Effects on growth and endocrine status of maintaining obese and lean Zucker rats at 22 degrees C and 30 degrees C from weaning.

Lean and genetically obese Zucker rats were maintained from 21 to 34 days of age at an ambient temperature of 22 degrees C or 30 degrees C. Maintenance at 30 degrees C normalised the lower rectal temperature of obese rats. At 30 degrees C obese rats reduced their food intake compared with obese animals at 22 degrees C but they were still hyperphagic compared with non-obese animals at 30 degrees C. At 30 degrees C obese rats accumulated as much lipid in their carcase, but deposited less protein than obese animals at 22 degrees C. Obese rats were hyperinsulinemic compared with non-obese animals at either temperature. Plasma T3 and T4 levels did not differ between phenotypes and T4 levels were lower in both phenotypes at 30 degrees C. Plasma corticosterone levels were higher in obese rats at both temperatures and this phenotypic difference was larger at 30 degrees C. In a second experiment rats were maintained at 22 degrees C from 21-34 days of age and obese rats pair-fed to the ad lib intake of non-obese animals. Obese rats still became more obese than non-obese animals and deposited less protein than either non-obese or ad lib fed obese animals. Pair-fed obese rats were hyperinsulinemic and had higher plasma corticosterone concentrations than non-obese animals. These findings suggest that the raised plasma corticosterone concentrations of obese rats kept at 30 degrees C, or not allowed to express their greater appetite, may cause reduced protein deposition.