Thermal compensation of peripheral oxygen transport in skeletal muscle of seasonally acclimatized trout.

Seasonal changes in ultrastructure of locomotory muscle were quantified after acclimatization to natural temperature and photoperiod. Only modest changes were seen in the volume density (V(v)) of mitochondria in slow fibers ranging from 0.21 +/- 0.01 (summer) to 0.24 +/- 0.01 (winter), despite an increase in fiber size from 945 +/- 19 to 1,594 +/- 46 microm(2), respectively, resulting in a significantly greater total mitochondrial volume at low temperatures. In contrast, intracellular lipid stores showed a marked change with season, from a maximum V(v) of lipid droplets of 0.16 +/- 0.01 in winter, progressively declining through spring and summer to a minimum of 0.07 +/- 0.01 in autumn. For both organelles, the surface density reflected changes in V(v), indicating little modification of structure. Seasonal effects may dominate those of environmental temperature on mitochondrial separation, which in winter and spring fish at 4(o)C averaged 0.64 +/- 0.06 and 1.20 +/- 0.07 microm, respectively. The extracellular transport of oxygen also varies with season, the peak capillary density in autumn (2,851 +/- 88 mm(-2)) resulting in a minimum tissue supply (domain) area of 529 +/- 9 microm(2) per capillary. As a consequence, the predicted intracellular PO(2) ( approximately 2.5 kPa) is similar throughout the year.