The relation of hepatic in vitro inactivation of corticosteroids to the circadian rhythm of plasma corticosterone.

Plasma corticosterone levels and the in vitro capacity of the liver to reduce the delta(4)3-ketone group of corticosterone were ascertained at 4 hr intervals in male rats maintained on a normal lighting schedule (12L:12D). The rates of delta(4)3-ketone reduction, as well as wet liver weight, were highest during the early portion (08.00 hr) of the light period when liver protein and plasma corticosterone concentrations were low. Shortly (2000 hr) after the beginning of the dark period plasma corticosterone reached peak levels, while hepatic inactivation of corticosterone was markedly depressed. This inverse relationship suggests that the rhythmicity in the capacity of the liver to inactivate corticosterone may contribute to the circadian periodicity of plasma corticosterone.