Capillary supply of skeletal muscles from acclimatized white-footed mice Peromyscus.

Winter-acclimatized white-footed mice (Peromyscus leucopus) can increase their aerobic heat production under cold stress by 70%. The possibility that changes in microvascular supply might account, in part, for some of this increased thermogenic capacity was examined in one of the primary thermogenic tissues, skeletal muscle. Capillaries were stained histochemically in four hindlimb muscles of freshly captured Peromyscus in summer and winter. Capillary density, mean fiber area, and mean capillaries in contact per muscle fiber were obtained from the soleus, plantaris, gastrocnemius, and semitendinosus. If results from all individual muscles are combined, mean fiber area is significantly smaller (8%) and mean capillary density is significantly greater (40%) in winter muscles. The mean number of capillaries in contact is not different, but the mean ratio of capillaries in contact per mean fiber area (index of potential perfusion) is significantly greater (25%) in winter muscles.