Circadian-time dependent differences of protein kinase C (PKC) activit
ies as well as its susceptibility to a phenobarbital caused suppressio
n in vivo (40 mg/kg b.i.d. for 3 days) were studied in outbred male Wi
star rats weighing 280-360 g. The well-known phenobarbital induction o
f cytochrome P-450 and 7-pentylresorufin-O-depentylase (PROD) was asso
ciated with inhibition of the hepatic PKC. The activities of the cytos
olic and membrane-bound PKC were significantly lower than in the respe
ctive controls. Statistically significant circadian-time differences w
ere found. The activities of both cytosolic and membrane-bound PKC of
control rats were highest in the evening (158% and 131%, respectively,
of the morning values). Furthermore, phenobarbital inhibited the enzy
me more strongly at 19.00 h than at 07.00 h. Our results suggest that
increase of hepatic cytochrome P-450 content as well as induction of P
ROD are inversely related with the function of PKC.