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Exchange protein activated by cAMP (Epac) mediates cAMP‐dependent but protein kinase A‐insensitive modulation of vascular ATP‐sensitive potassium channels
Author(s) -
Purves Gregor I.,
Kamishima Tomoko,
Davies Lowri M.,
Quayle John M.,
Dart Caroline
Publication year - 2009
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2009.173534
Subject(s) - protein kinase a , forskolin , adenylyl cyclase , activator (genetics) , bapta , chemistry , phosphatase , microbiology and biotechnology , potassium channel , intracellular , gi alpha subunit , pinacidil , calcineurin , biophysics , g protein , medicine , kinase , endocrinology , signal transduction , biochemistry , biology , enzyme , receptor , glibenclamide , diabetes mellitus , transplantation
Exchange proteins directly activated by cyclic AMP (Epacs or cAMP‐GEF) represent a family of novel cAMP‐binding effector proteins. The identification of Epacs and the recent development of pharmacological tools that discriminate between cAMP‐mediated pathways have revealed previously unrecognized roles for cAMP that are independent of its traditional target cAMP‐dependent protein kinase (PKA). Here we show that Epac exists in a complex with vascular ATP‐sensitive potassium (K ATP ) channel subunits and that cAMP‐mediated activation of Epac modulates K ATP channel activity via a Ca 2+ ‐dependent mechanism involving the activation of Ca 2+ ‐sensitive protein phosphatase 2B (PP‐2B, calcineurin). Application of the Epac‐specific cAMP analogue 8‐pCPT‐2′‐ O ‐Me‐cAMP, at concentrations that activate Epac but not PKA, caused a 41.6 ± 4.7% inhibition (mean ± s.e.m .; n = 7) of pinacidil‐evoked whole‐cell K ATP currents recorded in isolated rat aortic smooth muscle cells. Importantly, similar results were obtained when cAMP was elevated by addition of the adenylyl cyclase activator forskolin in the presence of the structurally distinct PKA inhibitors, Rp‐cAMPS or KT5720. Activation of Epac by 8‐pCPT‐2′‐ O ‐Me‐cAMP caused a transient 171.0 ± 18.0 n m ( n = 5) increase in intracellular Ca 2+ in Fura‐2‐loaded aortic myocytes, which persisted in the absence of extracellular Ca 2+ . Inclusion of the Ca 2+ ‐specific chelator BAPTA in the pipette‐filling solution or preincubation with the calcineurin inhibitors, cyclosporin A or ascomycin, significantly reduced the ability of 8‐pCPT‐2′‐ O ‐Me‐cAMP to inhibit whole‐cell K ATP currents. These results highlight a previously undescribed cAMP‐dependent regulatory mechanism that may be essential for understanding the physiological and pathophysiological roles ascribed to arterial K ATP channels in the control of vascular tone and blood flow.