Changes in the capillarity of skeletal muscle in the growing rat.

Capillary density (CD), capillary to fiber ratio (C/F), fiber cross sectional area (FCSA) and fiber composition were measured in the soleus and the gastrocnemius (medial head) muscles of rats weighing between 99 and 666 g. Muscle samples obtained from the anesthetized animal were rapidly frozen (-130 degrees C) sliced transversely at 16--18 micrometers, and treated histochemically by the ATPase method after preincubation at pH's of 4.0 and 4.4 to visualize capillaries and typify fibers. In both muscles the FCSA was positively related to body weight (BW) and muscle weight. At a given BW, the FCSA of the soleus was greater than that of the gastrocnemius. In both muscles CD decreased hyperbolically with FCSA (soleus: CD = 1.0613 X 10(6)/FCSA + 298.71; gastrocnemium: CD = 1.0349 X 10(6)/FCSA + 240.74). At the same time a positive linear correlation between C/F and FCSA was found (soleus: C/F = 3.92 X 10(-4) FCSA + 0.82; gastrocnemius: C/F = 2.90 X 10(-4) FCSA + 0.93). At a given FCSA, CD and C/F were greater in the soleus than in the gastrocnemius because of differences in fiber composition between the two muscles. The soleus had only oxidative fibers (STO and FTOG) whereas the gastrocnemius had 54% glycolytic fibers (FTG). The very large variability in CD and C/F values reported in the literature could, in part, be due to the differences in capillarity observed with maturation. A change in fiber composition with BW was observed in the soleus, but no systematic change occurred in the gastrocnemius.