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Departments of 1 Pharmacology and 2 Medicine and 3 Biomedical Sciences Ph.D. Program, School of Medicine, University of California at San Diego, La Jolla, California 92093-0636
Cardiac fibroblasts (CFs) are
an important cellular component of myocardial responses to
injury and to hypertrophic stimuli. We studied G protein-coupled
receptors to understand how CFs integrate signals that activate
Gq,
Gs, and
Gi. We predicted that the second messenger pathways present in CFs were distinct from those in cardiac
myocytes and that unique signaling interactions existed in the CFs. ANG
II, bradykinin, ATP, and UTP stimulated inositol phosphate (IP)
production 2.2- to 7-fold. Each of these agonists elevated
intracellular Ca2+ concentration
([Ca2+]i)
via release from the intracellular
Ca2+ storage compartment.
Endothelin-1 (ET-1), carbachol, and norepinephrine failed to increase
either IP production or
[Ca2+]i.
Although agonists that activated IP and
Ca2+ transients had no effect on
cAMP production when administered alone, these agents potentiated the
2-adrenergic response two- to
fourfold. Hormones known to inhibit adenylyl cyclase activity in
cardiac myocytes, such as ET-1 and carbachol, failed to lower the
-adrenergic response in fibroblasts. Order of potency and inhibitor
data indicate that the functional receptor subtypes in these cells are
2,
P2Y2, and
AT1 for isoproterenol, ATP, and ANG II, respectively. We conclude that CFs express functional G
protein-linked receptors that couple to
Gq and
Gs, with little or no coupling to
Gi. The expression of receptors
and their coupling to Gq- but not
to Gi-linked responses
distinguishes the signaling in CFs from that in myocytes. Furthermore,
agonists that activate Gq in CFs
potentiate stimulation of Gs, an
example of signaling cross talk not observed in adult myocytes. These
data suggest that G protein-mediated signaling in CFs is unique and may
contribute to the specificity of hormone and drug action on individual
cell types within the heart.
cyclic adenosine 3',5'-cyclic monophosphate; inositol phosphates; intracellular calcium; potentiation; receptor signaling
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