Suppression of the daily rhythm in tyrosine transaminase activity by acute elevation of norepinephrine.

Abstract

THE enzyme tyrosine transaminase has a 24 h rhythm of activity in the rat liver which varies over a four-fold range each day1. This periodicity persists in the absence of pituitary or adrenal glands2–4, indicating that the driving oscillation of the rhythm does not reside in the pituitary-adrenal axis. Recent work in this laboratory5 has shown that depletion of tissue norepinephrine is associated with a three-fold increase in hepatic tyrosine transaminase activity in the fasted, adrenalectomized rat. This rise in enzyme activity was abolished by repletion of norepinephrine. These findings suggest that the neurohumour norepinephrine is involved in the generation of the daily rhythm in the activity of this enzyme. The results presented here show that a rise in tissue norepinephrine, either by inhibition of degradation or by increase in synthesis, is associated with a suppression of the diurnal rhythm of the rat hepatic enzyme.