Food and light as separate entrainment signals for rat liver enzymes

The rat liver enzymes, tyrosine aminotransferase and ornithine decarboxylase, and to a lesser extent glucokinase, pyruvate kinase and glucose-6-phosphate dehydrogenase, undergo diurnal oscillations when rats are entrained to a daily schedule of 12 hr of darkness and either 2 or 8 hr of access to food during the dark period. These enzymes increase in activity after feeding, but the patterns of activity versus time after feeding differ for rats with food available for 2 hr in the middle of the dark period, compared with the patterns for rats with food available for 2 or 8 hr at the beginning of the dark period. Therefore, induction of these enzymes involves interaction between dietary factors and hormonal factors related to light and dark periods. It appears that both the onset of darkness and the beginning of the light period are signals that interact with dietary and hormonal factors. While no single protocol can be optimal for all purposes, it is suggested that the 2 + 22 protocols in trained animals with other variables controlled may be more useful than other protocols, since in contrast to the 8 + 16 or ad libitum regimens, the animals receive their food during an interval of time that can be specified.