Exercise during puberty and NMU induced mammary tumorigenesis in rats

Previously we have shown that exercising rats prior to administration of 50 mg NMU/kg (high dose) significantly decreased tumor multiplicity, but not incidence or latency. We hypothesized that the dose of NMU saturated the system such that the exercise intervention was overshadowed. Therefore, the purpose of this investigation was to determine if exercise would have a more pronounced effect with a moderate dose of 37.5 mg NMU/kg. Female Sprague Dawley rats were divided into sedentary and exercised groups. The rats were exercised five times per week from 21 to 50 days of age on a progressive treadmill training program with a final workload of 18 m/min at 15% incline for 60 min a day. At 50 days of age all rats were given an i.p. injection of NMU at 37.5 mg/kg body weight. The experiment was terminated 22 weeks post carcinogen administration. Although there was no difference in terms of tumor incidence, multiplicity, or latency between the two groups, the tumor growth rate (0.107+/-0.025 vs. 0.043+/-0.009 g/day) and final tumor weight (3.2+/-0.74 vs. 1.2+/-0.34 grams) were significantly higher in the exercised animals. IGF-I receptor (9.4+/-96 vs. 10.5+/-57 per microg protein) and estrogen receptor (18.4+/-1.14 vs. 19.6 + 1.6 per microg protein) levels were not significantly different in tumors from exercised animals compared to those from sedentary animals. For both sets of tumors, correlation between estrogen receptor content and growth rate is positive, while the correlation between IGF-I receptor content and growth rate is negative. The results of this study suggest that exercise prior to NMU administration does not affect tumor burden but does alter tumor growth rate, which is not due to differences in estrogen receptor or IGF-I receptor content.