Cold Adaptation and Stenothermy in Antarctic Notothenioid Fishes
99
of in vitro activities of enzymes of ATP-generating pathways. There are
several reasons why in vitro enzymatic activities can be expected to yield
insights into metabolic compensation and to avoid the pitfalls associated
with studying whole-organism respiration. First, in vitro enzymatic
activities are not subject to artifacts arising from the response of a fish to
confinement in a metabolic chamber. Second, at least for certain organs,
enzymatic activities are likely to be independent of nutritional and
reproductive states. Third, in some organs, the fish's mode of life, for
example, its locomotory habit and general activity level, should not be a
strong determinant of ATP-generating ability. The organ in which
enzymatic activities seem most likely to provide a clear indication of
metabolic compensation to temperature is brain [13]. We hypothesize that
the metabolic activity of brain should be largely independent of a fish's
mode of life and its reproductive and nutritional status. The activities of
enzymes in brains of temperate zone fishes with widely different modes of
life, for instance, pelagic versus benthic existence, are remarkably similar
[14]. Long-term food deprivation led to significant changes in enzymatic
activities in muscle but not in brain of the scorpaenid fish Sebastolobus
alascanus [15]. Size-dependent effects on enzymatic activity have been
observed in brain and other tissues, but these effects can readily be taken
into account in analyses [16]. Thus, if temperature compensation of ATP
production is a critical feature of adaptation to temperature, it should be
manifested especially clearly in brain.
Our comparisons of the in vitro activities (International Units per gram
fresh (wet) weight of tissue) of two enzymes involved in ATP generation,
citrate synthase (CS) of the citric acid cycle and lactate dehydrogenase
(LDH) of the glycolytic pathway, in homogenates of brains from several
Antarctic notothenioid and tropical fishes provided clear evidence for
partial temperature compensation of brain ATP-generating capacity [13].
Rates of CS and LDH activity were measured at a common temperature,
10 0c. A QIO of2.0 was used to adjust the 10°C rates to the rates expected
at each species' normal physiological temperatures (-1.9 °C for Antarctic
fish and 25-30 °C for tropical species). Rather than observing the
approximately 8-fold difference in activity at physiological temperatures
that would be predicted by QIO relationships if no compensation were
present, LDH activity was only 2.6-fold higher and CS 4.9-fold higher in
the tropical species. The factors that underlie this incomplete temperature
compensation of brain ATP-generating capacity remain to be discovered.
It seems possible that the energy costs of maintaining ion disequilibrium
might be reduced at low temperatures, but this hypothesis has not been
tested [17]. In contrast to brain tissue, CS and LDH activities in
locomotory muscle varied widely among Antarctic and tropical species in
99
of in vitro activities of enzymes of ATP-generating pathways. There are
several reasons why in vitro enzymatic activities can be expected to yield
insights into metabolic compensation and to avoid the pitfalls associated
with studying whole-organism respiration. First, in vitro enzymatic
activities are not subject to artifacts arising from the response of a fish to
confinement in a metabolic chamber. Second, at least for certain organs,
enzymatic activities are likely to be independent of nutritional and
reproductive states. Third, in some organs, the fish's mode of life, for
example, its locomotory habit and general activity level, should not be a
strong determinant of ATP-generating ability. The organ in which
enzymatic activities seem most likely to provide a clear indication of
metabolic compensation to temperature is brain [13]. We hypothesize that
the metabolic activity of brain should be largely independent of a fish's
mode of life and its reproductive and nutritional status. The activities of
enzymes in brains of temperate zone fishes with widely different modes of
life, for instance, pelagic versus benthic existence, are remarkably similar
[14]. Long-term food deprivation led to significant changes in enzymatic
activities in muscle but not in brain of the scorpaenid fish Sebastolobus
alascanus [15]. Size-dependent effects on enzymatic activity have been
observed in brain and other tissues, but these effects can readily be taken
into account in analyses [16]. Thus, if temperature compensation of ATP
production is a critical feature of adaptation to temperature, it should be
manifested especially clearly in brain.
Our comparisons of the in vitro activities (International Units per gram
fresh (wet) weight of tissue) of two enzymes involved in ATP generation,
citrate synthase (CS) of the citric acid cycle and lactate dehydrogenase
(LDH) of the glycolytic pathway, in homogenates of brains from several
Antarctic notothenioid and tropical fishes provided clear evidence for
partial temperature compensation of brain ATP-generating capacity [13].
Rates of CS and LDH activity were measured at a common temperature,
10 0c. A QIO of2.0 was used to adjust the 10°C rates to the rates expected
at each species' normal physiological temperatures (-1.9 °C for Antarctic
fish and 25-30 °C for tropical species). Rather than observing the
approximately 8-fold difference in activity at physiological temperatures
that would be predicted by QIO relationships if no compensation were
present, LDH activity was only 2.6-fold higher and CS 4.9-fold higher in
the tropical species. The factors that underlie this incomplete temperature
compensation of brain ATP-generating capacity remain to be discovered.
It seems possible that the energy costs of maintaining ion disequilibrium
might be reduced at low temperatures, but this hypothesis has not been
tested [17]. In contrast to brain tissue, CS and LDH activities in
locomotory muscle varied widely among Antarctic and tropical species in