4. THYROID FUNCTION AND ITS CONTROL IN FISHES
249
(see Y. A. Fontaine, 1967). Until it is definitively identified, this substance
is tentatively called the “heterothyrotropic factor” ( HTF). Rodesch and
Fontaine (19f39) found that although HTF is strongly thyrotropic in teleosts, it is a relatively weak thyroid stimulant in mammals. In an in vitro
test for thyrotropic activity based on 1311 metabolism by cultured sheep
thyroid cells, Rodesch and Fontaine found mammalian TSH most active,
mammalian HTF less active, and carp TSH least active. Y. A. Fontaine
and Burzawa-Grard (1969) fractionated the TSH and HTF activities in
rat and beef pituitaries and found that the HTF activity could not be
isolated from gonadotropic activity. This suggests strongly that the teleost
thyroid is responding to mammalian gonadotropin. Pickford and Grant
(1968) have confirmed the HTF activity of mammalian pituitary material
in hypophysectomized Fundulus and in starved trout, showing that it is
referable to a substance other than TSH.
It would appear, then, that the teleost thyroid responds to several
glycoprotein hormones, but in the evolution both of the pituitary hormones and the thyroid mechanisms responsive to them, that further
differentiation and specificity may have occurred. There is a basis here for
some important further research.
An interesting parameter was added to the question of species
responsiveness to TSHs of different sources by M. Fontaine and Fontaine
(1957; see also Y. A. Fontaine, 1958). They found in assays for TSH
activity in trout at different temepratures, that the rat pituitary is relatively more active at 20°C than at 10°C. Eel pituitary, on the other hand,
is more active at 10°C than at 20°C. This remarkable finding deserves
confirmation and further study.
IV. THYROID FUNCTION AND THYROID HORMONE SYNTHESIS
Stated most simply, the function of thyroid gland is to manufacture
thyroid hormone. As will be made clear presently, the thyroid hormones
of fish are relatively small molecules and they are the same in all vertebrates, namely, thyroxine ( T x ) and triiodothyronine ( T3). Other iodocompounds may be manufactured by the thyroid, and even released from
the thyroid tissue, but in amounts and activities that have no apparent
physiological value.
The process of thyroid hormone formation has been studied very
closely because it can be followed to a large extent by the use of radioiodine as a characteristic label. The first phase of the process involves
concentration of iodine (iodide) within the secretory units (follicles) by
249
(see Y. A. Fontaine, 1967). Until it is definitively identified, this substance
is tentatively called the “heterothyrotropic factor” ( HTF). Rodesch and
Fontaine (19f39) found that although HTF is strongly thyrotropic in teleosts, it is a relatively weak thyroid stimulant in mammals. In an in vitro
test for thyrotropic activity based on 1311 metabolism by cultured sheep
thyroid cells, Rodesch and Fontaine found mammalian TSH most active,
mammalian HTF less active, and carp TSH least active. Y. A. Fontaine
and Burzawa-Grard (1969) fractionated the TSH and HTF activities in
rat and beef pituitaries and found that the HTF activity could not be
isolated from gonadotropic activity. This suggests strongly that the teleost
thyroid is responding to mammalian gonadotropin. Pickford and Grant
(1968) have confirmed the HTF activity of mammalian pituitary material
in hypophysectomized Fundulus and in starved trout, showing that it is
referable to a substance other than TSH.
It would appear, then, that the teleost thyroid responds to several
glycoprotein hormones, but in the evolution both of the pituitary hormones and the thyroid mechanisms responsive to them, that further
differentiation and specificity may have occurred. There is a basis here for
some important further research.
An interesting parameter was added to the question of species
responsiveness to TSHs of different sources by M. Fontaine and Fontaine
(1957; see also Y. A. Fontaine, 1958). They found in assays for TSH
activity in trout at different temepratures, that the rat pituitary is relatively more active at 20°C than at 10°C. Eel pituitary, on the other hand,
is more active at 10°C than at 20°C. This remarkable finding deserves
confirmation and further study.
IV. THYROID FUNCTION AND THYROID HORMONE SYNTHESIS
Stated most simply, the function of thyroid gland is to manufacture
thyroid hormone. As will be made clear presently, the thyroid hormones
of fish are relatively small molecules and they are the same in all vertebrates, namely, thyroxine ( T x ) and triiodothyronine ( T3). Other iodocompounds may be manufactured by the thyroid, and even released from
the thyroid tissue, but in amounts and activities that have no apparent
physiological value.
The process of thyroid hormone formation has been studied very
closely because it can be followed to a large extent by the use of radioiodine as a characteristic label. The first phase of the process involves
concentration of iodine (iodide) within the secretory units (follicles) by