4. THYROID FUNCTION AND ITS CONTROL IN FISHES
261
There are so many physiological phenomena in teleosts in which thyroid appears to play some role, or has been claimed to, that it is necessary
to present them in some logical order. First, will be considered the metabolic actions of thyroxine; second, structural effects; and third, effects on
central nervous system and behavior.
Respiratory stimulation by thyroxine is its best known action, yet the
bulk of evidence, frequently reviewed (e.g., Pickford and Atz, 1957) indicates that thyroxine has no such action in teleosts. Thyroidectomy of
parrot fish or of rainbow trout does not alter oxygen Consumption (Matty,
1957; Fromm and Reineke, 1957). Antithyroid drugs (Matthews and
Smith, 1947) had no effect in Fundulus; however, Osborn (1951) claims
that thiouracil lowers oxygen consumption by Campostom, a minnow,
and makes it thereby more viable in water low in oxygen. To add confusion to this picture, Sage (1965, 1968) has found that thiourea depresses
oxygen consumption of the guppy, Lebistes reticulatus. Thyroxine reverses the respiratory depression of thiourea, but when administered alone
( without thiourea ) thyroxine then depresses oxygen consumption.
Thyroxine injections frequently produce different respiratory effects
in the hands of different investigators. Most commonly, thyroxine has
evoked no respiratory response in Carassius auratus, Lebktes reticulatus,
Opsanus tau, Rhodeus amarus, or Salmo gairdneri in doses that may produce other physiological effects (see review in Pickford and Atz, 1957).
Muller (1953) stimulated oxygen consumption in goldfish with dosages
of 0.5-1.0 mg-a tremendous dose in an animal that may produce less
than 1 pg of thyroxine per day (Leloup and Fontaine, 1960) in its own
thyroid. Chavin and Rossmore (1956) using the same species, the goldfish, were unable to produce a change in 0, consumption using 0.5 mg
doses of thyroxine, and in their tests thiouracil also was without action.
It is of particular interest that although thyroxine has no consistent
effect on teleost oxygen consumption it produces at
M hormone concentrations, mitochondrial swelling in vitro in fish tissues (Greif and
Alfano, 1964) just as it does in mammals. According to some interpretations of the cellular action of thyroxine, such hormone-evoked mitochondrial changes may alter enzymic relations sufficiently on the mitochondrial
membrane to uncouple oxidative phosphorylation. This would lead to an
increased oxygen consumption in order to yield a particular amount of
useful chemical energy. The fact that mitochondrial changes can be
evoked without respiratory consequences in fish may be a good argument
against this interpretation of thyroxine’s cellular action.
A direct intervention of thyroxine in carbohydrate metabolism of fish
tissues has been claimed by Hochachka (1962). He finds that thyroxine
or T, in vitro, acting upon brook trout liver slices or homogenates, stimu-
261
There are so many physiological phenomena in teleosts in which thyroid appears to play some role, or has been claimed to, that it is necessary
to present them in some logical order. First, will be considered the metabolic actions of thyroxine; second, structural effects; and third, effects on
central nervous system and behavior.
Respiratory stimulation by thyroxine is its best known action, yet the
bulk of evidence, frequently reviewed (e.g., Pickford and Atz, 1957) indicates that thyroxine has no such action in teleosts. Thyroidectomy of
parrot fish or of rainbow trout does not alter oxygen Consumption (Matty,
1957; Fromm and Reineke, 1957). Antithyroid drugs (Matthews and
Smith, 1947) had no effect in Fundulus; however, Osborn (1951) claims
that thiouracil lowers oxygen consumption by Campostom, a minnow,
and makes it thereby more viable in water low in oxygen. To add confusion to this picture, Sage (1965, 1968) has found that thiourea depresses
oxygen consumption of the guppy, Lebistes reticulatus. Thyroxine reverses the respiratory depression of thiourea, but when administered alone
( without thiourea ) thyroxine then depresses oxygen consumption.
Thyroxine injections frequently produce different respiratory effects
in the hands of different investigators. Most commonly, thyroxine has
evoked no respiratory response in Carassius auratus, Lebktes reticulatus,
Opsanus tau, Rhodeus amarus, or Salmo gairdneri in doses that may produce other physiological effects (see review in Pickford and Atz, 1957).
Muller (1953) stimulated oxygen consumption in goldfish with dosages
of 0.5-1.0 mg-a tremendous dose in an animal that may produce less
than 1 pg of thyroxine per day (Leloup and Fontaine, 1960) in its own
thyroid. Chavin and Rossmore (1956) using the same species, the goldfish, were unable to produce a change in 0, consumption using 0.5 mg
doses of thyroxine, and in their tests thiouracil also was without action.
It is of particular interest that although thyroxine has no consistent
effect on teleost oxygen consumption it produces at
M hormone concentrations, mitochondrial swelling in vitro in fish tissues (Greif and
Alfano, 1964) just as it does in mammals. According to some interpretations of the cellular action of thyroxine, such hormone-evoked mitochondrial changes may alter enzymic relations sufficiently on the mitochondrial
membrane to uncouple oxidative phosphorylation. This would lead to an
increased oxygen consumption in order to yield a particular amount of
useful chemical energy. The fact that mitochondrial changes can be
evoked without respiratory consequences in fish may be a good argument
against this interpretation of thyroxine’s cellular action.
A direct intervention of thyroxine in carbohydrate metabolism of fish
tissues has been claimed by Hochachka (1962). He finds that thyroxine
or T, in vitro, acting upon brook trout liver slices or homogenates, stimu-