Received for publication June 17, 2008.
Revised August 11, 2008.
Accepted for publication August 13, 2008.

Phosphodieasterase 4 and phosphatase 2A differentially regulate cAMP/PKA signaling for cardiac myocyte contraction under stimulation of ₁ adrenergic receptor

Abstract

Activation of adrenergic receptor (AR) induces a tightly controlled cAMP/PKA activity to ensure an agonist dose-dependent and saturable contraction response in animal heart. We have found that stimulation of ₁AR by isoproterenol induces maximal contraction responses at the dose of 10^-6 M in cardiac myocytes, however cAMP accumulaton continues to increase with higher agonist concentrations. Dose dependent cAMP accumulation is tightly controlled by negative regulator phosphodiesterase 4 (PDE4) that hydrolyzes cAMP. At 10^-9 M isoproterenol, cAMP accumulation is minimal due to the hydrolysis of cAMP by PDE4, which leads to small increase in PKA phosphorylation of phospholamban and troponin I (TnI), and contraction responses. Inhibition of PDE4 activity with rolipram enhances cAMP accumulation, yields maximal PKA phosphorylation of phospholamban and TnI, and myocyte contraction responses. In contrast, at 10^-5 M isoproterenol, despite the negative effect of PDE4, cAMP accumulation is sufficient for maximal PKA phosphorylation of phospholamban and TnI. Inhibition of PDE4 with rolipram enhances cAMP accumulation, but not PKA phosphorylation and contraction responses. Interestingly, activities of both PKA and phosphatase 2A (PP2A) are enhanced under ₁AR activation with 10-5 M isoproterenol, and PP2A is recruited to PKA/AKAP complex. Inhibition of PP2A with okadaic acid further enhances the phosphorylation of phospholamban and TnI as well as contraction responses induced by 10^-5 M isoproterenol. Therefore, PP2A plays a key role to limit PKA phosphorylation of phospholamban and TnI for myocyte contraction responses under ₁AR stimulation.

Key words: Adrenergic, Adenylyl cyclases, cAMP, Phosphodiesterases, Protein Kinase A, Protein ser/thr Phosphatases, Phosphorylation/Dephosphorylation