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Department of Physiology, University of Tennessee School of Medicine, Memphis, Tennessee 38163
The ability of adenosine
A1 receptors to activate type 2a protein phosphatase (PP2a)
and account for antiadrenergic effects was investigated in rat
myocardial preparations. We observed that the adenosine A1
receptor agonist N6-cyclopentyladenosine (CPA)
significantly reduces the isoproterenol-induced increase in left
ventricular developed pressure of isolated heats, and this effect is
blocked by pretreatment of hearts with the PP2a inhibitor cantharidin.
CPA alone or given in conjunction with isoproterenol stimulation
decreases phosphorylation of phospholamban and troponin I in
ventricular myocytes. These dephosphorylations are blocked by an
adenosine A1 receptor antagonist and by PP2a inhibition
with okadaic acid. Adenosine A1 receptor activation was
also shown to increase carboxymethylation of the PP2a catalytic subunit
(PP2a-C) and cause translocation of PP2a-C to the particulate fraction
in ventricular myocytes. These results support the hypothesis that
adenosine A1 receptor activation leads to methylation of PP2a-C and subsequent translocation of the PP2a holoenzyme. Increases in localized PP2a activity lead to dephosphorylation of key cardiac proteins responsible for the positive inotropic effects of
-adrenergic stimulation.
troponin I; phospholamban
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