Exercise during adolescence and its effects on mammary gland development, proliferation, and nitrosomethylurea (NMU) induced tumorigenesis in rats

The purpose of this investigation was to determine the effects of moderate intensity exercise training during peripuberty on nitrosomethylurea (NMU) induced mammary tumorigenesis and to assess the effects of this activity on mammary epithelial cell proliferation and mammary gland development in rats. Female Sprague Dawley rats were divided into two groups, sedentary and exercised. 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 fifty days of age eight rats from each group were sacrificed to determine the effects of exercise on mammary gland labelling index and development scores. The remaining animals were given NMU at a concentration of 50 mg/kg body weight. The experiment was terminated 24 weeks post carcinogen administration, and cancer incidence, multiplicity, and latency were analyzed. The total tumor number per group was reduced by exercise (58 vs. 33 carcinomas p < 0.05). This corresponded to a significantly higher number of tumors/animal in the sedentary rats compared to those that were exercised prior to NMU administration. However latency period (124.9 +/- 4.3 vs. 125.2 +/- 6.4) and final incidence (68.9% vs. 61.5%) were not significantly different in sedentary vs. exercised rats. There were no significant differences in mammary gland developmental scores (4.1 +/- 0.24 vs. 4.4 +/- 0.26) or labelling index of mammary epithelial cells in either ducts or lobuloaveolar units (ductal 41.5 +/- 4.7 vs. 39.4 +/- 5.7; lobuloalveolar 38.5 +/- 4.1 vs. 47.7 +/- 5.7) between the two groups. The results of this study suggest that although exercise prior to carcinogen administration has an effect on mammary tumorigenesis, these effects do not appear to be related to exercise-induced changes in mammary gland development.