Energy turnover and heat exchange in mature lean and obese Zucker rats acutely exposed to three environmental temperatures for 24 hours

Differences between lean (FA/?, n = 6) and obese (fa/fa, n = 6) mature male Zucker rats' energy turnover and heat storage were compared during a 24-h period when the animals were exposed to ambient temperatures of 30, 15 or 5 degrees C. Energy turnover was examined through measurements of heat production rates via indirect calorimetry and heat loss rates via direct calorimetry. Heat storage rates were calculated as the difference between heat production and heat loss rates. Predicted heat storage rates were also calculated as the product of the change in core temperature and the calculated specific heat of the animal based on body composition (carcass) analysis. A minimal heat loss rate was determined for each animal representing a period of least activity. Various comparisons were made: between groups (lean/obese), temperature (30, 15, 5 degrees C), calorimetry method (indirect/direct), period (light/dark), heat storage (experimental/predicted), and minimal heat loss. Immediately before a test, pretest weight and colonic temperature were obtained. Then, the animal was placed into the calorimeter chamber and remained there unrestrained for 24 h. Normal light/dark periods were maintained. On removal from the calorimeter, core temperature and body weight measurements were again obtained. Upon completion of all tests, body composition was analyzed and surface area determined. Energy turnover, i.e. both heat production and heat loss in the lean and obese animals differed among the 30, 15 and 5 degrees C exposures. The obese animals had relatively greater heat production rate and heat loss rate (kcal/day or kcal/kg (FFM)/day than the lean animals at 30, 15 and 5 degrees C. But, on a relative basis, the increments in heat production in the cold environments were greater for the lean animals. Both the lean and obese animals tended to be more active during the dark period when at 30 degrees C, but the difference was less at 15 degrees C and even less at 5 degrees C. Experimental heat storage rates did not differ significantly from predicted values at any of the temperatures with the possible exception of the animals at 5 degrees C. It was concluded that the mature obese Zucker rats had no major discernible defect in thermoregulation as revealed by rates of heat production and loss, although three of the obese rats did elicit a drop in colonic temperature during exposure to 5 degrees C, i.e. their excessive subcutaneous adiposity and thermal insulation did not prevent a fall in colonic temperature.(ABSTRACT TRUNCATED AT 400 WORDS)