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

Breast cancer patients' maximal O uptake (V̇Omax) 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 V̇Omax, 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 V̇Omax, 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 V̇Omax, rats were placed within a plexiglass metabolic chamber connected to CO and O 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 × 10 in 50 µl saline). Tumor growth was monitored daily and ~7 days following palpation (24 days post-injection), V̇Omax and echocardiography measurements were repeated. Tumor mass and volume were 2.1 ± 0.6 g and 1685 ± 428 (range 256-3749) mm, respectively. Body mass (217 ± 6 vs 218 ± 6 g), V̇Omax (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 V̇Omax (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 V̇Omax or cardiac function, but reduced exercise tolerance.