Measurement and seasonal variations of black bear adipose lipoprotein lipase activity

The black bear (Ursus americanus) provides a unique model for the study of adipose physiology because it exhibits seasonal periods of rapid weight gain and weight loss without marked changes in its metabolic rate. To better understand fat cycling in this model, we obtained plasma and gluteal adipose tissue from five captive adult bears at approximately 20-day intervals from October 1 1992 through March 31 1993. The study included a predenning and denning period for each animal. Sampling during the predenning period followed a 12-h fast. Bears were anorectic while denning. Adipose LPL activities and plasma insulin concentrations were determined for each time point. Predenning LPL activities (4.83 ± 0.64 μmol/h/g) were significantly greater than those seen during the denning period (1.82 ± 0.65, p < 0.001). A biphasic pattern of fasting LPL activity was seen in four of the five bears during the predenning period. Fasting insulin concentrations showed no such pattern of variation during the study period (mean = 25.1 ± 1.36 pmol/l; range 1.1-6.0). We found no evidence of a linear relationship between LPL activity and insulin levels (p = 0.139). Neither LPL activity nor insulin concentrations were related to changes in weight (p = 0.257 and p = 0.7104, respecitively). We conclude that LPL activity can be measured in black bear adipose tissue and that fall (predenning) activities are significantly higher than those seen during the winter (denning period). Furthermore, the seasonal regulation of LPL involves some factor(s) in addition to insulin.