Increased capillary supply in skeletal muscle of guinea pigs acclimated to cold.

The ATPase technique was used to visualize blood capillaries and to study fiber composition in 10-micrometer transverse sections of guinea pig gastrocnemius and soleus muscles. A control group of newborn, weanling, juvenile and adult male guinea pigs (GP) (BW = 89-1274 g) was studied in a 20-24 degrees C environment (22 degrees C GP) while 2-3 week old animals were exposed continuously to 5 degrees C for 2-18 weeks before sacrifice (5 degrees C GP) (BW = 239-1074 g). Body weight gain was not affected by cold exposure; however, the gastrocnemius and soleus muscles of the 5 degrees C GP grew at a slower rate than did the muscles of the 22 degrees C GP. The equations relating fiber cross sectional area (FCSA) and muscle weight (MW) were not different between the 22 degrees C GP and 5 degrees C GP for the soleus and gastrocnemius. Therefore, in both muscles at the same BW, FCSA was smaller in the 5 degrees C GP than in the 22 degrees C GP. In both of the two muscles of each group, capillary density (CD) decreased hyperbolically with increasing FCSA, while the capillary to fiber ratio (C/F) and the average number of capillaries around each fiber (CAF) increased linearly with increasing FCSA. The regression lines for CD, C/F and CAF versus FCSA for both muscles were parallel between groups, but at any FDSA, the CD, C/F and CAF were greater in the 5 degrees C GP than in the 22 degrees C GP. Percent fiber composition of the gastrocnemii of the 22 degrees C GP and 5 degrees C GP were not different; however, at the same FCSA each fiber type had a greater capillary supply in the 5 degrees C GP. The increased capillarity in the gastrocnemius and soleus muscles of the 5 degrees C GP suggests an improved capacity for oxygenation, a response which would correlate well with the increased oxygen utilization during prolonged cold exposure.