Increased capillarity in skeletal muscle of growing guinea pigs acclimated to cold and hypoxia.

Capillarity was evaluated on transverse sections of frozen gastrocnemius and soleus muscles of young, growing guinea pigs exposed to the combined stresses of cold (6 degrees C) (C) and hypoxia (ambient PO2 = 85 Torr) (H) for up to 16 weeks and these data were compared to those obtained in a control group of guinea pigs kept in Denver (22 degrees C, ambient PO2 = 133 Torr). Capillarity was assessed from measurements of capillary density and capillarity density to fiber density ratios. Mean (R) and maximal (R95) diffusion distances were measured by the closest individual method. The body growth rate of guinea pigs exposed to C + H was the same as that in the control condition. The gastrocnemius muscle grew at the same rate as in the control guinea pigs. Exposure to C + H produced a significant (P less than 0.001) increase in the capillary density and the C/F of the gastrocnemius, reducing the mean and the maximal diffusion distances. However, the soleus muscles of the guinea pigs in C + H did not grow at the same rate and relative to body size the soleus muscles of these guinea pigs in C + H were smaller due to their smaller fiber cross-section area; consequently, there was a relatively larger capillarity in these muscles. It is hypothesized that the increased muscle capillarity in animals exposed to C + H results from a marked lowering of the tissue PO2 which may result from a leftward shift of the Hb-O2 dissociation curve.