418
0. B. COWEY AND J. R. SARGENT
In contrast to the red muscle, which is apparently used constantly for
normal cruising in fish, the white muscle is used only for short bursts
of sudden activity (Bone, 1966). Red muscle oxidizes fatty acids
efficiently by aerobic mechanisms (/3 oxidation) whereas white muscle
oxidizes stored glycogen by anaerobic glycolysis. One mole of glucose-6phosphate (derived from glycogen) is oxidized to two moles of lactic
acid with the appearance of three moles of ATP. During severe exercise
in rainbow trout, muscle glycogen is depleted extremely rapidly, one
half of the resting level being used up in the first two minutes of
activity (Black et al., 1962, 1966). Recovery is characterized by the
persistence of lactate and, to a lesser extent, pyruvate in muscle. Regeneration of glycogen is very slow and incomplete. A similar
situation has been very well documented in the plaice (Wardle, personal
communication), where it has also been shown that the lactic acid is
retained within the white muscle cells and that such retention is not due
to a diminished flow of either blood or lymph through the muscle. This
situation contrasts strikingly to that found in mammalian skeletal
muscle which rapidly releases its lactic acid into the blood for subsequent oxidation by other body tissues, especially liver and kidney (the
gluconeogenic tissues in mammals). It is possible that lactic acid in
white muscle of fish is either slowly oxidized to carbon dioxide and
water or converted to glycogen by white muscle itself. However, it is
equally possible that lactic acid is slowly removed from muscle into the
blood to be slowly metabolized by the low levels of lactic acid dehydrogenase known to be present in the livers of several fish including plaice
(Dando, 1969). In this context it is unfortunate that little evidence
concerning the possible presence of the gluconeogenic enzymes (e.g.
fructose 1, 6 diphosphatase) in fish has been presented. The apparent
inability of many fish to cope efficiently with lactic acid produced in
white muscle implies that carbohydrate metabolism is much less
important quantitatively in fish under normal circumstances than in
mammals. Such a situation certainly reflects the relatively low carbohydrate content in natural fish diets.
The enzyme catalysing the conversion of pyruvate to lactate, namely
lactic dehydrogenase, occurs in several isoenzymic forms. One of these,
present largely in cardiac muscle, is inhibited by high concentrations of
pyruvate at temperatures of 25"-37°C ; another isoenzyme is present
mainly in skeletal muscle and it shows little pyruvate inhibition at
these temperatures. I n mammals this has been interpreted (Kaplan
and Goodfriend, 1964) as a mechanism for ensuring complete oxidation
of pyruvate in heart muscle. The situation in fish may not be so
straightforward because marked pyruvate inhibition of lactic dehydro-
0. B. COWEY AND J. R. SARGENT
In contrast to the red muscle, which is apparently used constantly for
normal cruising in fish, the white muscle is used only for short bursts
of sudden activity (Bone, 1966). Red muscle oxidizes fatty acids
efficiently by aerobic mechanisms (/3 oxidation) whereas white muscle
oxidizes stored glycogen by anaerobic glycolysis. One mole of glucose-6phosphate (derived from glycogen) is oxidized to two moles of lactic
acid with the appearance of three moles of ATP. During severe exercise
in rainbow trout, muscle glycogen is depleted extremely rapidly, one
half of the resting level being used up in the first two minutes of
activity (Black et al., 1962, 1966). Recovery is characterized by the
persistence of lactate and, to a lesser extent, pyruvate in muscle. Regeneration of glycogen is very slow and incomplete. A similar
situation has been very well documented in the plaice (Wardle, personal
communication), where it has also been shown that the lactic acid is
retained within the white muscle cells and that such retention is not due
to a diminished flow of either blood or lymph through the muscle. This
situation contrasts strikingly to that found in mammalian skeletal
muscle which rapidly releases its lactic acid into the blood for subsequent oxidation by other body tissues, especially liver and kidney (the
gluconeogenic tissues in mammals). It is possible that lactic acid in
white muscle of fish is either slowly oxidized to carbon dioxide and
water or converted to glycogen by white muscle itself. However, it is
equally possible that lactic acid is slowly removed from muscle into the
blood to be slowly metabolized by the low levels of lactic acid dehydrogenase known to be present in the livers of several fish including plaice
(Dando, 1969). In this context it is unfortunate that little evidence
concerning the possible presence of the gluconeogenic enzymes (e.g.
fructose 1, 6 diphosphatase) in fish has been presented. The apparent
inability of many fish to cope efficiently with lactic acid produced in
white muscle implies that carbohydrate metabolism is much less
important quantitatively in fish under normal circumstances than in
mammals. Such a situation certainly reflects the relatively low carbohydrate content in natural fish diets.
The enzyme catalysing the conversion of pyruvate to lactate, namely
lactic dehydrogenase, occurs in several isoenzymic forms. One of these,
present largely in cardiac muscle, is inhibited by high concentrations of
pyruvate at temperatures of 25"-37°C ; another isoenzyme is present
mainly in skeletal muscle and it shows little pyruvate inhibition at
these temperatures. I n mammals this has been interpreted (Kaplan
and Goodfriend, 1964) as a mechanism for ensuring complete oxidation
of pyruvate in heart muscle. The situation in fish may not be so
straightforward because marked pyruvate inhibition of lactic dehydro-