228
Energy Provision During Exercise
Aerobic Capacity
P.L.M. van Dijk et al.
Sustained swimming is powered by slow aerobic muscle fibers that rely
almost exclusively on aerobic energy provision. An increase of the
maximum sustainable speed during cold adapation should therefore go
along with an increase of the aerobic capacity. How do fish enhance their
aerobic capacity in the cold? Several studies on temperate fish have
shown a significant increase of the mitochondrial volume density in
muscle cells during cold acclimation [17,18]. A similar trend is observed
in Antarctic fish. Mitochondrial densities in slow fibers of several
notothenioids are higher than those in temperate species at warm
acclimation temperatures [19,20]. Besides a compensatory increase in
oxidative capacity a second functional implication has been ascribed to
changes in mitochondrial density. Increased number of mitochondria and
concomitant reduction of the diffusion pathlength between capillaries or
cytosolic compartments and these organelles may compensate for the
reduced diffusion coefficients of metabolites and oxygen at low
temperatures [17,21]. A recent study on diffusion limitations in fish
muscle has shown, however, that diffusional fluxes of neither ATP nor
phosphocreatine limit muscular function at low temperatures even in the
absence of increased mitochondrial densities [22].
Only few studies have investigated the impact of cold acclimation or
adaptation on the functional properties of mitochondria. There is some
evidence that both cold acclimation and adaptation lead to an increase in
mitochondrial aerobic capacity [23]. In the short-homed sculpin
Myxocephalus scorpius a decrease of the acclimation temperature from 15
°C to 5 °C doubled maximal rates of pyruvate oxidation per milligram of
mitochondrial protein [24]. A comparison of oxidative capacities of
Antarctic, temperate and tropical fish showed only moderate cold
compensation [25]. While respiration rates of mitochondria from tropical
tilapia extrapolate to zero at around 2°C, mitochondria of the Antarctic N
coriiceps still oxidize substrates at considerable rates at -1.5 °C. However,
the aerobic capacity of tropical and temperate fish was considerably
higher than that of Antarctic fish when compared at their habitat
temperatures.
The mechanisms that underlie the increase of oxidative capacity of
mitochondria are still the subject of investigation. Cristae density does not
change during thermal acclimation of fish [17,21] and is probably not
enhanced in Antarctic fish species [26].
Energy Provision During Exercise
Aerobic Capacity
P.L.M. van Dijk et al.
Sustained swimming is powered by slow aerobic muscle fibers that rely
almost exclusively on aerobic energy provision. An increase of the
maximum sustainable speed during cold adapation should therefore go
along with an increase of the aerobic capacity. How do fish enhance their
aerobic capacity in the cold? Several studies on temperate fish have
shown a significant increase of the mitochondrial volume density in
muscle cells during cold acclimation [17,18]. A similar trend is observed
in Antarctic fish. Mitochondrial densities in slow fibers of several
notothenioids are higher than those in temperate species at warm
acclimation temperatures [19,20]. Besides a compensatory increase in
oxidative capacity a second functional implication has been ascribed to
changes in mitochondrial density. Increased number of mitochondria and
concomitant reduction of the diffusion pathlength between capillaries or
cytosolic compartments and these organelles may compensate for the
reduced diffusion coefficients of metabolites and oxygen at low
temperatures [17,21]. A recent study on diffusion limitations in fish
muscle has shown, however, that diffusional fluxes of neither ATP nor
phosphocreatine limit muscular function at low temperatures even in the
absence of increased mitochondrial densities [22].
Only few studies have investigated the impact of cold acclimation or
adaptation on the functional properties of mitochondria. There is some
evidence that both cold acclimation and adaptation lead to an increase in
mitochondrial aerobic capacity [23]. In the short-homed sculpin
Myxocephalus scorpius a decrease of the acclimation temperature from 15
°C to 5 °C doubled maximal rates of pyruvate oxidation per milligram of
mitochondrial protein [24]. A comparison of oxidative capacities of
Antarctic, temperate and tropical fish showed only moderate cold
compensation [25]. While respiration rates of mitochondria from tropical
tilapia extrapolate to zero at around 2°C, mitochondria of the Antarctic N
coriiceps still oxidize substrates at considerable rates at -1.5 °C. However,
the aerobic capacity of tropical and temperate fish was considerably
higher than that of Antarctic fish when compared at their habitat
temperatures.
The mechanisms that underlie the increase of oxidative capacity of
mitochondria are still the subject of investigation. Cristae density does not
change during thermal acclimation of fish [17,21] and is probably not
enhanced in Antarctic fish species [26].