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JEZ 0543
EXPERIMENTAL ZOOLOGY 284:492–499 (1999)
Evaluation of Thyroid Hormone Economy in
Elasmobranch Fishes, With Measurements of
Hepatic 5¢-Monodeiodinase Activity in Wild Dogfish
Department of Zoology, University of Guelph, Guelph, Ontario, Canada
N1G 2W1
Department of Biomedical Sciences, University of Guelph, Guelph, Ontario,
Canada N1G 2W1
This paper reviews the current understanding of thyroid hormone economy and
homeostasis in elasmobranch fishes and considers those measures of the activity of the hypothalamic-pituitary gland-thyroid gland-peripheral tissue axis that are necessary for adequate assessment of thyroid hormone physiology. In particular, we focus on the value of measuring hepatic
5′-monodeiodinase (5′-MDA) activity as an indicator of the animal’s cellular production rate of the
active thyroid hormone, triiodo-L-thyronine (T3). We also examine the characteristics of hepatic 5′MDA activity, in vitro, in adult female dogfish (Squalus acanthias) collected from Passamaquoddy
Bay, New Brunswick, Canada, and in the embryos that they were carrying. T3 production from T4
by hepatic homogenates in vitro was time- and temperature-dependent, and was enhanced by the
presence of a thiol donor. Michaelis constant (Km) and maximum reaction velocity (Vmax) values
were 3.8 × 10–7 M and 0.29 nM T3/mg protein/hr, respectively. The inclusion of trimethylamine-Noxide (TMAO) or a mixture of urea, TMAO, betaine and sarcosine significantly enhanced T3 production. Hepatic 5′-MDA activity was depressed in fish fasted for 7 days. J. Exp. Zool. 284:492–499,
1999. © 1999 Wiley-Liss, Inc.
The thyroid gland or its progenitor, the endostyle, is a characteristic of all vertebrates, and its
essential structure has been highly conserved
throughout the vertebrate lineage (Leatherland,
’94). The gland is composed of follicles, formed by
a single layer of thyrocytes, surrounding a colloidfilled lumen. The principal constituent of the colloid is the protein thyroglobulin, which contains
within its structure the iodinated tyrosine units,
monoiodotyrosine (MIT) and diiodotyrosine (DIT).
When combined, these form the two hormones
that are released from the thyroid gland, tetraiodothyronine, or L-thyroxine (T4) and triiodo-Lthyronine (T3).
Our understanding of the processes associated
with thyroid hormone synthesis and release is
based largely on studies of mammals (Lissitzky
et al., ’90), and few studies have been made in
other classes of vertebrates; however, the information that is available suggests that the processes are very similar among vertebrates.
Hormone synthesis and release
The protein that ultimately forms the iodinated
thyroglobulin is synthesized by the thyrocytes and
secreted, by exocytosis, into the lumen of the follicle. As the protein is secreted, it is iodinated at
specific tyrosine sites, resulting in the formation
of MIT and DIT units (Xiao et al., ’96). Further
processing of the protein results in specific coupling of MIT and DIT units, to form T4 (two DIT
units) or T3 (DIT + MIT). However, at this stage
T4 and T3 are still part of the thyroglobulin matrix, and must be released from thyroglobulin in
order to become hormonally viable.
Both the initial iodination of the tyrosines and
the coupling of the DIT and MIT residues are catalyzed by thyroid peroxidase and NADPH oxidase.
These enzymes are integral proteins of the apical
membrane of the thyrocytes. They have their ac-
Grant sponsor: NSERC.
*Correspondence to: Dr. J.F. Leatherland, Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Guelph,
ON N1G 2W1, Canada. E-mail:
tive site projecting into the lumen, and consequently, the iodination process per se occurs
within the lumen (Golstein et al., ’95). The iodide
required for iodination is provided by the thyrocytes, which actively sequester iodide from the
blood by means of electrolytic pumps located on
the basal cell membrane (Golstein et al., ’92). The
iodide is then converted oxidatively to iodine and
attached to the tyrosine units. The thyrocytes synthesize the peroxidase and NADPH oxidase, and
secrete the hydrogen peroxide that is needed for
the oxidative process.
The thyroid hormones, T4 and T3, are released
from the thyroglobulin by proteolytic digestion of
the protein (Lissitzky et al., ’90). Colloid is taken
up, by endocytosis, at the apical pole of the
thyrocytes and membrane-defined vesicles appear
within the cytoplasm. Lysosomes, also synthesized
by the thyrocytes, fuse with the colloid-filled
vesicles, the thyroglobulin is proteolytically digested, and T4 and T3 are released. The hormones
diffuse into the cytoplasm of the thyrocytes and
then out of the cell. MIT and DIT residues are
also released from the thyroglobulin during proteolysis, but are largely degraded and the iodide
and amino acids recycled intracellularly.
Hormone transport in the blood, peripheral
metabolism of thyroid hormone and site of
T3 action in target cells
In all vertebrates studied to date, a significant
amount (up to 99% in some mammals) of the
thyroid hormones in the blood are attached to
specific transport proteins (Hayashi and Refetoff,
’95). This binding of the hormones to transport
proteins allows a greater concentration of hormone to be carried in the blood than could be
accomplished if the hormone were in simple
solution. In addition, loss of hormone via the kidney (and gills in aquatic vertebrates) is prevented, and there is a readily available supply
of hormone present in the blood to meet the
sometimes rapidly changing needs of the animal.
An additional advantage for transporting thyroid hormones in a protein-bound form is that
as blood perfuses an organ system, as free hormone leaves the blood to enter target cells, bound
hormone dissociates from the transport protein.
Thus, there is a source of available hormone in
the blood in all regions of an organ system; this
is particularly important for large organ systems,
such as the liver.
However, only the unbound (free) thyroid hormone in the blood is available for interaction with
receptors in target cells. Consequently, a dynamic
equilibrium is established between free and protein-bound thyroid hormones in the blood. Factors
that change the levels of the transport proteins,
or interfere with the binding of hormone to these
proteins will bring about changes in the dynamic
equilibrium of free and bound hormones, and thus
alter total thyroid hormone levels in the blood.
Although both T3 and T4 are present in the
blood of vertebrates, thyroid hormone-sensitive
tissues have only T3 receptors. Thus, only T3 has
biological activity and T4 acts as a pro-hormone,
available for enzymatic conversion (monodeiodination) to form T3. In those taxa studied to date,
most of the T3 present in the general circulation
is generated by the monodeiodination of T4 in nonthyroidal tissues. The enzyme involved, 5′-monodeiodinase (5′-MDA), catalyzes the removal of one
iodide from the outer tyrosine ring of T4, and has
been characterized in representatives of all
classes of vertebrates, except elasmobranch fishes
(mammals: Visser, ’78; Toyoda et al., ’97; birds:
McNabb et al., ’86; amphibians: Galton and
Hiebert, ’87; teleost fishes: Leatherland et al., ’90b;
lampreys: Leatherland et al.,’90a).
T4 can also be subjected to monodeiodination by
removal of an iodide from the inner tyrosine ring,
a reaction that is catalyzed by 5-monodeiodinase
(5-MDA) (Fig. 1). The product, called reverse T3
(rT3), has no proven biological activity to date.
However, the degradation of T4 via a metabolite
that has no biological activity allows for the elimination of excess T4 without generating the potent
hormone, T3. The selective expression of 5′- and
5-MDA thus offers a cell the means by which the
intracellular needs of T3 can be finely regulated.
The cells of some organ systems manufacture
sufficient T3 to meet only their own immediate
needs. However, other peripheral tissues, such as
the liver and kidney, not only supply their own
intracellular needs, but also release T3 into the
general circulation.
T3 interacts with its target cell at receptor
(T3R) sites that are located at specific positions
on the nuclear DNA (Brent, ’94; Oppenheimer et
al., ’95; Wagner et al., ’95). Additional nonnuclear sites of interaction have also been proposed, although the question as to whether these
are receptor-mediated is still controversial. The
nuclear T3R forms a heterodimer with the retinol-X receptor, and in the presence of T3, the
receptor is activated and, depending on the specific target cell, will either stimulate or inhibit
the expression of specific genes.
that code for 5′- and 5-MDA, and by the selective
activation of these enzymes, as required. “Nonthyroidal” hormones, such as growth hormone
(GH), can induce changes in 5′- and possibly 5MDA activity. As a result, the overall control of
production of the biologically active thyroid hormone T3 imparts a considerable stability (homeostasis) to the regulation of the amount of hormone
that is available in the blood and in specific target tissues.
Evaluation of thyroid hormone
homeostasis and economy
Fig. 1. Schematic diagram showing the “typical” vertebrate model of the regulation of blood T4 levels by TSH regulation of thyroid gland synthesis and release, and the
production of T3 and rT3 by outer- and inner ring monodeiodination of T4, respectively.
Regulation of thyroid hormone secretion
Thyroid gland activity in mammals is largely
under the direct control of the pituitary hormone,
thyrotropin (TSH). TSH stimulates many of the
mechanisms associated with the synthesis of thyroglobulin, as well as the processes associated with
the release of the hormones from the thyroglobulin. The synthesis and release of TSH are, in turn,
stimulated by the hypothalamic hormone, thyrotropin-releasing hormone (TRH). In addition, several
neurotransmitters influence the synthesis and/or
release of TRH. This is the so-called hypothalamus-pituitary gland-thyroid gland (HPT) axis.
There are marked species differences with regard
to the dominant factors in the HPT axis of different vertebrates. For example, in some teleost
fishes, TRH may play only a minor role, with
dopamine, a potent inhibitor of TSH release, playing the major regulatory function (Leatherland,
’94). However, the fundamental HPT axis has been
well conserved throughout vertebrate classes.
In vitro studies have shown that there is a negative feedback regulation of TRH secretion by TSH.
Moreover, in these experimental situations, TSH
itself may be autoregulatory. In addition, T4, and
to a lesser extent T3, exert a potent negative feedback control over the synthesis and secretion (release) of TSH, probably acting both at the level of
the hypothalamus and the thyrotrophs. Thus,
there is considerable stability in the maintenance
of blood T4 levels. Furthermore, the production of
T3 from T4 by peripheral tissues is itself highly
regulated, by the selective expression of the genes
The various negative feedback control systems
that link the hypothalamus, pituitary gland and
thyroid gland provide a significant stabilization
of blood thyroid hormone levels. Furthermore, although the total blood hormone concentrations
may be affected by several factors, including
changes in the concentration of transport proteins,
the free T3 concentrations in the blood tend to be
adjusted dynamically to satisfy the needs of the
animal. Added to this is the control of T3 and rT3
production by target tissues themselves. These
regulatory processes, acting in concert, impart considerable stability to the overall thyroid hormone
economy of the animal, and T3 homeostasis is
maintained within fine limits.
In order to monitor changes in the animal’s thyroidal status it is necessary to measure several
parameters of thyroid physiology (Table 1). Measurement of only one parameter may not provide
sufficient information to facilitate interpretation.
Thyroid hormone economy and
homeostasis in elasmobranchs
Although it is assumed that many of the processes governing thyroid hormone homeostasis in
elasmobranchs are similar to those found in mammals, there is very little direct evidence available.
The morphology of the elasmobranch thyroid
gland has received some attention (Olivereau, ’49;
Woodhead, ’66; Honma et al., ’87), and the sensitivity of the elasmobranch gland to environmental goitrogens is well known (Leatherland et al.,
’99). In addition, blood total thyroid hormone levels have been measured in a few elasmobranch
species, and attempts have been made to relate
changes in thyroid hormone concentrations to season and the reproductive state of the animals (e.g.,
Crow et al., ’99; Volkoff et al., ’99). However, compared with other vertebrate classes, the physiology of thyroid function in elasmobranch fishes has
received very little attention. Consequently, there
TABLE 1. Variables to be measured for the assessment
of thyroid function*
Expression of mRNA for TRH and other hypothalamic
Thyroid gland
Epithelial cell height and colloid content1
Rate of iodide incorporation2
Thyroid peroxidase activity
Blood plasma or serum
Total thyroid hormone concentration3
Free thyroid hormone concentration
Transport protein concentration
Thyrotrophin concentration
TRH (and other hypothalamic hormone) concentration
Thyroid hormone clearance
Peripheral (non-thyroidal) tissues
Tissue 5′-monodeiodinase (5′-MDA) activity4
Tissue 5-monodeiodinase (5-MDA) activity
Expression of mRNA for 5′- and 5-MDA
Expression of mRNA for T3 receptor (T3R)
T3R content of target tissues
*Superscripts indicate that some work has been documented on this
aspect of thyroid hormone physiology for elasmobranch fishes:
Olivereau (’49), Woodhead (’66), Honma et al. (’87); 2Dimond (’63);
Crow et al. (’99), Volkoff et al. (’99); 4Leary et al. (this paper).
are major gaps in knowledge (Table 1), and interpretation of the available data has to be made
with care. Of particular note is the absence of any
work related to the characterization of MDAs in
peripheral tissues of elasmobranchs. The following section reports on such a study of hepatic 5′MDA in dogfish, Squalus acanthias, collected from
the wild.
Hepatic 5¢-monodeiodinase activity
in Squalus acanthias
To date, there have been no studies of peripheral monodeiodination of T4 in any elasmobranch
species. As discussed above, this is a key aspect
of thyroid hormone homeostasis since monodeiodination of T4 by peripheral tissues represents the
major T3 generating process, and some of the earliest responses to changing physiological state of
the animal are to be found in the altered MDA
activity in peripheral tissues. For example, in teleostean fishes, 5′-MDA activity falls rapidly when
animals are fasted (Farbridge and Leatherland,
’92; Leatherland, ’94). As a consequence of this
lability, measurements of MDA activity in tissues
of fish taken from the wild have proven difficult
because most wild species will not readily adapt
to captive conditions.
The study of MDA activity in elasmobranchs is
further complicated by the fact that appropriate
enzyme assay conditions, vis-à-vis the potential ef-
fects of organic osmolytes on tissue enzyme activity, have not been determined. These osmolytes,
such as urea and trimethylamine-N-oxide (TMAO),
are known to be required for optimal enzyme activity for some metabolic enzymes (Yancey and
Somero, ’80; Lin and Timasheff, ’94). Thus, we undertook to investigate the requirements for, and
effects of these osmolytes on hepatic 5′-MDA activity in adult female dogfish collected from the north
Atlantic Ocean, all of which showed evidence of recent feeding activity.
The animals were sacrificed and the enzyme assays performed shortly after their capture to minimize any loss of enzyme activity associated with
food deprivation. Additional animals were sampled
after a fasting period to evaluate whether the response to food deprivation in this species is comparable to that of salmonid teleostean species. We
also measured hepatic 5′-MDA activity in the embryos taken from the pregnant dogfish. These studies were of considerable interest since parallel
studies in teleostean species (tilapia, Oreochromis
mossambicus: Reddy et al., ’92; rainbow trout,
Oncorhynchus mykiss and Arctic charr, Salvelinus
alpinus: P.K. Reddy, M.N. Khan and J.F. Leatherland, unpublished data) have failed to detect 5′MDA activity in yolksac embryos; enzyme activity
appeared only after full yolk absorption.
Five adult female dogfish (2.68 ± 0.25 kg, range
2.1–3.4 kg) were caught by otter trawl from
Passamaquoddy Bay, New Brunswick, and placed
temporarily in a live well aboard the trawling vessel until transfer to 2 m3 aquaria supplied with
continuous running sea water at 15°C at the
Huntsman Marine Science Centre. Three animals
were sampled following a 6 hr acclimation, and
the two remaining animals were sampled after 7
days; the fish were not fed during this period. Five
embryos, obtained from three of the pregnant females, were also used in the study.
For comparative purposes, we examined the effects of organic osmolytes on 5′-MDA activity in
both rainbow trout and dogfish liver homogenates.
Trout were chosen because the kinetics of 5′-MDA
are well studied in this species, and in vivo organic osmolyte levels are very low. Three immature rainbow trout (410 ± 9 g, range 394–424 g)
were acclimated for at least 2 months to laboratory conditions in constantly running aerated well
water at 10°C, and were fed daily, ad libitum, with
a commercial trout chow (Martin’s Feed Mills,
Blood samples were taken by heparinized syringe from the caudal vein of the dogfish only, the
plasma was separated by centrifugation and
stored frozen at –80°C. The dogfish were pithed
and the rainbow trout were anaesthetized in
MS222 before dissection of the liver. We used the
hepatic 5′-MDA assay described previously (Flett
and Leatherland, ’89). Briefly, pieces of freshly dissected liver were homogenized in three volumes
of ice-cold phosphate buffer (0.1 M Na2HPO4/
KH2PO4; pH 7.0) using a Polytron homogenizer
(setting 6, three × 5 sec bursts). The homogenates
were centrifuged at 150g for 15 min and the lipid
layer was removed (dogfish). Unless otherwise
stated, the reaction vessel contained 50 µl of homogenate, 150 µl of phosphate buffer and dithiothreitol (DTT) and T4 to final concentrations of
10 mM and 50 nM, respectively. After 20 min at
15°C, the reaction was stopped by the addition of
four volumes of ice-cold ethanol. The reaction
tubes were refrigerated overnight at 4°C and then
centrifuged at 2000g for 12 min. Appropriate volumes of the supernatant were transferred into
glass tubes, dried at room temperature, and the
T3 content was measured by radioimmunoassay
(RIA). Blanks containing either no homogenate
(blank 1) or with ethanol added prior to the introduction of the T4 substrate (blank 2) were prepared with each enzyme assay. The blank 2 values
were subtracted from the test reaction vessels before calculating the reaction rate as pM of T3
formed per mg of homogenate protein per hour.
Homogenate protein concentration was measured
by the biuret method using human albumin as
the standard. The T3 content of each tube and
plasma T3 and T4 concentrations were measured
using commercial RIAs (Amersham Amerlex
Assay, Kodak Clinical Diagnostics, Toronto, ON)
previously validated for use with fish tissues
(Leatherland and Farbridge, ’92).
Hepatic 5′-MDA kinetics were examined in
pooled homogenates of the 6-hr-acclimated adult
female dogfish. The following parameters were
evaluated: incubation time from 0 to 60 min, substrate concentration from 0 to 125 nM, DTT concentration from 0 to 100 mM, pH from 6.5 to 8.0,
temperature from 0 to 40°C, organic osmolytes
[urea or TMAO alone (400 and 200 mM, respectively) or a mixture of osmolytes that simulate
the concentrations found in elasmobranch liver
and muscle cells (urea, TMAO, betaine and sarcosine at 400, 180, 13 and 7 mM, respectively)]
(Yancey and Somero, ’80; Robertson et al., ’91).
The same osmolyte concentrations were used for
the measurement of hepatic 5′-MDA activity in
trout, and in both the trout and dogfish preparations, the effect was examined over a 0–125 nM
range of substrate concentrations.
Regression lines were calculated for LineweaverBurk plots and the correlation coefficients for each
line tested for significance (P < 0.05). Km and Vmax
values were derived from the Lineweaver-Burk
regression lines.
In the adult dogfish liver homogenates, T3 production from the T4 substrate was time-dependent,
increasing rapidly over the first 20 min and reaching a plateau within 60 min (Fig. 2a). The Km and
Vmax values were 3.8 × 10–7 M and 0.29 nM T3/mg
protein/hr. The reaction rate was temperature-dependent over a range of 0–40°C (Fig. 2b), had a
pH optimum of 7.5 at 15°C (Fig. 2c), and was linearly dependent on DTT concentrations over a
range of 0–100 nM (Fig. 2d).
The inclusion of organic osmolytes in the reaction vessel markedly reduced the Vmax and Km values for the hepatic 5′-MDA of rainbow trout (Table
2). However, in the dogfish, the Vmax and Km values were reduced (12 and 51%, respectively) in
the presence of urea, and enhanced (418 and
190%, respectively) in the presence of TMAO
(Table 2). In the presence of the organic osmolyte
mixture, hepatic 5′-MDA Vmax and Km values increased (125%) and decreased (31%), respectively,
when compared with the preparation incubated
with buffer alone (Table 2).
Plasma T3 and T4 concentrations and hepatic
T3 production rates of the dogfish fasted for 7 days
were markedly lower than those of the fish
sampled on the day of capture (Table 3). Moreover, the hepatic 5′-MDA activity of the yolksac
embryos was considerably lower than that of the
6 hr acclimated adults, and in the same range as
that of the 7-day-fasted fish (Table 3).
This study shows that the kinetic characteristics of hepatic 5′-MDA activity of the dogfish resemble those of the high affinity hepatic 5′-MDA
of trout (e.g., Flett and Leatherland, ’89; MacLatchy and Eales, ’92). The dogfish enzyme appeared to be relatively thermally stable, and, as
in the trout, its activity was enhanced in the presence of the thiol, DTT. The apparent enzyme pH
optimum of 7.5 at 15°C is similar to the reported
intracellular pH of elasmobranch muscle cells
(Heisler et al., ’76). The study also shows that in
Fig. 2. Kinetics of T3 production from T4 substrate by liver homogenates from adult female
dogfish: effects of temperature (a), temperature (b), pH (c), and dithiothreitol (DTT) concentration (d). Data represent mean ± SEM of 3 replicates.
order to measure 5′-MDA activity in this species,
an appropriate organic osmolyte or mixture of
osmolytes needs to be added to the assay buffer.
Whereas the organic osmolytes inhibited rainbow trout hepatic 5′-MDA activity, as evidenced
by the depressed Vmax value, the Vmax value for the
5′-MDA in adult dogfish was enhanced in the presence of either TMAO alone or the 2:1 urea to methylamine organic osmolyte mixture (Table 2).
Although the addition of these organic osmolytes
to the reaction mixture caused small pH changes
(up to pH 7.2), the changes in Vmax and Km of the
preparations incubated with the osmolytes cannot
be attributed to the small pH shift (see Fig. 2c).
The fact that organic osmolytes influence hepatic enzyme activity in dogfish but not trout suggests that they play a physiological role in 5′-MDA
function in elasmobranch fishes. Some studies
have shown that urea and TMAO affect protein
structure by altering the solubility of nonpolar residues and of the polypeptide backbone (Mashino
and Fridovich, ’87; Lin and Timasheff, ’94). Furthermore, both osmolytes have been shown to
exert enzyme-specific and species-specific stimulatory or inhibitory effects on enzyme activities
(Yancey and Somero, ’80; Mashino and Fridovich,
’87; Yancey, ’93).
As with fasting salmonid fishes (Farbridge and
Leatherland, ’92), the hepatic 5′-MDA activity
and plasma thyroid hormone concentrations of
the adult female dogfish declined when the fish
were held without food for 7 days (Table 3). The
fall in plasma T3 levels, presumably in part resulting from a decreased peripheral T3 production rate, is likely a caloric sparing strategy,
comparable to that seen in ration-restricted teleost fishes (Leatherland, ’94).
The finding of hepatic 5′-MDA activity (albeit
TABLE 2. Effect of organic osmolytes on the Vmax and Km
values of hepatic 5¢-monodeiodinase of dogfish
and rainbow trout*
Buffer alone
Buffer + urea
(400 mM)
Buffer + TMAO
(200 mM)
Buffer + mix3
*Vmax and Km values were derived from a Lineweaver-Burk plot of
the effect of substrate concentration (0–125 nM range). The correlation coefficients of the regression lines for all dogfish trials and the
buffer alone trout trial were >0.94, those for the trout trials with
organic osmolytes were 0.85, 0.79 and 0.56 for the urea, TMAO, and
mixed osmolyte trials, respectively; all regression lines were significantly different (P < 0.01) from 0.
pmol T3/mg protein/hr.
nM T4.
Urea (400 mM); TMA0 (200 mM); betaine (13 mM) and sarcosine
(7 mM).
at low levels) in dogfish yolksac embryos is of
considerable interest. In several comparable
studies made of embryonic teleost fish, 5′-MDA
activity (whole body) was present only after the
yolk had been absorbed (Reddy et al., ’92). The
yolk of some teleost fish eggs contains significant levels of thyroid hormone. The hormones
appear to be of maternal origin and may inhibit
the development of a functional HPT axis, and
may delay the onset of hepatic 5′-MDA activity
(Tagawa et al., ’90; Leatherland and Barrett, ’93;
Leatherland, ’94). The finding of 5′-MDA activity in the dogfish embryos suggests that a similar inhibition of the enzymes involved in T 4
monodeiodination is not present.
TABLE 3. Plasma L-thyroxine (T4) and triiodo-L-thyronine
(T3) concentrations and hepatic 5¢-monodeiodinase (5¢-MDA)
activity in adult female dogfish and hepatic 5¢-MDA activity
in dogfish embryos1
Adult females
[6 hour retention
in holding aquarium]
Adult females
[7 day retention
in holding aquarium]
0.02 ± 0.003
Plasma thyroid hormone concentrations (nM) and hepatic 5′-MDA
activity (pmol/mg protein/hr) of adult dogfish are shown as individual
values; hepatic MDA activity of the embryos is shown as mean ±
SEM (n = 5); nd, not detectable; nm, not measured.
There is clearly much that needs to be done
to examine the various components of thyroid
function in elasmobranch fishes. To date only a
handful of studies have been made of blood hormone levels at different times of development
and of season, and the present report is the first
to describe the presence of monodeiodinase activity in any elasmobranch tissues. Any study
of the effect of environmental factors on thyroid
hormone physiology of elasmobranch fishes has
to include measurements of several parameters
in order for the results to be interpreted with
confidence. This study shows that 5′-MDA is
present in the liver of dogfish, and it is likely
also present in other tissues, and that changes
in enzyme activity reflect the altered physiological status of the animal. They are therefore useful indicators for any studies of thyroid hormone
economy or thyroid hormone homeostasis in
elasmobranch fishes.
The work was supported by grants from NSERC
to J.S.B. and J.F.L. Thanks to A.W. Bially for assisting with the data collection and to S. BrettWelsh for technical support.
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