Page 120 - 80_02

C.
Hernández-­-Sánchez,
P.
Vázquez,
Flora
de
Pablo

We
set
out,
then,
to
demonstrate
the
presence
of
receptors
which
may

mediate
the
action
of
catecholamines,
likely
produced
by
the
cardiomyocytes
of
the

primitive
heart
tube.
Adrenergic
and
dopaminergic
receptors
are
7-­-
transmembrane
domain
G
protein-­-coupled
receptors.
The
family
of
adrenergic

receptors
are
composed
by
a-­-
and
ß-­-receptors.
There
are
two
types
of
a-­-receptors:

The
stimulation
of

a-­-1-­-receptors

(a-­-1a,
a-­-1b
and
a-­-1d
subtypes)
activates
the
Gaq

subunit
of
G
proteins
and,
therefore,
Phospholipase
C
is
activated.
In
contrast,
the

a-­-2-­-receptors
(a-­-2a,
a-­-2b,
and
a-­-2c
subtypes)
are
coupled
to
Gai/o
subunit
and,

therefore,
inactivate
adenyl
cyclase
(AC).
In
contrast,
the
ß-­-adrenergic-­-receptors

family
includes
three
subtypes:
ß-­-1,
ß-­-2,
ß-­-3,
which
activate
the
subunit
Gas
and

subsequently
increase
the
intracellular
cAMP
levels.
However,
the
ß-­-2
adrenergic

receptors
may
activate
the
Gai
and
Gß?
subunits
of
G
protein
in
some
cases.

The
dopaminergic
receptors
are
clasified
into
two
subfamilies.
The
D1-­-
receptors
(D1,
D5)
are
coupled
to
Gas
proteins
and
their
activation
leads
to

estimulation
of
AC
and
subsequent
increase
in
cAMP
levels.
On
the
contrary,
the

D2-­-receptors
(D2,
D3,
D4)
are
coupled
to
Gai
proteins
and
their
activation
leads
to

inhibition
of
AC
and
decrease
of
cAMP
levels.
Members
of
the
D1
and
D2
receptors

can
form
heterodimers
that
are
coupled
to
Gaq
proteins
and
their
activation
leads

to
stimulation
of
phospholipase
C
and
intracellular
calcium
release.

Our
preliminary
studies
showed
the
presence
of
adrenergic
(both
a
and
ß

types)
and
dopaminergic
receptors
(D1
and
D2
subfamilies)
in
the
heart
tube

(Figure
6)
suggesting
that
catecholamines

have
the
possibility
to
act
through

them.
Further
studies
are
needed
to
confirm
the
role
of
the
specific
receptor

subtypes
and
signaling
pathways
in
cardiogenesis.

4.
L-­-DOPA
AND
DOPAMINE
INDUCE
CARDIAC
DIFFERENTIATION

Differentiation
of
contracting
cardiomyocyte
requires
the
action
of
a

combination
of
transcription
factors,
including
Nkx2.5
and
Tbx5
(17).
The
pattern

of
th
expression
suggested
that
catecholamines
may
play
a
role
in
cardiac

development.
Implanting
microbeads
soaked
in
L-­-DOPA
and
dopamine,
laterally
to

one
of
the
bilateral
heart
fields
in
cultured
chick
embryos,
we
could
show
that
both

L-­-DOPA
and
dopamine
induce
cardiac
muscle
differentiation.
Cardiac
transcription

factors
Nkx2.5
and
Tbx5
were
expressed
in
the
ectopic
tissue
generated
near
the

microbead
(Figure
7A).
Additionally,
the
sarcomeric
protein
AMHC1
(atrial
myosin

heavy
chain),
marker
of
terminal
cardiac
differentiation,
was
also
induced,
as

reflected
by
in
situ
hybridization
and
immunostaining
with
MF20
antibody
(Figure

7A).
Moreover,
the
cells
of
the
induced
ectopic
tissue
developed
myofibrils

organized
into
sarcomeres,
similar
to
those
found
in
the
cardiomyocytes
of
the

primitive
heart
tube
(Figure
7B).
The
link
of
TH
to
cardiac
differentiation
programs

356