Impaired activation of thermogenesis in the corpulent rat

The capacity for non-shivering thermogenesis was measured in groups of 12 week-old congenic lean and corpulent LA/N-cp rats of both sexes to determine if their obese state might be associated with an impairment in energy expenditure via non-shivering thermogenesis. Body weights of the corpulent phenotypes were 1.6 to 1.8 times greater than those of the lean phenotype. Measurements of resting oxygen consumption were similar in lean and in corpulent rats, and were greater in female than in male rats. Isoproterenol stimulation resulted in a significant increase in oxygen consumption in lean rats, while the rates of oxygen consumption of isoproterenol-stimulated corpulent rats were unchanged. Acute exposure of male rats to a 5 degrees C cold environment resulted in significant decreases in colonic and in rectal temperature in both phenotypes, but body temperatures recovered more rapidly in lean than in corpulent rats. Urinary VMA excretion was greater in lean than in corpulent rats and increased following cafeteria-feeding in lean but not in corpulent rats. These observations are consistent with an impaired mechanism of sympathetically-mediated thermogenesis in the corpulent phenotype of the LA/N-cp rat, and which may be a contributing factor in the development of their obese state via a decreased capacity for energy expenditure.