Endocrine differences between the Wistar and Sprague-Dawley laboratory rat: influence of cold adaptation.

Sprague-Dawley (SD) rats kept at room temperature do have a higher growth rate and food conversion compared to Wistar rats. Mean hormone level in blood samples collected every four hours during a 24 hour period do differ between both strain of rats. So, the mean circulating levels of T3, T4 and corticosterone are significantly lower, whereas prolactin concentrations are higher in SD-rats. Following acute cold exposure comparable increases in plasma T3 and T4 are seen in both strains after 3-5 hours. Prolactin plasma concentrations are decreased after 5 and 9 hours but only in SD-rats, whereas they remain unchanged in Wistars. Acclimation to 4 degrees C during 3 and 8 weeks resulted in a decreased growth rate of the SD-rats, which becomes comparable to Wistar animals, but food conversion is lower and food intake higher in SD-rats. A decrease in circulating levels of T4 is present in Wistars resulting in a significantly lower figure at 8 weeks of cold acclimation, whereas at the same time T3 is increased. Prolactin is decreased in SD-rats and increased in Wistars causing at 8 weeks of cold acclimation higher levels in Wistars. Corticosterone levels remain higher in Wistar rats and are comparable to rats kept at room temperature. It is concluded that the differences in growth rate, food conversion and food intake are related to the endocrine differences and responses observed.