Relationship between the size of the capillary bed and oxidative capacity in various cat skeletal muscles

The size of the capillary bed, assessed by capillary density (CD), capillary per muscle fibre ratio (C/F), total capillary length, surface area and volume was related to the oxidative capacity, assessed by the volume density of mitochondria and \(\dot V\) O₂max in cat muscles with a different composition of glycolytic and oxidative fibres: predominantly glycolytic gracilis, purely oxidative soleus and gracilis transformed towards oxidative by chronic low frequency (10 Hz) electrical stimulation. Maximal blood flow and lactate output were measured in the muscles during isometric contractions. When capillary supply was estimated by C/F ratio, there was a close correlation between various parameters only in stimulated gracilis. The combined data of all muscles showed a significant correlation between the total volume of mitochondria, \(\dot V\) O₂max and total capillary surface area. Capillary volume showed a tight correlation with maximal blood flow in both control and stimulated gracilis, but not in soleus. Maximal blood flow was correlated withVO₂max in oxidative muscles (stimulated gracilis and soleus) but not in control glycolytic gracilis. Moreover normal gracilis did not show any relationship between the volume density of mitochondria and the size of the capillary bed. The latter was inversely correlated with the output of lactate which was greater in muscles with a lower C/P ratio. The data on gracilis indicates that the capillary bed can adapt to the increased demand for oxygen and a greater oxidative capacity induced by long-term activity imposed on a glycolytic muscle, while it may be more important for the removal of lactate in the glycolytic muscles under their normal activity. The factors involved in the regulation of blood flow in control soleus — when the morphological size of the vascular bed is not related to blood flow — are discussed.