Significant differences in responses to intraperitoneally injected tyramine or cold exposure (-10 degrees C) were observed in young (3-4 mo) male Sprague-Dawley (S-D) and Fischer 344 (F344) rats cold-acclimated (CA) or non-cold-acclimated (non-CA). Non-CA S-D and F344 rats respond to tyramine by increased O2 uptake and elevation of colonic temperature, but, as we had reported previously, the optimal doses of tyramine required was significantly different for these two strains, i.e., 2 mg/kg for F344 vs. 20 mg/kg for S-D. The response to tyramine was significantly potentiated by cold acclimation in both strains of rats. Despite the different dosages of tyramine given to the animals, similar increases in O2 uptakes and colonic temperatures were observed. Differences between these two strains were also evident in their responses to a cold challenge (3 h, -10 degrees C). The ranking for cold tolerance (-10 degrees C) was as follows: CA F344 greater than non-CA F344 greater than CA S-D greater than non-CA S-D. These differences between the two strains suggest that interpretations regarding the response of rats to stressful conditions must be made with caution and appreciation of strain differences.
