Rat pineal N-acetyltransferase activity: stimulation, exhaustion, and recovery.

Rat pineal N-acetyltransferase activity (NAT) has a circadian rhythm, with a peak in the dark time. The rhythm of NAT was studied in young rats (26-40 days old). In particular, the possibility of maintenance of prolonged low NAT or prolonged high NAT was studied. NAT (given in nanomoles per pineal gland/h) had peak activity (33.0) in the dark time. Forty-eight hours of constant light suppressed the peak and maintained low NAT (0.5 or less). In vivo, in constant light, NAT was stimulated (to 13.1-29.4) by single injections of isoproterenol. Similarly, NAT was stimulated by isoproterenol or (Bu)2cAMP in vitro. In vivo, multiple injections of isoproterenol stimulated NAT for 14 h (to peak 31.9), but NAT was exhausted and fell to less than 5 over the next 8 h. Similarly, isoproterenol and (Bu)2cAMP exhausted NAT in vitro. High NAT was not maintained. NAT recovered [could be restimulated by dark in vivo, isoproterenol in vivo or in vitro, or (Bu)2cAMP in vitro]. Exhaustion was not permanent. Light can keep NAT at low levels, but dark cannot maintain peak NAT beyond 14 h. The results are interpreted as meaning that the duration of NAT is fixed.