced by light at night was prevented by isoproterenol; phenylephrine, however, did not prevent the fall and the effect of isoproterenol + phenylephrine was similar to that obtained with isoproterenol alone. On the other hand, the light-inhibited NAT activity recovered after the injection of isoproterenol; phenylephrine did not elicit any effect, but the injection of both isoproterenol and phenylephrine simultaneously caused a greater NAT response than that induced by isoproterenol alone. 3. When injected during the day, phenylephrine had no effect on either pineal 5′-D or NAT activities; however, the injection of either isoproterenol alone or isoproterenol + phenylephrine elicited 5-fold and 10-fold increases in nocturnal, light-suppressed 5′-D and NAT activities, respectively. During the day, phenylephrine did not potentiate the effects of isoproterenol on NAT activity as it did at night. When the effects of isoproterenol on the 5′-D activity were compared to rats exposed to light during the day and at night, the activity of 5′-D reached a higher level at night than during the day.