Tumor bearing in untreated breast cancer decreases exercise tolerance without lowering maximal oxygen uptake in rats.

Breast cancer patients' maximal O2 uptake (V̇O2max) values average 60-80% of age-predicted values which is often attributed to adjuvant therapy rather than risk factors, comorbidities, or the tumor and associated factors (e.g., pro-inflammatory cytokines). It is crucial to understand the physiological mechanisms behind exercise intolerance in breast cancer patients to enhance targeted interventions; however, the effect of breast cancer, as an isolated condition on V̇O2max, exercise tolerance, and resting cardiac function has not been investigated. We hypothesized that breast cancer, in the absence of underlying conditions or chemotherapy, would lower V̇O2max, exercise tolerance, and cardiac function in proportion to tumor mass. Female Fischer-344 rats (~6-8 months, n = 8) were acclimatized to treadmill running for 5 days at 25 m/min for 5 min/day. To measure V̇O2max, rats were placed within a plexiglass metabolic chamber connected to CO2 and O2 analyzers. Tests began at 25 m/min and increased (5 m/min) until exhaustion. Cardiac function was determined by echocardiography before rats received a mammary intraductal injection of rat adenocarcinoma cells (MATBIII, 6 × 103 in 50 µl saline). Tumor growth was monitored daily and ~7 days following palpation (~24 days post-injection), V̇O2max and echocardiography measurements were repeated. Tumor mass and volume were 2.1 ± 0.6 g and 1685 ± 428 (range 256-3749) mm3, respectively. Body mass (217 ± 6 vs 218 ± 6 g), V̇O2max (72.1 ± 2.7 vs 70.0 ± 2.8 ml/kg·min; P > 0.05), and all measures of cardiac function were unchanged following tumor formation, with no significant correlation between tumor mass and V̇O2max (P > 0.05). However, time to exhaustion (376 ± 20 vs 297 ± 25 s), final treadmill speed (48 ± 1 vs 42 ± 2 m/s), distance run (209 ± 16 vs 152 ± 18 m), and total work (45 ± 3 vs 32 ± 4 m·kg) were significantly reduced with tumor bearing. Contrary to our hypothesis, breast cancer did not affect V̇O2max or cardiac function, but reduced exercise tolerance.