β2-adrenergic receptor-coupled phosphoinositide 3-kinase constrains cAMP-dependent increases in cardiac inotropy through phosphodiesterase 4 activation.
BACKGROUND
Emerging evidence suggests that phosphoinositide 3-kinase (PI3K) may modulate cardiac inotropy; however, the underlying mechanism remains elusive. We hypothesized that β(2)-adrenergic receptor (AR)-coupled PI3K constrains increases in cardiac inotropy through cyclic adenosine monophosphate (cAMP)-dependent phosphodiesterase (PDE) activation.


METHODS
We tested the effects of PI3K and PDE4 inhibition on myocardial contractility by using isolated murine cardiac myocytes to study physiologic functions (sarcomere shortening [SS] and intracellular Ca(+) transients), as well as cAMP and PDE activity.


RESULTS
PI3K inhibition with the reversible inhibitor LY294002 (LY) resulted in a significant increase in SS and Ca(2+) handling, indicating enhanced contractility. This response depended on G(iα) protein activity, because incubation with pertussis toxin (an irreversible G(iα) inhibitor) abolished the LY-induced hypercontractility. In addition, PI3K inhibition had no greater effect on SS than both a PDE3,4 inhibitor (milrinone) and LY combined. Furthermore, LY decreased PDE4 activity in a concentration-dependent manner (58.0% of PDE4 activity at LY concentrations of 10 μM). Notably, PI3K(γ) coimmunoprecipitated with PDE4D. The β(2)-AR inverse agonist, ICI 118,551 (ICI), abolished induced increases in contractility.


CONCLUSIONS
PI3K modulates myocardial contractility by a cAMP-dependent mechanism through the regulation of the catalytic activity of PDE4. Furthermore, basal agonist-independent activity of the β(2)-AR and its resultant cAMP production and enhancement of the catalytic activity of PDE4 through PI3K represents an example of integrative cellular signaling, which controls cAMP dynamics and thereby contractility in the cardiac myocyte. These results help to explain the mechanism by which milrinone is able to increase myocardial contractility in the absence of direct β-adrenergic stimulation and why it can further augment contractility in the presence of maximal β-adrenergic stimulation.
Gregg
Christopher J
CJ
Johns Hopkins Medical Institutions, Baltimore, MD 21287, USA.
Steppan
Jochen
J
Gonzalez
Daniel R
DR
Champion
Hunter C
HC
Phan
Alexander C
AC
Nyhan
Daniel
D
Shoukas
Artin A
AA
Hare
Joshua M
JM
Barouch
Lili A
LA
Berkowitz
Dan E
DE
D000818:Animals; D000242:Cyclic AMP / Q000378:metabolism*; D054703:Cyclic Nucleotide Phosphodiesterases, Type 4 / Q000378:metabolism*; D004789:Enzyme Activation / Q000502:physiology; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008810:Mice, Inbred C57BL; D009200:Myocardial Contraction / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*; D020298:U937 Cells
D018343:Receptors, Adrenergic, beta-2; D000242:Cyclic AMP; D019869:Phosphatidylinositol 3-Kinases; D054703:Cyclic Nucleotide Phosphodiesterases, Type 4; C531980:PDE4D protein, mouse; D019206:GTP-Binding Protein alpha Subunits, Gi-Go