Maintenance of heterogeneity of capillary spacing is essential for adequate oxygenation in the soleus muscle of the growing rat

OBJECTIVES: Normal muscle growth is accompanied by capillary proliferation, which usually lags behind the increase in muscle size, causing a decline in mean capillary density (CD). It is not known, however, how the capillary distribution is affected and what impact it has on the oxygenation of the muscle. METHODS: The capillarization of soleus muscles of rats (64-425 g) was determined with the method of capillary domains. As well as quantifying CD, capillary to fiber ratio (C:F), and fiber size, this method provides a measure of the heterogeneity of capillary spacing. Capillary locations were used to mathematically model oxygenation levels within the muscle. RESULTS: The increase in muscle mass was largely attributable to 5-fold increase in fiber size, accompanied by a more than 3-fold rise in C:F. The mismatch between rates of angiogenesis and muscle growth resulted in a decrease in CD. However, the heterogeneity of capillary spacing was unaffected (heterogeneity index logRSD: 0.091 +/- 0.013; mean +/- SD) as was muscle PO2, with modal values between 4 and 60 mmHg (0.5 and 8 kPa). CONCLUSIONS: Angiogenesis during normal muscle growth does not maintain CD, but with similar heterogeneity of capillary spacing it preserves the potential for adequate intramuscular oxygenation.