HIND LEG HEAT-BALANCE IN OBESE ZUCKER RATS DURING EXERCISE

To analyse the effect of obesity on exercise-derived heat dissipation, lean and obese Zucker rats were exercised on an inclined treadmill un til they would no longer run with gentle prodding. We measured their o xygen consumption, water vapour loss, the concentrations of adenosine tri-and diphosphate, creatine phosphate, and lactate in quick-frozen l eg muscles, and the temperature of muscle, skin and blood in the aorta . We determined blood flow to leg muscle, fat and skin by measuring th e entrapment of fluorescent microspheres. From the measurements we cal culated heat flow rates between hind leg muscle, blood, fat and skin a nd the environment. The obese rats weighed twice as much as the lean ( 340-400 g and 175-200 g respectively) and ran half as fast (113 +/- 7 m versus 257 +/- 17 m). The differences between the two groups for bas al oxygen consumption (lean: 6.7 +/- 0.9 mu mol/min, obese: 5.0 +/- 1. 9 mu mol/min) and exercising oxygen consumption (lean: 37.8 +/- 5.6 mu mol/min, obese: 22.2 +/- 3.8 mu mol/min) were not significant. Both g roups stopped running after the same time at their maximal speed (lean : 3.5 +/- 0.3 min, obese: 4.2 +/- 0.2 min). During exercise, lean rats had higher increases in core temperature (lean: 0.7 degrees C, obese: 0.4 degrees C) and muscle temperatures (lean: 1.3 degrees C, obese: 0 .7 degrees C) than the obese rats. The calculated heat flows indicated a predominant conductive transfer of heat from muscle through the ski n in lean rats but a higher proportion of heat transfer to the blood i n obese rats. II is concluded that muscle heat accumulation did not ca use fatigue in either case.