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Department of Physiology, University of Birmingham, Birmingham B15 2TT, United Kingdom
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 (Vv) 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 µm2, 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 Vv 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 Vv, indicating little modification of structure.
Seasonal effects may dominate those of environmental temperature on
mitochondrial separation, which in winter and spring fish at
4oC averaged 0.64 ± 0.06 and 1.20 ± 0.07 µm,
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 ± 9µm2 per capillary. As a consequence, the
predicted intracellular PO2 (~2.5 kPa) is
similar throughout the year.
capillary supply; intracellular lipid; mitochondrial separation; Oncorhynchus mykiss; oxygen tension
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