Abstract
In cardiac myocytes there is evidence that activation of some receptors
can regulate protein kinase A (PKA)-dependent responses by stimulating
cAMP production that is limited to discrete intracellular domains.
We previously developed a computational model of compartmentalized
cAMP signaling to investigate the feasibility of this idea. The model
was able to reproduce experimental results demonstrating that both
beta(1)-adrenergic and M(2) muscarinic receptor-mediated cAMP changes
occur in microdomains associated with PKA signaling. However, the
model also suggested that the cAMP concentration throughout most
of the cell could be significantly higher than that found in PKA-signaling
domains. In the present study we tested this counterintuitive hypothesis
using a freely diffusible fluorescence resonance energy transfer-based
biosensor constructed from the type 2 exchange protein activated
by cAMP (Epac2-camps). It was determined that in adult ventricular
myocytes the basal cAMP concentration detected by the probe is approximately
1.2 muM, which is high enough to maximally activate PKA. Furthermore,
the probe detected responses produced by both beta(1) and M(2) receptor
activation. Modeling suggests that responses detected by Epac2-camps
mainly reflect what is happening in a bulk cytosolic compartment
with little contribution from microdomains where PKA signaling occurs.
These results support the conclusion that even though beta(1) and
M(2) receptor activation can produce global changes in cAMP, compartmentation
plays an important role by maintaining microdomains where cAMP levels
are significantly below that found throughout most of the cell. This
allows receptor stimulation to regulate cAMP activity over concentration
ranges appropriate for modulating both higher (e.g., PKA) and lower
affinity (e.g., Epac) effectors.
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