Lipid metabolism
75
Ng
et
ale
(1984)
observed
that
estradiol
increased
the
activity of
two
lipogenic enzymes, malic enzyme and isocitrate dehydrogenase, but had no
effect
on
glucose—6—phosphate
dehydrogenase
(G6PDH)
in
the
grouper,
Epinephalus
akaarae
Variable
effects
of
estradiol
on.
G6PDH
have
been
reported
in
another
teleost,
Platichthys
flesus
(Sand
et
al»,
1980;
Petersen et
ale, 1983)e
Ng et ala (1984) further suggested that the acetyl
CoA
substrate
for
lipogenesis could be furnished from the elevated plasma
levels
of
amino
acids
induced
by both estradiol and
This
latter
suggestion
is
consistent
with
findings of Nagai and Ikeda (1972,
1973) with carp discussed aboveo
Apart from a growing body of knowledge concerning thyroid involvement
(Plisetskaya
et
alu,
1983),
hormonal
control
of
lipid
mobilization
in
teleosts,
including cyprinids,
is
poorly understood°
As
mentioned
above,
mobilization of
lipids from carp visceral adipose tissue has been found to
be insensitive to catecholamines°
The
situation
regarding
lipid. mobilization
from.
skeletal.
muscle
is
also
uncleare
In
general,
the
high lipid content of fish red muscle is
believed to provide energy substrate for that highly aerobic
White
muscle,
on
the
other
hand,
burns
largely
carbohydrate
and
has
a
high
anaerobic
capacityu
(For review,
see
Driedzic
and
Hochachka,
In
salmonids,
the
lipid
stores
in
white
muscle,
which
are
largely
extracellular
(Greene 1913), form an important metabolic reserve that can
be
mobilized
during fasting (Robinson and Mead,
1973)
and
smoltification
(Sheridan et ale
1983)°
The mechanism of mobilization
from this
store
is
unknown
and
Sergent
(1976)
has
suggested
that
it
may
be
mobilized
as
triglyceride as well as FFA«
In
cyprinids, substantial intracellular and
extracellular
fat
is
found
in
red
muscle
(George
and
Bokdawala,
19643
Bokdawala and George, 1967).
In their
histochemical studies, these authors
did
not
report any significant intracellular or extracellu1ar fat reserves
in
white
muscle
but
did
report
an
extracellu1ar
lipase
in
white
muscle
(George
and
Bokdawala.
1964)…
The
role
of
such
za
lipase
remains
to
be
determinede
The
coincidence
of
somatic
lipid
depletion
with
ovarian
growth
suggests
that
lipid mobilization
in
teleosts
may
be
stimulated
by
sex
steroids°
The
various
published
studies
have,
however,
yielded
inconsistent
De Vlaming et alu, (1977) found variable effects of
estradiol on body lipid reserves in
crysoleucas°
Injection of estrogens
'
caused
elevations
in
serum.
FFA
levels
in
some
teleosts
(Petersen
and
Korsgaard,
1978;
Korsgaard
and
Petersen,
1979)
but
Wiegand
and
Peter
(1980a) were not able to reproducibly demonstrate this in goldfish.
These
latter authors did find that testosterone could elevate plasma FFA and that
estrogens
elevated
plasma
of
other
and
Peter,
1980b)u
Testosterone
caused depletion of muscle lipid in Ëf
akaara (Ng et
al., 1984) although variable effects were obtained in salmonids (Donaldson
et
a1°, 1979)°
The equivocal results obtained may be a function of species
and
protocol differences or they may be due to sex steroids not being the
sole,
or
even.
the
primary,
stimuli
for
lipid. mobilization.
during
gonad
recrudescenceo
The rising plasma levels of corticosteroids coincident with
gonad recrudescence
in
some
teleosts (Peter et al., 1978) and with other
functions
associated
with
lipid mobilization (Wingfield and Grimm,
1977;
Specker and Schreck, 1982) suggest that these hormones may play a role in
lipid mobilizationa
Corticosteroids are permissive to lipolysis in mammals
(Goodman, 1970)°
_
75
Ng
et
ale
(1984)
observed
that
estradiol
increased
the
activity of
two
lipogenic enzymes, malic enzyme and isocitrate dehydrogenase, but had no
effect
on
glucose—6—phosphate
dehydrogenase
(G6PDH)
in
the
grouper,
Epinephalus
akaarae
Variable
effects
of
estradiol
on.
G6PDH
have
been
reported
in
another
teleost,
Platichthys
flesus
(Sand
et
al»,
1980;
Petersen et
ale, 1983)e
Ng et ala (1984) further suggested that the acetyl
CoA
substrate
for
lipogenesis could be furnished from the elevated plasma
levels
of
amino
acids
induced
by both estradiol and
This
latter
suggestion
is
consistent
with
findings of Nagai and Ikeda (1972,
1973) with carp discussed aboveo
Apart from a growing body of knowledge concerning thyroid involvement
(Plisetskaya
et
alu,
1983),
hormonal
control
of
lipid
mobilization
in
teleosts,
including cyprinids,
is
poorly understood°
As
mentioned
above,
mobilization of
lipids from carp visceral adipose tissue has been found to
be insensitive to catecholamines°
The
situation
regarding
lipid. mobilization
from.
skeletal.
muscle
is
also
uncleare
In
general,
the
high lipid content of fish red muscle is
believed to provide energy substrate for that highly aerobic
White
muscle,
on
the
other
hand,
burns
largely
carbohydrate
and
has
a
high
anaerobic
capacityu
(For review,
see
Driedzic
and
Hochachka,
In
salmonids,
the
lipid
stores
in
white
muscle,
which
are
largely
extracellular
(Greene 1913), form an important metabolic reserve that can
be
mobilized
during fasting (Robinson and Mead,
1973)
and
smoltification
(Sheridan et ale
1983)°
The mechanism of mobilization
from this
store
is
unknown
and
Sergent
(1976)
has
suggested
that
it
may
be
mobilized
as
triglyceride as well as FFA«
In
cyprinids, substantial intracellular and
extracellular
fat
is
found
in
red
muscle
(George
and
Bokdawala,
19643
Bokdawala and George, 1967).
In their
histochemical studies, these authors
did
not
report any significant intracellular or extracellu1ar fat reserves
in
white
muscle
but
did
report
an
extracellu1ar
lipase
in
white
muscle
(George
and
Bokdawala.
1964)…
The
role
of
such
za
lipase
remains
to
be
determinede
The
coincidence
of
somatic
lipid
depletion
with
ovarian
growth
suggests
that
lipid mobilization
in
teleosts
may
be
stimulated
by
sex
steroids°
The
various
published
studies
have,
however,
yielded
inconsistent
De Vlaming et alu, (1977) found variable effects of
estradiol on body lipid reserves in
crysoleucas°
Injection of estrogens
'
caused
elevations
in
serum.
FFA
levels
in
some
teleosts
(Petersen
and
Korsgaard,
1978;
Korsgaard
and
Petersen,
1979)
but
Wiegand
and
Peter
(1980a) were not able to reproducibly demonstrate this in goldfish.
These
latter authors did find that testosterone could elevate plasma FFA and that
estrogens
elevated
plasma
of
other
and
Peter,
1980b)u
Testosterone
caused depletion of muscle lipid in Ëf
akaara (Ng et
al., 1984) although variable effects were obtained in salmonids (Donaldson
et
a1°, 1979)°
The equivocal results obtained may be a function of species
and
protocol differences or they may be due to sex steroids not being the
sole,
or
even.
the
primary,
stimuli
for
lipid. mobilization.
during
gonad
recrudescenceo
The rising plasma levels of corticosteroids coincident with
gonad recrudescence
in
some
teleosts (Peter et al., 1978) and with other
functions
associated
with
lipid mobilization (Wingfield and Grimm,
1977;
Specker and Schreck, 1982) suggest that these hormones may play a role in
lipid mobilizationa
Corticosteroids are permissive to lipolysis in mammals
(Goodman, 1970)°
_