(Received for publication, August 7, 1995; and in revised form, November 6, 1995)
From the
Prostaglandin F (PGF
)
stimulates protein synthesis of skeletal and smooth muscle cells in
culture and is elevated in the heart during compensatory growth. We
hypothesized that PGF
stimulates hypertrophic growth
of neonatal rat cardiac myocytes. Prostaglandin F
increased [
H]phenylalanine incorporation by
cultured ventricular myocytes in a dose-dependent manner (EC
= 11 nM), suggesting action through a
PGF-specific receptor. Semiquantitative reverse transcriptase
polymerase chain reaction revealed that PGF receptor mRNA is expressed
in ventricular myocytes > A7R5 vascular smooth muscle cells
cardiac fibroblast-like cells. The protein content of cardiomyocyte
cultures was increased by 10 nM PGF
and
11
-PGF
but was unchanged by 10 nM PGD
, PGE
, PGF
,
carbaprostacyclin, U-46619, or 12- or 15-hydroxyeicosatrienoic acid.
Stimulation of myofibrillar gene expression by PGF
was
demonstrated by Northern and Western blot analysis for myosin light
chain-2 (MLC-2) and by transient transfection experiments with MLC-2
luciferase expression plasmids. In addition, myofibrillogenesis was
increased by PGF
as assessed by immunocytochemical
staining with MLC-2 antisera. Prostaglandin F
did not
affect myocyte proliferation or [
H]thymidine
incorporation, thus myocyte growth occurred by hypertrophy.
Proliferative and hypertrophic growth of cardiac fibroblast-like cells
were unaffected by PGF
. We conclude that
PGF
stimulates hypertrophic growth of neonatal rat
ventricular myocytes in culture and speculate that PGF
plays a role in myocardial adaptation to chronic hypertrophic
stimuli, recovery from injury, and cardiac ontogeny.
During embryonic development the myocardium enlarges by the proliferation of cardiac myocytes. Shortly after birth, cardiac myocytes lose their capacity for mitogenesis, and further growth of the myocardium to meet the increasing hemodynamic demand of an elevated blood pressure and blood volume occurs by enlargement of existing muscle cells (hypertrophy). Similarly, the restoration of myocardial contractile performance lost to ischemia or viral infection is dependent on the recovery of injured myocytes and on the compensatory enlargement of agonist myocytes. Thus, an understanding of the controlling factors of myocardial protein synthesis and growth has implications for cardiac ontogeny, adaptation to chronic physiologic and pathophysiologic stimuli, and recovery from injury.
Cardiac
hypertrophy is produced by a variety of stimuli in culture and in
vivo(1) , including, but not limited to, mechanical
stretch(2, 3) ,
neurotransmitters(4, 5) , and
hormones(6, 7) . As the biochemistry of myocardial
growth is experimentally revealed, some common intracellular signaling
pathways appear among primary stimuli. For example, stretch-induced
cardiomyocyte hypertrophy is mediated, in part, by the local production
of angiotensin II(8) , and several hypertrophic stimuli,
angiotensin II, norepinephrine, and endothelin-1, act through G protein-coupled receptors(9, 10, 11) and activate mitogen-activated protein
kinases(12, 13) . Direct evidence of G
involvement in cardiac growth was provided by microinjection of
G
neutralizing antibodies to block the hypertrophic
response of neonatal rat ventricular myocytes to the
-adrenergic agonist phenylephrine(9) . Thus
cardiac hypertrophy appears to be mediated, at least for several
stimuli, by agonists of G
protein-coupled receptors.
In
addition to promoting myocardial growth, angiotensin II,
norepinephrine, and endothelin-1 are vasoactive substances.
Interestingly, Katz (14) speculated that angiotensin II evolved
from a primitive growth factor and assumed additional regulatory roles
in the cardiovascular system, such as stimulation of aldosterone
production and smooth muscle contraction. It is conceivable that other
vasoactive substances went through a similar evolutionary process,
especially in consideration of the finding that Ca signaling is important for angiotensin II activation of
mitogen-activated protein kinase in cardiac myocytes(13) .
Prostaglandin F (PGF
) (
)is a vasoactive substance that stimulates protein
synthesis in skeletal and smooth muscle cells in
culture(15, 16) . Moreover, PGF
regulates, in part, stretch-induced skeletal myoblast
growth(16) , and the effects of exogenous PGF
on vascular smooth muscle hypertrophy are most likely mediated by
a PGF-specific receptor(15) . As to the heart, PGF
was increased in the left ventricles of rabbits by acute pressure
overload(17) , and PG synthase inhibitors blocked cardiac
growth induced by hypertension (18) and clenbuterol (19) . These observations and others suggested that
PGF
may play a role in the control of cardiac muscle
growth. Recently, the prostaglandin F receptor (FP receptor) was cloned (20) and found to be coupled to a G
protein in
Chinese hamster ovary cells (21) and to activate
mitogen-activated protein kinase and mitogen-activated protein kinase
kinase in NIH-3T3 cells(22) . Expression of FP receptor mRNA in
murine heart was also demonstrated, although the specific cell type
that expresses the receptor was not explored(20) .
The
present experiments were designed to test the hypothesis that
PGF stimulates hypertrophy of neonatal rat ventricular
myocytes. Cultured cardiomyocytes were incubated with PGF
and observed for evidence of cellular growth and myofibrillar
development. Results show that PGF
stimulates
hypertrophy of ventricular myocytes and the induction and expression of
myofibrillar genes. The potency of PGF
, in relation to
the dissociation constant for the FP receptor and in comparison to
other eicosanoids, suggests that PGF
-induced growth of
the heart is mediated by its specific receptor, which we found is
expressed in ventricular myocytes. We speculate that PGF
regulates myocardial protein synthesis during development,
compensatory hypertrophy, and/or recovery from injury.
Nonmuscle cardiocyte (NMC) cultures were
prepared as follows. The top layer of cells from the Percoll separation
was collected, washed twice with Buffer A, and plated onto 150
25-mm culture dishes (Falcon) at about 1500 cells/mm
in
DMEM/F12 supplemented with 20% fetal bovine serum (Gemini Bio-Products)
and antibiotics. To obtain cultures free of ventricular myocytes, cells
at 75% confluence were serially passaged three times. Cells were lifted
by incubation for several minutes at 37 °C in Puck's saline A
containing 0.05% trypsin and 1 mM EDTA. Serum was added to 20%
to inhibit trypsin, and cells were collected by centrifugation. Cells
were replated in culture dishes at a surface area of 1:3. After the
third passage, when cells reached 33% confluence, they were rinsed
twice with serum-free medium and switched into serum-free DMEM/F12 with
antibiotics for 8-20 h before treatment with eicosanoids. NMC
cultures prepared under these conditions consist primarily of
fibroblasts(25, 26) .
Embryonic rat thoracic aorta smooth muscle cells (A7R5) were obtained from American Type Culture Collection (Rockville, MD) at the vendor's 11th passage. Cells were grown in DMEM supplemented with 10% fetal bovine serum and harvested for RNA within the 16th passage.
Rat aortic endothelial cells (passage 4) were a generous gift from Dr. L. J. Ignarro (UCLA). Cells were maintained on gelatin-coated dishes in DMEM containing 4.5 mg/ml glucose and 25 mM HEPES supplemented with 20% fetal bovine serum, 0.75% sodium pyruvate, 1% endothelial cell growth supplement (Becton Dickinson), penicillin (100 units/ml), and streptomycin (100 µg/ml). Heparin (10 units/ml) was added at 50% confluence to retard growth of smooth muscle cells. Cells were harvested for RNA within the eighth passage.
Primers for polymerase chain
reaction (PCR) were designed using GenBank sequences for FP receptor
(D28581) and -actin (J00691). Oligo 4.0 software (National
Biosciences, Plymouth, MN) was used to suggest upper and lower primers
and to evaluate primers for melting temperatures, secondary priming
sites, and inter- and intraprimer complementation. The primers were
screened against the GenBank nonredundant combined nucleotide data base
using the BLAST network service provided by the National Center for
Biotechnology Information. The following primer pairs, spanning at
least one intron, were synthesized by a Beckman Oligo 1000 Synthesizer
(Beckman Instruments, Fullerton, CA): FP receptor upper primer,
5`-CTCGGCATCTCATTCTCGTG-3`, lower primer 5`-GCCTCACTAGATGCTTGCTG-3`;
-actin upper primer, 5`-CGTTGACATCCGTAAAGACCTCTA-3`, lower primer,
5`-TAAAACGCAGCTCAGTAACAGTCCG-3`. The PCR reactions (50 µl)
contained 0.2 mM dNTPs, 1.5 mM MgCl
, 25
pmol of each primer, 1.25 units of Taq polymerase (Life
Technologies), 20 mM Tris (pH 8.4), and 50 mM KCl.
Amplification cycles were performed in a Perkin-Elmer DNA thermal
cycler model 480 (Perkin-Elmer, Norwalk, CN) under the following
conditions: FP receptor primers, 30 s at 94 °C, 30 s at 55 °C,
and 90 s at 72 °C for 30 cycles;
-actin primers, 30 s at 94
°C, 30 s at 65 °C, and 60 s at 72 °C for 27 cycles. All
runs included an initial 1-min denaturation at 94 °C and a final
7-min extension at 72 °C. Products were separated on a NuSieve 3:1
agarose (FMC BioProducts, Rockland, ME) gel in 90 mM Tris
borate (pH 8.0) and 2 mM EDTA, stained with ethidium bromide
and photographed under UV illumination. To verify their authenticity,
restriction digests of the FP receptor and
-actin PCR products
were analyzed by agarose gel electrophoresis (data not shown). The FP
receptor and
-actin PCR products generated from genomic DNA
templates were larger than from cDNA (data not shown).
Figure 1:
Prostaglandin F stimulates [
H]phenylalanine incorporation
by ventricular myocytes. Neonatal rat ventricular myocytes were
incubated in serum-free medium and stimulated with 10
to 10
M PGF
for 24
h. During the last 4 h of stimulation, myocytes were switched into
medium containing 0.36 mML-phenylalanine and 5
µCi/ml of L-[2,3,4,5,6-
H]phenylalanine. Cells were
harvested and counted for [
H]phenylalanine
incorporation as an estimate of the relative rates of protein
synthesis. Values are presented as the mean ± S.E. of nine
observations. *, statistically different (p < 0.01) from
the unstimulated control group by
resampling.
Figure 2:
Concentration (A) and time (B) effects of prostaglandin F on the protein
content of ventricular myocyte cultures. The protein content of
neonatal rat ventricular myocyte cultures was determined on dishes of
cells incubated in serum-free medium with or without
PGF
. A, concentration effects of
10
to 10
M PGF
are shown for myocytes harvested after 48 h
of treatment. B, time effects are shown for myocytes harvested
6-72 h after stimulation with 10
M
PGF
. To correct for the variation in cellular density
between experiments, values are normalized to the mean of the
unstimulated group and are presented as the mean ± S.E. of
10-11 observations taken from two (A) or three (B) separate cultures. *, statistically different (p < 0.001) from the unstimulated group within time by
resampling.
Since primary myocyte cultures are contaminated by
fibroblasts, as well as smooth muscle cells and endothelial cells, and
considering that angiotensin II stimulates the growth of both cardiac
myocytes and fibroblasts, we were compelled to show that the changes in
protein content we observed for PGF were due to its
effects on myocytes and not on other cell types in these
Percoll-enriched myocyte cultures. In addition, because eicosanoids are
known to regulate growth, we were curious to know if other eicosanoids
of the cardiovascular system were capable of stimulating growth of
ventricular myocytes or NMC. For example, U-46619 (a stable analog of
thromboxane A
), 12(S)-hydroxyeicosatetraenoic acid
(HETE), and 15(S)-HETE stimulate hypertrophic growth and/or
mitogen-activated protein kinase activity of vascular smooth muscle (15, 35, 36, 37) , PGE
and PGE
are angiogenic(38) , and PGD
stimulates hepatocyte DNA synthesis(39) . We also tested
11
-PGF
because it has an affinity for the FP
receptor equal to PGF
(20) and is a PGF
synthase product of PGD
(40) . First we determined
whether PGF
or other cardiac eicosanoids stimulated
growth of NMC cultures. NMC were incubated for 24-48 h in
serum-free medium containing 10
M eicosanoids. Protein content of the NMC cultures was unaffected by
12(S)- or 15(S)-HETE, U-46619, PGI
,
carbaprostacyclin (a stable analog of PGI
),
PGD
, PGE
, PGE
,
PGF
, 11
-PGF
, or PGF
(data not shown). Thus, the PGF
-stimulated
increase in the protein content of ventricular myocytes cultures is due
to protein accumulation by myocytes and not by NMC. We next determined
if any of the above eicosanoids, in addition to PGF
,
stimulated ventricular myocyte growth. Myocytes were incubated for 24 h
in serum-free media containing 10
,
10
, or 10
M eicosanoids.
The protein content of ventricular myocytes was increased by
10
M PGD
, PGE
,
PGF
, and 11
-PGF
(Fig. 3,
PGF
data not shown) and was unaffected by any
concentration of 12(S)- or 15(S)-HETE, U-46619,
PGE
, PGI
, or carbaprostacyclin (PGE
data not shown). Moreover, 11
-PGF
was the
only eicosanoid besides PGF
to produce an increase in
protein content at 10
M, and the increase
produced by the two FP receptor-specific agonists at 10
M was equal to that produced by 100-fold higher
concentrations of PGD
, PGE
, and
PGF
. These results support the assumption that the
growth-stimulatory effects of PGD
, PGE
, and
PGF
at 10
M are mediated
by FP receptor binding, as does the relative potency of prostanoids as
a stimulus for myocyte growth (PGF
=
11
-PGF
> PGF
> PGD
= PGE
> U-46619 =
carbaprostacyclin), which agrees well with the binding specificity
order for the FP receptor assayed by
[
H]PGF
displacement in
cDNA-transfected COS-1 cells(20) . Thus of the eicosanoids
tested, only FP receptor agonists stimulated growth of neonatal rat
ventricular myocyte cultures, and growth was due to the effects on
myocytes and not NMC.
Figure 3:
Effects of exogenous eicosanoids on the
protein content of ventricular myocyte cultures. Neonatal rat
ventricular myocytes in serum-free medium were treated with
10, 10
, or 10
M eicosanoids. Cells were harvested 24 h later, and the
protein content of each dish was determined. To correct for the
variation in cell density between experiments, values are normalized to
the mean of the untreated control group and are presented as the mean
± S.E. of eight observations taken from two separate cultures.
*, statistically different (p < 0.005) from the
unstimulated group by resampling.
We next determined if PGF mediates its affects on myocyte growth via hypertrophy or
hyperplasia. The rate of DNA synthesis by myocytes and NMC was
estimated by their incorporation of [
H]thymidine
from the culture medium. Prostaglandin F
(10
M) did not affect
[
H]thymidine incorporation of ventricular
myocytes or NMC (data not shown). In addition, the effects of
PGF
on cellular proliferation were determined by cell
counting. Ventricular myocytes and NMC were incubated in serum-free
medium with or without 10
M PGF
for 24 h. The average number of cells/dish was not significantly
affected in either cell type by PGF
stimulation (data
not shown). Thus, PGF
-mediated growth of neonatal rat
ventricular myocyte primary cultures is due to myocyte hypertrophy.
Figure 4:
Prostaglandin F receptor mRNA is present
in ventricular myocytes. Semiquantitative reverse transcriptase
polymerase chain reaction for rat FP receptor and rat -actin was
performed on RNA isolated from primary cultures of neonatal rat
ventricular myocytes (VM), serially passaged rat NMC, which
consist primarily of fibroblasts, A7R5 embryonic rat thoracic aorta
smooth muscle cells (VSMC), and water (H
O). Reaction
products and a 100-bp DNA ladder were separated on an agarose gel and
stained with ethidium bromide. The two water controls for the FP
receptor and rat
-actin were run together and display no visible
bands. The gel was photographed and developed by direct positive
processing. Primers span introns and produce larger products from
genomic DNA (data not shown).
Figure 5:
Prostaglandin F stimulates protein content (A) and
[
S]methionine incorporation (B) of
ventricular myocytes in the absence of spontaneous contraction.
Neonatal rat ventricular myocytes were incubated in serum-free medium
containing 4 mM KCl or, to arrest spontaneous beating, 50
mM KCl. Myocytes were stimulated or not stimulated with
10
M PGF
for 48 h and
assessed for changes in protein content (A) or L-[
S]methionine incorporation (B). To correct for the variation in cellular density between
experiments, values are normalized to the unstimulated 4 mM KCl group and are presented as mean ± S.E. of nine (B) or 15 (A) observations from two (B) or
three (A) separate cultures. *, statistically different (p < 0.001) from the unstimulated control group within medium
treatment by resampling.
Figure 6:
Uridine incorporation by ventricular
myocytes is stimulated by prostaglandin F. Neonatal
rat ventricular myocytes and nonmuscle cardiocytes were stimulated with
10
M PGF
for 24 h. The
medium was pulsed with 1 µCi/ml of
[5,6-
H]uridine for the last 2 h of stimulation.
Cells were harvested and counted for [
H]uridine
incorporation as an estimate of the relative rates of RNA synthesis.
Values are normalized within cell type to their respective unstimulated
group and are presented as the mean ± S.E. of 11 observations
from three separate cultures. *, statistically different (p < 0.002) from the unstimulated control group within cell type
by resampling.
The effects of PGF on myofibrillar gene
expression were demonstrated by Northern and Western blot analysis for
MLC-2. Myocytes were incubated for 6, 12, 24, 48, or 72 h with or
without 10
M PGF
. The
relative cellular content of MLC-2-specific protein between treatment
groups was approximated by the product of the total cellular protein
mass (µg/dish) and the proportion of MLC-2-specific protein (MLC-2
band intensity per µg of total protein). The relative
MLC-2-specific mRNA was similarly estimated. Over the course of the
experiment, the combined increase of MLC-2 mRNA and protein content in
PGF
-simulated cells was approximately 2-3-fold
(data not shown).
The finding that MLC-2 protein expression was
higher in stimulated cardiomyocytes, and our informal observation via
phase contrast microscopy that PGF-stimulated myocytes
had larger cytoplasmic surface areas and were more stellate, suggested
that the myofibrils of PGF
-treated cells may be more
highly developed than unstimulated control cells. Accordingly, we
performed immunocytochemistry on myocytes incubated for 48 h with
various concentrations of PGF
to assess the effects of
PGF
on myofibrillogenesis. The results of these
experiments are shown in Fig. 7. Unstimulated myocytes were
smaller, frequently round in shape, and stained diffusely for MLC-2. In
contrast, PGF
-stimulated myocytes were larger and more
stellate, and MLC-2 staining was concentrated at what appears to be the
A band (but not the H zone) of numerous organized myofibrils. Our
qualitative observations suggest that myofibrillogenesis was responsive
to PGF
at concentrations as low as 10
M and was not improved by PGF
concentrations above 10
M.
Figure 7:
Myofibrillogenesis in ventricular myocytes
is enhanced by prostaglandin F. Neonatal rat
ventricular myocytes were incubated for 48 h in serum-free medium (A) or in serum-free medium containing 10
(B), 10
(C), 10
(D), or 10
M PGF
(E) or 10
M phenylephrine (F). Immunocytochemical staining was performed with myosin
light chain-2 primary and rhodamine-conjugated secondary antisera.
Cells were photographed under fluorescent light at 1000
magnification.
In this study we show that PGF stimulates
hypertrophic growth of neonatal rat ventricular myocytes and boosts the
expression of myofibrillar genes. The expression of FP receptor mRNA by
ventricular myocytes and the low PGF
concentration
required to increase protein synthesis suggest that the growth effects
of PGF
on the heart are mediated by its specific
receptor. To our knowledge, this is the first demonstration that an
eicosanoid stimulates myocardial growth. The potential of these
findings for biological significance to the intact animal is discussed
below.
First, are the PGF concentrations used to
stimulate protein synthesis in our cultures consistent with FP
receptor-mediated regulation in vivo? Van Bilsen et al.(43) argued that for a PG-induced experimental result to
have biological significance, the PG must be effective at the same
order of magnitude as its dissociation constant, which for most PGs is
in the nanomolar range. We found that PGF
stimulated
the incorporation of [
H]phenylalanine by
cardiomyocytes at concentrations as low as 0.1 nM and obtained
50% of maximal stimulation at 11 nM. Our EC
value
compares favorably with dissociation constants of 9-47
nM for high affinity PGF
binding sites
determined on luteal cells taken from cycling, pregnant, and
pseudopregnant pigs (41) and the dissociation constant of 1.3
nM for COS cells transiently transfected with a murine FP
receptor expression vector(20) . In addition, our EC
for [
H]phenylalanine incorporation is
similar to those calculated for the PGF
-stimulated
accumulation of intracellular Ca
(49 nM) and
[
H]inositol phosphates (29 nM) in
vascular smooth muscle cells (15) and agrees well with that
calculated for phosphatidylinositol hydrolysis in transfected COS-1
cells (10 nM)(20) . Thus our effective PGF
concentrations for protein synthesis are within the same order of
magnitude as the FP receptor dissociation constants and agree with
PGF
concentrations required by other cells to activate
intracellular signaling pathways associated with cardiac hypertrophy.
Second, does PGF attain levels in the myocardium
sufficiently high to stimulate protein synthesis? Prostaglandins are
not stored but are sequentially synthesized and released in response to
stimuli to act as autocrine or paracrine factors in the heart. Humoral
effects of prostaglandins are negligible because the lungs remove
nearly all prostaglandins from the blood. It is difficult to estimate
PGF
concentrations near the myocardial FP receptor
from release rates into culture medium or coronary veins incognizant of
such variables as PG turnover and receptor density, especially in a
mixed population of cell types. Although of arguable validity, studies
in which myocardial biopsies were taken to determine the average PG
concentration in tissue homogenates provide a direct estimate of
myocardial PG concentration (17, 44) . For example, in
left ventricle samples taken from rabbits subjected to 15-60 min
of pressure overload by coarctation of the ascending aorta,
PGF
levels increased to about 40 nmol/kg, wet weight,
which was 7-10 times above basal levels (17) . Thus, it
is plausible that when actively synthesized, PGF
concentrations in the myocardium exceed the dissociation constant
for the FP receptor and may stimulate myocardial protein synthesis.
Third, does PGF-induced myocardial growth depend on
myocyte production of PGF
? Endothelial cells and
fibroblasts exist in close proximity to cardiac myocytes, and the PGs
they produce may affect neighboring myocardial cells. Culture medium
concentrations of PGF
from unstimulated primary
cultures of three types of adult rat heart cells revealed the following
order of production: endothelial cells > fibroblast-like cells >
myocytes(45) . Similar results were reported for neonatal rat
fibroblast-like cells and myocytes(46) . It is worth noting
that cells other than myocytes may be responsible for PG production,
because primary signals for myocardial growth may affect nonmuscle
cardiocytes, which then produce PGs that act as paracrines on myocytes.
Consideration of the initial stimuli of PGF
production
leads to the last question.
Fourth, in what situations might
PGF play a role in cardiac growth/hypertrophy?
Prostaglandin F
may stimulate myocardial growth during
chronic overload, ischemia, or development in utero.
Prostaglandin F
levels in the myocardium were
increased by acute (17) or chronic (44) hemodynamic
overload. In addition, PG synthase inhibitors blocked experimentally
induced cardiac hypertrophy(18, 19) . The gain in
heart weight to body weight ratios of rats after 4 weeks of pulmonary
hypertension (18) or 1 week of
-adrenergic stimulation
with clenbuterol (19) was completely blocked with aspirin (18) or with fenbufen(19) . Although these results
point to a possible role for PGs in the regulation of cardiac growth,
other cardiovascular effects such as a reduction in blood viscosity (18) cannot be ruled out. Prostaglandin synthesis is also
greater in cardiomyocytes(47) , endothelial cells (48) and fibroblasts (49) subjected to hypoxia. The
role of PGF
in hypoxia may be to aid recovery of
injured myocytes by stimulating protein synthesis, as well as to
stimulate growth of uninjured myocytes to restore a decrement in
myocardial contractility. It is interesting to note that PG production
was elevated in fibroblasts derived from healing, infarcted
myocardium(49) . In addition to enhancing myocyte growth,
PGF
stimulates the release of atrial natriuretic
peptide from the myocardium(50) . Thus the effects of
PGF
may be 2-fold. In response to cardiac overload or
hypoxia, PGF
stimulates myocardial protein synthesis
to maintain or enhance the pumping capacity of the heart, while at the
same time PGF
stimulates atrial natriuretic peptide
release to reduce blood volume and decrease the demand on the heart. In
addition to myocardial adaptation to chronic hemodynamic change and
injury, PGF
may play a role in cardiac development, as
it appears to for skeletal muscle(51) . Prostaglandin
F
levels were elevated in fetal chick thigh muscle
just prior to secondary myogenesis and decreased in association with
cell cycle withdrawal and myotube formation(51) . The specific
role of prostaglandins during development may be to mediate the effects
of peptide growth factors(52) , such as basic fibroblast growth
factor, which displays developmental regulation(53) , activates
MAP kinase in the heart(12) , and alters the cardiac
phenotype(54) .
In conclusion, we found that exogenous
PGF stimulates hypertrophic growth of neonatal rat
ventricular myocytes in culture. The FP receptor, which appears to
mediate PGF
-induced myocyte growth, is expressed most
highly in the heart by cardiac muscle cells. We speculate that
PGF
plays a regulatory role in myocardial adaptation
to chronic hypertrophic stimuli, recovery from injury, or cardiac
ontogeny.