100
G.N. Somero et al.
concert with differences in locomotory habits: active swimmers had high
activities of both enzymes compared to sluggish species (KawaU and
Somero, unpublished observations). As would be predicted by the analyses
of Holeton [11] and Clarke [8], no apparent temperature-related
differences in muscle ATP-generating capacity were observed (KawaU and
Somero, unpublished observations).
Cold Adaptation and Stenothermy in Mitochondrial
Function
The ability to respire - regardless of rate - at extremely low temperatures is
dependent upon mitochondrial function, a highly temperature-sensitive
process [18,19,6] that, in eurythermal species, is subject to the effects of
acclimation [20]. There are several properties of mitochondrial respiration
that provide insights into the thermal tolerance limits of mitochondrial
ATP generation. These properties include 1) the effectiveness of coupling
of electron transport to phosphorylation of ADP (as measured by the
Acceptor Control Ratio (ACR) , the degree to which respiration is
stimulated by introducing ADP into the respiration medium), and 2) the
temperatures at which sharp discontinuities ("breaks") in slope are
observed in Arrhenius plots of log respiration rate versus the reciprocal of
absolute temperature [Arrhenius Break Temperatures (ABTs)]. We studied
these two properties of mitochondria isolated from livers of a nototheniid
fish, Trematomus bernacchii, and several temperate and tropical species
(Weinstein and Somero, in preparation).
Our results provide further illustrations of the degree of cold adaptation
present in the notothenioids and the restricted range of temperatures over
which their physiological systems can function. Mitochondria from T.
bernacchii exhibited a high capacity for ATP synthesis (high ACR values)
only at temperatures below approximately 18°C; at higher temperatures
electron transport rapidly became uncoupled to ATP production. The
temperatures at which sharp breaks in the slopes of Arrhenius plots
occurred also reflected the low adaptation temperature of the Antarctic fish
(Fig. 1).
The Arrhenius Break Temperature for mitochondria from T. bernacchii
is the lowest reported for any organism, and falls well below the value
predicted by the regression equation based on ABT data from a wide range
of vertebrates and invertebrates (Fig. 1) [18-20]. Moreover, 2 weeks of
acclimation to 4 °C led to no change in the ABT, a result which contrasts
with the plasticity in this trait found in more eurythermal animals ([20],
Weinstein and Somero, in preparation).
G.N. Somero et al.
concert with differences in locomotory habits: active swimmers had high
activities of both enzymes compared to sluggish species (KawaU and
Somero, unpublished observations). As would be predicted by the analyses
of Holeton [11] and Clarke [8], no apparent temperature-related
differences in muscle ATP-generating capacity were observed (KawaU and
Somero, unpublished observations).
Cold Adaptation and Stenothermy in Mitochondrial
Function
The ability to respire - regardless of rate - at extremely low temperatures is
dependent upon mitochondrial function, a highly temperature-sensitive
process [18,19,6] that, in eurythermal species, is subject to the effects of
acclimation [20]. There are several properties of mitochondrial respiration
that provide insights into the thermal tolerance limits of mitochondrial
ATP generation. These properties include 1) the effectiveness of coupling
of electron transport to phosphorylation of ADP (as measured by the
Acceptor Control Ratio (ACR) , the degree to which respiration is
stimulated by introducing ADP into the respiration medium), and 2) the
temperatures at which sharp discontinuities ("breaks") in slope are
observed in Arrhenius plots of log respiration rate versus the reciprocal of
absolute temperature [Arrhenius Break Temperatures (ABTs)]. We studied
these two properties of mitochondria isolated from livers of a nototheniid
fish, Trematomus bernacchii, and several temperate and tropical species
(Weinstein and Somero, in preparation).
Our results provide further illustrations of the degree of cold adaptation
present in the notothenioids and the restricted range of temperatures over
which their physiological systems can function. Mitochondria from T.
bernacchii exhibited a high capacity for ATP synthesis (high ACR values)
only at temperatures below approximately 18°C; at higher temperatures
electron transport rapidly became uncoupled to ATP production. The
temperatures at which sharp breaks in the slopes of Arrhenius plots
occurred also reflected the low adaptation temperature of the Antarctic fish
(Fig. 1).
The Arrhenius Break Temperature for mitochondria from T. bernacchii
is the lowest reported for any organism, and falls well below the value
predicted by the regression equation based on ABT data from a wide range
of vertebrates and invertebrates (Fig. 1) [18-20]. Moreover, 2 weeks of
acclimation to 4 °C led to no change in the ABT, a result which contrasts
with the plasticity in this trait found in more eurythermal animals ([20],
Weinstein and Somero, in preparation).