Selected Contribution: Skeletal muscle capillarity and enzyme activity in rats selectively bred for running endurance

Abstract

To attempt to explain the difference in intrinsic (untrained) endurance running capacity in rats selectively bred over seven generations for either low (LCR) or high running capacity (HCR), the relationship among skeletal muscle capillarity, fiber composition, enzyme activity, and O2 transport was studied. Ten females from each group [body wt: 228 g (HCR), 247 g (LCR); P= 0.03] were studied at 25 wk of age. Peak normoxic maximum O2 consumption and muscle O2 conductance were previously reported to be 12 and 33% higher, respectively, in HCR, despite similar ventilation, arterial O2 saturation, and a cardiac output that was <10% greater in HCR compared with LCR. Total capillary and fiber number in the medial gastrocnemius were similar in HCR and LCR, but, because fiber area was 37% lower in HCR, the number of capillaries per unit area (or mass) of muscle was higher in HCR by 32% ( P < 0.001). A positive correlation ( r = 0.92) was seen between capillary density and muscle O2 conductance. Skeletal muscle enzymes citrate synthase and β-hydroxyacyl-CoA dehydrogenase were both ∼40% higher ( P < 0.001) in HCR (12.4 ± 0.7 vs. 8.7 ± 0.4 and 3.4 ± 0.2 vs. 2.4 ± 0.2 mmol · kg−1 · min−1, respectively), whereas phosphofructokinase was significantly ( P = 0.02) lower in HCR (27.8 ± 1.2 vs. 35.2 ± 2.5 mmol · kg−1 · min−1) and hexokinase was the same (0.65 ± 0.04 vs. 0.65 ± 0.03 mmol · kg−1 · min−1). Resting muscle ATP, phosphocreatine, and glycogen contents were not different between groups. Taken together, these data suggest that, in rats selectively bred for high-endurance exercise capacity, most of the adaptations for improved O2 utilization occur peripherally in the skeletal muscles and not in differences at the level of the heart or lung.