Differential effects of cold exposure on muscle fibre composition and capillary supply in hibernator and non-hibernator rodents.

Changes in the composition of fibre types and the capillary supply of skeletal muscle (tibialis anterior) were quantified in rats and hamsters subjected to 8-10 weeks of cold exposure and reduced photoperiod (10 degrees C, 1 h light-23 h dark). Muscle mass decreased in both species (by 12% and 17%, respectively). Following acclimation to cold there were no specific changes in fibre cross-sectional area (FCSA) in rats, whereas in hamsters there was a substantial atrophy of Type II, but not Type I fibres. In rat muscle there was little difference between the two groups in average capillary to fibre ratio (C:F) (1.76 +/- 0.15, normothermia, N; 1.69 +/- 0.05, hypothermia, H) and average capillary density (CD) (188 +/- 14 mm(-2), N; 201 +/- 12 mm(-2,) H). Similarly, the average C:F was unaltered in hamsters (2.75 +/- 0.11, N; 2.72 +/- 0.15, H), although the 30 % smaller fibre size observed with hypothermia resulted in a corresponding increase in average CD, to 1539 +/- 80 mm(-2) (P < 0.01). However, there was a coordinated regional adaptation to cold exposure in hamsters resulting in capillary rarefaction in the glycolytic cortex and angiogenesis in the oxidative core. Following acclimation of rats to cold there was a reduction in the supply area of individual vessels (capillary domain), particularly in the cortex (9310, N; 8938 microm2, H; P < 0.05). In contrast, hypothermic hamsters showed only a small decrease in mean domain area in the cortex (948 microm2, N; 846 microm2, H; n.s.) but a marked reduction in the core (871 microm2, N; 604 microm2, H; P < 0.01). Rats showed little or no change in local capillary supply (LCFR) to fast fibres on acclimation to cold, while in hamsters the LCFR of Type IIb fibres showed a decrease in the cortex (2.7, N; 2.3, H) and an increase in the core (3.0, N; 3.3, H) during acclimation to cold. These data suggest that during a simulated onset of winter rats maintain FCSA and capillary supply as part of an avoidance strategy, whereas hamsters increase muscle capillarity in part as a consequence of disuse atrophy.