SKELETAL MUSCLE CAPILLARITY, ENZYME ACTIVITY, AND OXYGEN TRANSPORT IN RATS SELECTIVELY BRED FOR ENDURANCE RUNNING PERFORMANCE

Abstract

Purpose: This study investigated the relationship between skeletal muscle capillarity, enzymes, and oxygen transport in untrained rats selectively bred over 7 generations as either high capacity (HCR) or low capacity (LCR) runners based on treadmill running performance. 10 of each group were tested (body wt: 228 g (HCR), 247 g (LCR), p = 0.03). Results: Peak power output was 33% higher in HCR, despite similar ventilation, arterial saturation and a cardiac output that was only 10% greater. Total capillary and fiber number in the medial gastrocnemius were similar in HCR and LCR, but because fiber area was lower in HCR, the number of capillaries per unit mass of muscle (Nc) was higher in HCR (p < 0.001). A positive correlation (r = 0.92) was seen between Nc and muscle oxygen conductance. Skeletal muscle enzymes CS and β-HAD were both significantly (p < 0.001) higher in HCR (CS: 12.4 ± 0.7 vs. 8.7 ± 0.4; β-HAD: 3.4 ± 0.2 vs. 2.4 ± 0.2 mmol/kg/min,) while PFK was significantly (p = 0.02) lower in HCR (27.8 ± 1.2 vs. 35.2 ± 2.5 mmol/kg/min) and HK was the same (0.65 ± 0.04 vs. 0.65 ± 0.03 mmol/kg/min). Resting muscle ATP, PCr, and glycogen contents were not different between groups. Conclusion: Taken together, these data suggest that in rats selectively bred for endurance exercise, most of the enabling adaptations occur peripherally in the skeletal muscles, matching structure to function, and not in differences at the level of the heart or lungs. Supported by NIH AR40155, HL17731, HL 64270, NSERC, HL39443.