Circadian Assessment of Lipids in the Hyperphagic Obese Rat Compared with Lean Litter-Mates

Time and feeding influences on cholesterol, triglyceride, glucose and insulin levels, and serum cholinesterase activity were assessed in a genetically-hyperlipidemic hyperphagic obese rat model, and compared with its lean litter-mate. Following a 28-day acclimation to a 12-hr light/dark cycle, blood samples were obtained every 2 hr from rats via tail bleed for a 24-hr period. Synchronization with other animal studies was established by endogenous serum Cortisol levels acrophase 18-20 hr after light onset (HALO) in both groups]. Triglycerides cholesterol, insulin and glucose levels were significantly elevated in obese versus lean rats. Obese rats were observed to feed throughout the 24-hr cycle, whereas lean litter-mates ate only during the dark cycle. No circadian rhythmicity was found in glucose levels with either rat group. Insulin levels were not correlated. Although triglyceride levels peaks at 13 HALO in lean rats, no pattern was observed in obese rats. Cholesterol levels were unchanged with time in either group. Cholinesterase activity followed a circadian rhythm in the lean, but not obese, rats with an acrophase estimated at 8 HALO. In contrast to previous reports, enzyme activity was not correlated with triglyceride levels in either rat group. Circadian similarities in insulin levels between rat groups suggest changes in insulin metabolism and/or secretion which are likely to be independent of feeding or activity. Conversely, triglyceride levels remained elevated throughout the 24-hr period in obese rats, whereas significant increases were observed in lean rats during the dark active cycle. These data suggest that triglyceride levels, and not insulin and cholesterol levels, are most likely dependent on feeding patterns.