Circadian rhythms of body temperature and liver function in fed and food-deprived goats.

Daily rhythms of body core temperature and liver function were recorded in goats maintained under various schedules of lighting and feeding. Concentration of urea in the blood was used as an index of digestion-driven hepatic activity, whereas concentration of cholesterol served as an index of autonomous hepatic activity. Body temperature exhibited robust circadian rhythmicity in the presence and absence of a light-dark cycle and/or a feeding regime. The rhythm was more responsive to shifts in feeding time than to shifts in the light-dark cycle. Urea concentration in the blood exhibited daily rhythmicity only in the presence of a daily feeding regime and, therefore, was driven by ingestive and digestive processes. The rhythm of cholesterol concentration persisted in the presence or absence of a light-dark cycle and/or a feeding regime, except when the feeding time was shifted under constant light. However, the cholesterol rhythm did not respond either to shifts in the light-dark cycle or, more importantly, to shifts in feeding time. Thus, based on this index of hepatic function, the liver cannot be identified as the site of the putative food-entrainable pacemaker.