The effect of anterior hypophysectomy on thyroid metabolism of radioactive lodine (I131) in male ducks

GEN~;:HAI.

The

AND

COMl’4HITIVE

Effect

2, 574-585 ( 1962)

ESDO(‘HINOLO~:~-

of Anterior Radioactive

Hypophysectomy Iodine (f131)

A. TIXIER-VIDAL Laboratoire

on Thyroid Metabolism in Male Ducks

of

AND I. ASSENMACHER’

d’Histophysiologie du CollBge de France, Paris, et Laborntoire Physiologie de la Faculte’ des Sciences de Montpellie,

de

Rereived June 8, 1962 Using radioiodine (I’“‘) it was found that complete or almost complete hypophyaectomy in ducks results, after a period of 12 days, in the following modifications in thyroid function: (1) The fixation of radioiodine by the thyroid is considerably reduced. (2) Plasma radioactivity is slightly increased. (3) Synthesis of iodotyrosines is not affected, but that of the iodothyronines is retarded; within 72 hours after the injection, however, the latter are as abundant as in the controls. Hormone synthesis is also qualitatively modified, with the almost complete inhibition of thyroxine formation, and with the nearly exclusive production of triiodothyronine. The results show, in comparison with the relevant data dealing with thyroid hormone synthesis in the hypophysectomized rat, that in the hypophysectomized duck the thyroid possesses a considerable degree of autonomous activity as far as the synthesis of triiodothyronine is concerned. It is concluded that triiodothyronine is the first hormone to be formed, and that its transformation to thyroxine is much reduced in the absence of the pituitary. MATERIAL

IXTRODUCTIOiV

It is known that hypophysectomy in birds, as in mammals, results in thyroid hypertrophy, aplasia of the thyroid epithelium, and occasionally in a loss of thyroid weight (Mitchell, 1929; Hill and Parkes, 1934; Nalbandov and Card, 1943; Payne, 1957; Benoit, 1950; Assenmacher, 1958). There are, however, no relevant data on the thyroid metabolism of radioiodine following hypophysectomy in these animals, most of the work concerned with this problem having been carried out on mammals, and then almost entirely on the rat. The object of the present study has therefore been to investigate the effect of hypophysectomy on thyroid function in the male duck, in which animals we have been studying thyroid function for the past few years (Tixier-Vidal and Assenmacher, 1958, 1959, 1960, 1961a,b,c; Assenmacher and Tixier-Vidal, 1959a,b). 1 With the technical assistance of Mme. J,aplant,e and Mmr. Cl. Kagan.

E.

ASD

SOURCE OF ANIMALS

METHODS AND DIET

Nineteen male Pekin ducks, aged 5 months (fully grown and at the onset of sexual maturity), were used for these experiments. Of these ducks, thirteen were hatched on April 12, 1960 in the Laboratory hatchery (Group I). At hatching they were put on a diet containing a large amount of iodide (KI), which is necessary for their health?development (3.57 mg iodide/kg of food for the first six weeks, and then 1.53 mg/kg of food until 2% months). Six other ducks, of the same age and race, were obtained from another hatchery when they were 2% months old (Group II). From this time on. until the beginning of the experiment (that is, for the next 2% months), both groups were kept under the same conditions and put on a diet deficient in iodide, although not completely lacking in it, since it contained 25% fish meal. OPERATIVE

TECHNIQUE AND POSTOPERATIVE CARE

Hypophysectomy specimens between 574

was performed on eleven September 19 and 23, 1960,

HYPOPHYSECTOMY

AND THYROID

following Martins’ technique (1933) as modified by Benoit (1937). With this technique, the posterior hypophyseal lobe remains in situ. Immediately after the operation, the animals were placed, with the controls, in a room maintained at a constant temperature (18°C * 1°C) and given as much food and water as they wanted. It is known that anterior hypophysectomy in the duck results in death by cachexy within about a month; the operated animals were therefore weighed daily. The injection of radioiodine was given before the animals had started to lose weight, but allowing a time lapse of 12 days after the operation. Of the eleven operated animals, only one showed a marked loss of weight and this one had to be rejected, since it was dying hy the end of the experiment. INJECTION

OF RADIOIODINE

AND AUTOPSY

Twelve days after the operation each hypophysectomized duck was given an intramuscular injection of radioiodine I’“’ in physiological serum (250 &/kg of body weight). The control ducks also received injections at the same time. The animals were killed by decapitation at 1 hr, 4 hr, 8 hr, 24 hr, 48 hr, and 72 hr after the injection. Blood was collected over heparine. The thyroids were dissected out and weighed, and the hypophyseal region was fixed in Bouin-Hollande so that the amount of anterior pituitary remaining could be determined. METHODS USED IN STUDYING METABOLISM OF 1131

THE

Plasma. The plasma was separated by centrifuging blood at 2500 revolutions/minute for 15 minutes. Samples of the plasma were weighed and the radioactivity measured in a scintillation counter. An aliquot of the original injected solution was measured in the same way so that plasma radioactivity could be expressed as a percentage of the injected dose. The plasma proteins were precipitated with 20% trichloracetic acid, separated by centrifuging, and washed twice in more trichloracetic acid. They were then brought back into solution with 2 N NaOH, diluted to the original volume, and their radioactivity measured in the scintillation counter. The following calculation of the proteinbound radioiodine (PBILZ1) was made:

pBp31 = Radioactivity of the plasma proteins total radioactivity of plasma Thyroids. Thyroidal

%

activity was measured with

IN DUCKS

575

a Tracerlab scintillation counter with an intervening lead filter. The rate of iodine fixation by the thyroid was determined by comparing the activity of the gland with the activity of an aliquot of the injected solution measured under the same conditions. Thyroid activity was expressed as a percentage of the injected dose, both for 100 mg of thyroid and for the thyroid as a whole. The thyroids were then subjected to trypsin (pancreatin powder Armour) hydrolysis for 72 hr, following the method of Roche et al. (1954). An aliquot was taken from each hydrolyzate for chromatographic analysis; knowing the weight and radioactivity of the whole thyroid, as little as 0.1 cc of the final hydrolyzate was sufficiently radioactive for a quantitative study. Three chromatographic methods were used: Butanol-acetic acid solvent and Butanol-2 N NH,OH solvent for ascending chromatography, and normal Pentanol2 N NH,OH solvent for descending chromatography. One autoradiograph was obtained from each chromatogram. The iodinated thyroid compounds were identified by comparing their positions on the autoradiograph with those of standard compounds ldiiodotyrosine (DIT), 3-53’triiodothyronine (T3), and thyroxine (T4)l spotted on each chromatogram at the same time. The spots were developed by t’reatment with cerium sulphate, which is more sensitive and specific than the ninhydrin method. The distribution of radioactivity along the chromatogram was measured with an automatic cylindrical counter and a Tracerlab integrator. The areas under the curves were measured with a Coradi planimeter. The activities of the different fractions separated by chromatography were expressed as a ratio of the total radioactivity of the chromatogram. STUDY OF THE PITUITARY

REMNANTS

Serial sections of the hypophyseal region were cut at 7 p and stained with PAS-alcian blue. The approximate volume of the remnants of anterior pituitary still remaining in the sella turcica was calculated in the following way: each cellular mass was approximated to an ellipsoid of which two axes (a and b) were measured with a micrometer eye-piece while the third axis (c) was calculated from the number of sections over which the cellular mass extended. The formula 4/37r a-b.c gave the approximate volume (V.a.1. With these results were calculated, in each case, the ratio (R) between the approximate volume of the remaining pituitary and the average volume of an entire duck pituitary measured in the same way (about 4.5 mm?.

576 RESULTS PITUITARY

REMNANTS

Table 1 gives the volumes of the anterior pituitary remnants (V.a.) and the ratio (23) between these measurements and the average volume of a duck pituitary. Of the eleven operated animals only one possessed a significant volume of remaining anterior pituitary, while in two specimens no cellular remnants remained at all. In the other eight specimens, there remained only very small isolated fragments of cellular tissue which would have escaped notice during an examination of the sella turcica under a dissection microscope. Moreover the histological appearance of these fragments suggested a profound reduction in functional activity, for the cells were small and frequently chromophobic, and were grouped into vesicles and interspersed with numerous blood corpuscles. Apart from specimen

WEIGHTS

OF BODY,

THYROID,

Treatment

Group

I

Controls

Hypophysectomieed

Group

II

Controls

Hypophysectomized

“*m=

-\I -. n(n

ZAZ - 1)

ADRENAL Classification number of ducks

AND

TABLE TESTIS

Body weight (kg)

APPROXIMATE ANI) RATIO VOLUME

1‘ABLE 1 VOLUME OF REMAINING PITUIT.IRY BETWEEN THIS AND APPROXIMATE OF ENTIRE PITCITARY UF DUCK

Classification number of duelis

va (mm31

2863 2846 2862

R

0 0 0.00078 0.003 0.006 0.027 0.03 0.046 0.049 0.106 1.02

2883 2849 2854 2844 2896 2884 2867

0 0 l/5800 1/1500 l/750 l/170 t/150 l/100 l/90 l/45 l/-i

No. 2867, therefore, one may consider hypophysectomy to have been complete or very nearly complete. 2 OF HYPOPHYSECTOMIZED Thyroid weight (mg)

2873 2860 2866 2871 2878 2861 mean 2849 2854 2867 2846 2844 2865 2862 mean

2.180 2.200 2.400 2.230 2.350 2.570 2.320 2.400 2.250 2.230 2.620 2.340 2.300 2.200 2.330

112.8 144.5 103.7 117.5 161.0 127.8 127.9 94.7 91.5 82.9 90.3 116.0 84.6 74.2 90.6

2891 2897 2886 mean 2896 2883 2884 mean

2.040 2.330 3.190 2.750 2.520 2.340 2.100 2.330

224.0 204.4 173.7 200.7 221.3 180.5 120.5 174.1

_+ 8.74”

k 4.95

rk 14.64

k 29.26

AND CONTROL

Adrenal weight (mg)

246.0 229.0 249.9 241.2 218.5 217.6 233.7 134.0 145.8 148.6 169.2 153.0 167.5 157.0 153.6 187.3 238.3 253.5 226.4 195.5 186.4 136.0 172.6

fk 5.72

+ 4.67

f 20.0

+ 18.49

Test;;

3.800 15.000 6.400 2.300 10.200 8.400 7.683 0.428 0.896 0.770 0.553 0.515 0.765 0.943 0.695 12.900 0.785 0.613 4.767 0.432 0.414 0.438 0.428

DUCKS wyight m

k 1.879

2~ 0.075

5~ 4.062

rk 0.0072

HYPOPHYBECTOMY WEIGHTS

OF THYROID, ADRENALS, TESTES (TABLE 2)

AND THYROID

AND

In both Groups I and II it was found that hypophysectomy resulted in the wellknown marked loss in testicular weight in those cases where maturation changes had already begun. In addition, hypophysectomy caused a significant loss in both thyroid and adrenal weight in animals from Group I ; this loss was as marked in specimen No. 2867, possessing a considerable volume of remaining pituitary, as in specimen No. 2844, in which hypophysectomy was complete. This fall in thyroid and adrenal weight was far less pronounced in animals from Group II, in which group the thyroid weight of the controls was higher than that. of the controls from Group I. METABOLISM

Fhation

OF RADIOIODINE

Table 3 and of the results. In

by the Thyroid,

Fig. 1 gives a summary

0%

IN DUCKS

577

rate of iodine fixation increases as a function of time following the injection. This increase is very rapid during the first 4 hr, but then becomes much slower. The rate of iodine fixation per 100 mg of thyroid was noticeably less in those animals from Group II which were obtained from a different source, even though they had been kept under the same conditions as animals from Group I for 21/ months before the beginning of the experiment. This difference is less obvious if one compares the rate of fixation by the entire thyroids since specimens from Group II had larger thyroids than those from Group I. Anterior hypophysectomy resu!ted in a great decrease in the thyroid fixation of radioiodine, even when a considerable volume of pituitary still remained (specimen No. 2867). Taking individual variation into account, the rate of iodine fixation per 100 mg of thyroid in the hypophysectomized ducks was only 2.5-570 that of the controls. As in the controls, the rate of fixation varied but little following the first 24 hr after injection.

b%-

hours aller ,npc,,on

FIG. 1. Uptake of I’” by thyroids of hypophysectomized and control ducks, expressed as percentage of the injected dose.

the controls, as we have noticed previously (Tixier-Vidal and Assenmacher, 1959) the

FIG. 2. PBFS and activity of 1000 gm of plasma, of hypophysectomiaed and of control ducks, expressed as percentage of the injected dose.

578

TIXIER-VII).lL

.lSD

hSSENM.1(:HEH

TABLE3 Ilypophysectomized HOWS after p31 injwtion

1

4 8 24 18 72

Clasuification number of ducks

lJ$$ H,r

2846 0 2865 l/5000 2854 l/150 2884" l/45 2883e l/750 2867 l/4 2944 1/lOO 2896" l/90 2862 l/5800 2849 l/170

by 100 mg t,hyroid

Tz;;z of 1’31

0.025 0.023 0.069 0.058

Controls Activity of plasma ,‘I 000 gmc

0.190

0.170

0.080 0.310 0.50 0.340 0.440 0.270 0.680

0.095 0.560

1.070 1.133 1.077 0.983 0.080

0.414

0.13‘4

0.394 0.075 0.200 0.645

0.005 0.006 0.004 0.003

pHI13’d

Classification number of ducks

0.75 2866 0.45 2871 0.70 2861 0.60 2886' 4.30 2897" 3.70 2873 29.00 2878 54.50 2891e 45.00 2860 53.00

ug$” by 100 w thyroid

0.73 2.42 3.72 2.50 2.20 4.80 4.30 2.0 5.25

‘;M;$ of 1’3’h

0.76 2.84 1.75 1.30 -1.45 5.40 6.90 1.25 7.60

;\ctivity of plasma 1000 grw

0.935 0.938 0.925 0.444 0.014 0.019

0.003 0.003 0.002

p~,m”

0.55 0.73 0.70 0.95 20.0 15.5

51.0 51.0 61.5

a R, see Tahle 1. b Thyroid uptake of P expressed as percentage of injected dose for 100 mg of thyroid and for the whole gland. c Radioaetivitv of plasma expressed as percentage of injected dose for 1000 gm of plasma. d PBI’31 = Ra”dioa&ivity of plasma proteins 7&I Total radioactivity of plasma of hypophysectomiaed and control ducks. e Group II.

Plasma Activity and PBP31 (Table 3, Fig. 2). In the controls, as we have described previously (Tixier-Vidal and Assenmacher, 1959), plasma radioactivity falls rapidly during the first 24 hr following the injection, but after this decreases very much more slowly. Inversely, the PBI131, which may be regarded as an indicator of the presence of labelled thyroid hormones in the plasma, only reaches a significant value after the first 24 hr, increasing most markedly 24-48 hr after injection. Plasma radioactivity does not change much once the PBI131 has reached its maximum value. In the operated animals the decrease in plasma radioactivity and the increase in PBI131 occurred much more slowIy than in the controls. However, 72 hours after the injection, the values were not greatly different from those of the controls. Hypophysectomy thus retards, but does not inhibit, the production of labelled hormanes. Labelled Iodinated Compounds in the Thyroid (Table 4, Fig. 3). The numerical results are given in Table 4, which shows

the proport.ions of the different labelled thyroid constituents after separation by trypsin hydrolysis. The results are expressed as a percentage of the total radioactivity of the sample used for the chromatogram. In the controls we found the same characteristics which we had noted previously in the duck (Tixier-Vidal and Assenmacher, 1959). Thyronine synthesis occurs rapidly, and 1 hr after injection thyronines constitute 12.3% of the iodinated compounds in the thyroid. The first hormone to be formed is 3-5-3’ triiodothyronine (T3). Thyroxine (T4) appears later (4 hr after the injection) and in smaller amounts; it does not reach as great a concentration as that of triiodothyronine until 48-72 hr after the injection (Fig. 4). Coincident with these changes, the ratio MIT/DIT decreaseswith time following the injection, due to a reduction in the concentration of i’&T and an increase in DIT. The inorganic iodine content of the thyroid remains very low. Two of the animals from Group II were noteworthy for a more rapid decrease in the ratio MIT/DIT and for a

HYPOPHYSECTOMY

AND

THYROID

IN

DUCKS

579

FIG. 3. Radioautographs of chromatograms showing the distribution of Ilzl in pancreatin hydrolyzates of thyroids from hypophysectomiaed and control ducks examined 4 hr, 24 hr, and 48 hr after I’= injection.

lower than normal concentration of iodothyronines. Anterior hypophysectomy has a marked ;mK---

------i

-----g

.

r”M$40.

.

ESFBz

--_-

IO,

.-

contrd5

o--m

hypoph&

44 FIG.

8

24

2

72

4

FIG. 4. Percentage of T3 in the total iodothyronines from thyroids of hypophysectomized and control ducks.

effect. on the proportions of certain of the iodinated compounds in the thyroid. The proportion of inorganic iodine was greater than normal during the first 24 hr following the injection, but after this time it became of the same order as in the controls. The rate at which t.he iodotyrosines were formed was not affected; 1 hr after the injection these were as abundant as in the controls. The formation of the iodothyronines, on the other hand, was considerably retarded; they did not appear until 4 hr after the injection, and were generally slightly less abundant then in the controls. Hormone synthesis is thus retarded. Indeed, the mechanism itself of hormone synthesis appears to be modified, since triiodothyronine was the only detectable hormone

a Group

72

II.

,,,

*.*

/.(,

,

/

25.5

2.6

2849

,

23.0

28.4

1.2

5.3

2896”

17.2

29.0

26.9

22.1

2862

4.2

2844

5.2

2883”

48

3.4

2867

24

4.8

2884”

8

5.4

2854

.

21 .o

22.0

7.2

2865

4

MIT

26.1

Iodine

PROPORTIONS

9.7

2846

1

Classification number of ducks

RELATIVE

,.

48.0

43.5

49.2

,, <

.,

73.5

66.5

77.6

68.7

73.7

44.7

51.5

83.7

65.9

65.5

65.0

74.9

DtT

MIT

56.8

43.8

44.5

43.0

48.8

DIT

Iodotyrosines

Hypophysectomized

/,

0.53

0.53

0.58

0.33

0.65

0.47

0.50

0.47

0.51

0.53

MIT DIT

.L

OF RADIOIODINATED AMINO EXPRESSED AS PERCENTAGE

“~.

16.3

14.9

7.9

12.9

.,

11.30

12.1

11.75

9.50

9.95

0

,,,

T3 + T4

..

Iodothyronines

. . . .,

;oo 0 100 15 85 c 100 6 94 8 92

0 100 0 100 0 100

&q

.

.

.a

..

2860

2891”

2878

2897”

2873

2886”

2861

2871

2866

.,.,

Classification number of ducks

.

*

4.0

4.3

4.3

3.5

3.5

3.5

1.15

3.9

6.1

Iodine

.

13.9

18.4

21.4

18.6

24.2

24.0

31.5

27.9

31.0

MIT

.

.

.

Iodotyronines

TABLE 4 ACIDS IN THYROIDS OF HYPOPHYSECTOMIZED OF TOTAL RADIOACTIVITY OF THE GLAND

,.

55

52

45.2

53.7

49.8

48.7

54.5

54.7

43.8

DIT

Controls

AND

.,

.

68.9

70.4

66.6

74.3

74.0

72.7

86.0

82.6

74.8

D&i-

MIT

..

CONTROL

..*.

.*.

0.25

0.35

0.45

0.33

0.50

0.49

0.58

0.51

0.70

MIT DIT

DUCKS,

..

11.2

12

17.1

6.2

15

13.8

10.4

10

12.3

T3 + T4

Iodothyronines

./

0 100 24 76 14 86 7 !a 40 60 38 62 33 67 45 55 51 49

T’: z

,

HYPOPHYSECTOMY

AND

THYROID

IN

DUCKS

581

In the hypophysectomized rat the radioactivity of the blood is distinctly higher than normal (Morton et al., 1942; Albert et al., 1952) ; this results both from the reduced ability of the thyroid to fix iodine and from a lowered rate of iodine clearance by the kidneys (Albert et al., 1952). Following hypophysectomy in the duck the blood radioactivity is slightly higher than in the controls, but the difference is far too small to compensate for the drop in thyroidal radioactivity. It must therefore be assumed that in the hypophysectomized duck, in contrast to what is observed in the hypophysectomized rat, there is a rapid elimination of radioiodine from the body. The organic fraction of plasma iodine is much reduced in the hypophysect,omized rat, even after 24 hr following the injection (Morton et al., 1942). After an injection of a very large dose of radioiodine (Taurog et al., 1960)) the level of labelled thyroxine in the plasma is even less than 1% of that observed in control rats injected with tracer doses of Y. In the duck, however, measurements of radioactivity made after the first 24 hr give a different result: after this time there is little difference between the hypophysectomized and control ducks DISCUSSION in either the values of PBI131 or in the level The influence of hypophysectomy on of radioactivity due to the plasma proteins. thyroid function in mammals and espe- This may be associated with the process of cially in the rat has been the subject of iodine “re-utilization” which would seem numerous investigations; the results of to be highly developed in the duck. We these studies must now be considered to- have, in fact, remarked on this ability gether with ours. Leblond et al. (1940) earlier (Tixier-Vidal and Assenmacher, were the first to show that hypophysectomy 1959) : in the unoperated duck, thyroidal resulted in a reduced ability to concentrate radioactivit,y remains constant over a radioiodine (11z8). This observation has period of several days following the injecsince been frequently confirmed using Il”l, tion. This prolonged retention of thyroidal both in the rat (Leblond and Sue, 1941; iodine has previously been noted in certain Morton et al., 1942; Cort,ell and Rawson, other birds-the chick (Wahlberg, 1955), 1944; Albert and Lorenz, 1951; Randall the laying hen (Pipes and Premachandra, et al., 1951; Taurog et al., 1957, 1958a,b) 1958), the wood-pigeon (de la Queriere and and in the mouse (Wollmann and Scow, Lachiver, 1957; Poivilliers de la Queriere,’ 1953). In this respect, therefore, the duck 1960)) and the White-throated sparrow and reacts in the same way as the rat and the adult weaver-finch (Kobayashi et al., mouse. It has previously been observed 1960). Kobayashi et al. (1960) consider t.hat the thyroid in the hypophysectomized that this phenomenon is perhaps the most chick embryo fixes less iodine towards the characteristic of thyroid function in birds. end of incubation than that of the con- ((‘an avian pattern of thyroidal function”). trols (Maraud et al., 1957). Whatever its mechanism, the same phe-’ up to 24 hr after the inject,ion and remained the predominant hormone after this time (Fig. 4). Correspondingly, except for one case (No. 2844)) the ratio MIT/DTT remained in the region of l/s. The rate of transformation of iodotyrosines to iodothyronines is thus distinctly reduced, and the synthesis of T4 to a great extent inhibited. In summary, therefore, the main modifications in thyroid function resulting from hypophysectomy are the following: 1. The fixation of radioiodine is greatly decreased. Quantitatively this is the most marked change. 2. Plasma radioactivity is increased. 3. The rate of iodothyronine synthesis is decreased but that of iodotyrosine synthesis is not affected. Hormone synthesis is qualitatively modified, thyroxine production being t.o a large extent inhibited in favor of an almost exclusive production of T3. Correlated with t.he reduced rate of hormone synthesis, the percentage of plasma radioactivity due to the plasma proteins increases far more slowly in hypophysectomized ducks than in the controls.

582

TIXIEK-VIDIL

.iYYl

nomenon is also encountered in hypophysectomized ducks. The problem of the effect of hypophysectomy on thyroid hormone synthesis m the rat has stimulated numerous investigations and led to many controversial interpretations. Most, authors are agreed that t,he synthesis of the iodotyrosines, MIT and DIT, is not affected; the incorporation of radioiodine into the iodotyrosines is as rapid after hypophysectomy as in the cont.rols. Opinions differ, however, over the question of iodothyronine synt.hesis. According to Morton et al. (1942) hypophysectomy acts as a powerful check to thyroxine synthesis. Albert and Lorenz (1951), who subjected the protein fraction of t,hc thyroid to sodium hydrolysis, found, on the contrary, that although thyroxine production occurs more slowly after hypophysectomy, the hormone nevertheless reaches a considerable level; the effect of hypophysectomy would therefore appear to be quantitative rather than qualitative. Roche et al. (1953) arrived at the same conclusion after subject,ing the thyroid to trypsin hydrolysis, which preserves far better the original constituents of the thyroid and also permits the isolation of triiodothyronine (T3) (Roche et al., 1952a,b; Gross and Pitt-Rivers, 1952). These workers have thus concluded that hypophysectomy retards iodothyronine synthesis, but’ that given sufficient time after the injection the hormones become almost as abundant as in the controls. They found furthermore that the relative proportions of T3 and T4 are not altered. In contrast to these results. Taurog et al. (1957, 1958a,b, 1960)) although using pancreatic hydrolysis, concluded that thyroid activity is much reduced. They found that hypophysectomy acts as a strong brake on T3 and T4 synthesis, and that the concentration of MIT is much increased, although that of DIT remains normal. According to these auhypophysectomy would thors, therefore, appear to affect primarily the conversion of MIT to DIT, and, as a consequence, the formation of hormones. Our own observations on hypophysectomized ducks have led us t.o conclude, like

ISSENM.4CHER

Roche et ctI. (1953), that the thyroid possesses a considerable degree of autonomous activity. As t.hese authors have found in the rat, we, too, have found in the hypophysectomized duck that, allowing sufficient time after the injection, the level of radioactive thyronines approaches that of the controls. Besides this, we have noticed another effect which has not, as far as we know, been reported for mammals: the almost complete inhibition of thyroxine synthesis following hypophysectomy, and the almost exclusive synthesis of T3. Furthermore, the MIT: DIT ratio cxpressed in terms of radioactivity remains in the region of 1: 2, and thus in the region of 1: 1 as far as the number of molecules are concerned, clearly indicating that in the absence of the pituitary, synthesis of T3 occurs by the coupling of MIT + DIT. Two theories may be advanced to explain the role of the anterior pituitary in thyroxine synthesis in ducks: either the pituitary has a specific effect on the transformation of T3 into T4 or, alternatively, it enhances the speed of enzyme reactions in the t,hyroid, favoring the rapid formation of T4 in the intact animals while t.he proportion of MIT to DIT is decreased (see Fig, 4). This second hypothesis seems to us to be the more likely one, in view of the indisputable presence of a low concentration of T4 in t.he two animals killed 72 hr after the inject,ion, and in which there remained only negligible remnants of pituitary. It is possible that, the absence of thyroxine in the thyroid of those ducks examined at earlier stages is the result of a more rapid liberation of this hormone into the blood. Unfortunately, the very low level of radioactivity in the blood in ducks (about 200 times lower than in rats) prevented us from isolating the plasma thyronines. Whatever its explanation, t,his phenomenon, which we have observed at the thyroid level, is very characteristic: in the absence of a pituit,ary the synthesis of T3 occurs normally by the coupling of MIT and DIT while the synthesis of T4 is both very slow and very feeble. These results are in harmony with those of Feuer (19591, who studied the appearance of

HYPOPHYSECTOMY

.4ND

thyroid hormones in the blood of the rat and the rabbit, both in control animals and in those treated with thyrotropic hormone. He concluded (1) that T3 is a precursor of T4, and (2) that t’he thyrotropic hormone accelerates the transformat,ion of T3 into T4. These results support both the suggestion advanced by Roche (Roche et al., 1953 and Roche and Michel, 1955) that T3 is a precursor of T4, and also the observations made by Lachiver and Leloup (1955)) who found that, in rats given diets of different iodine contents the ratios MIT/DIT and T3/T4 change in the same way. The peculiar abundance of triiodothyronine in our control ducks merits some further notice. The presence of this hormone has previously been noted in the chick (Vlijm, 1958; Shellabarger and PittRivers, 1958; Kobayashi and Gorbman, 1960), the duck (Tixier-Vidal and Assenmacher, 19581, and in the White-throated sparrow and the weaver-finch (Kobayashi and Gorbman, 1960; Kobayashi et al., 1960). It is generally present only in low concentrations in these species except in the duck, in which, at certain times of the year, it is found to be abundant even at 72 hr after an injection of radioiodine, as was the case, for example, in the control ducks of the present experiment. Except in the young cockerel (Vlijm, 1958)) in which, as in the duck, the decrease of T3 coincides with the increase of T4, there appears to be no correlation, in those species of birds which have been studied, between the level of T4 and that of T3, the latter always remaining low no matter at. what interval after the injection it is measured. To explain this peculiarity in ducks one may wonder, in view of the work of Leloup and Lachiver (1955)) whether, as these authors noted for the rat, the preponderance of T3 is not perhaps determined by the iodine content of the diet. The relevant data dealing with the effects of dietary iodine content on the proportions of the different iodinated thyroid constituents seem, however, to be far less understood for birds than they are for mammals. The few researches made into this question (Kobayashi and Gorbman, 1960; Kobapashi et al., 1960) suggest that,

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variations in iodine intake have but little effect on the proportions of the different thyroid compounds. This appears to be the case also in ducks (unpublished observations). Furthermore, although T3 is abundant in the duck thyroid, the level of MIT is always less than that of DIT, which is the reverse situation to that observed by Leloup and Lachiver (1955) in the thyroid of rats which are deficient in iodine and in which the ratio MIT/DIT was found to be about 2 and T3/T4 about 1.5. Lastly, the rate of iodine fixation by the thyroid in the control ducks was low, which would seem to indicate a high level of iodine (I”“l in the thyroid rather than a deficiency in thyrotropic hormone. It should be remembered that although the ducks had been kept on a diet poor in iodine for 21,$ months preceding the experiment, they had previously received considerable doses of iodide mixed in with their food. It appears, therefore, that in birds the thyroid is able both to fix and retain large doses of iodine (cf. bibliography in Vlijm, 1958; Poivilliers de la Queriere, 1960). In conclusion, then, it does not seem that the high rate of T3 production in the thyroid of the control ducks is a result of iodine deficiency. But whatever the explanation of this may be, it would seem to const,itute a characteristic peculiar to ducks, and it no doubt explains why the lack of thyroxine synthesis was so marked after hypophysectomy. REFERENCES 8., .IND LORENZ, N. (1951). Effect of hypophysectomy on the intrathyroidal metabolism of I’“‘. Proc. Sot. Erptl. Biol. Med. 77, 204-205. ALBERT, A., TENNEY, A., AKD LORENZ, N, (1952). The effect of hypophysectomy on the renal clearance of I13’. Endocrinology 50, 327-330. ASSENMACHER, I. (1958). Recherches sur le contrale hypothalamique de la fonction gonadotrope pr6hypophysaire chez le Canard. Arch. mat. microscop. morphol. exptl. 47, 448-557. ASSENMACHER, I., AND TIXIER-VIDAL, A. (1959a). Etude B I’aide du radioiode Ii31 du fonctionnement thyroydien chew le Canard P&kin $ au tours de l’automne et de l’hiver. I.-Fixation thyroi’dienne et iode marquk fix6 aux proteines (P.B.I.). Corn@. rend. sot. biol. 153, 1994-1999. ALBERT,

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2035-2038. TXIER-VIDAL, A., AND ASSENMACHER, I. (1959). Etude des syntheses iodees thyroidiennes chez le Canard Pkkin 8 maintenu B temperature constante. Premiers rksultats sur l’influence de la lumiere et de l’obscuritk permanentes. Compt. rend. sot. biol. 153, 721-726. TLKIER-VIDAL, A., AND ASSENMACHER, I. (1969). Etude & l’aide du radioiode ‘Y du fonctionnement thyroidien chez le Canard PQkin 3 au

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