De Arcangelis Vania, Soto Dagoberto, Xiang Yang
Department of Molecular and Integrative Physiology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA.
Mol Pharmacol. 2008 Nov;74(5):1453-62. doi: 10.1124/mol.108.049718. Epub 2008 Aug 14.
Activation of the beta adrenergic receptor (betaAR) induces a tightly controlled cAMP/protein kinase A (PKA) activity to ensure an agonist dose-dependent and saturable contraction response in animal heart. We have found that stimulation of beta(1)AR by isoproterenol induces maximal contraction responses at the dose of 1 microM in cardiac myocytes; however, cAMP accumulation continues to increase with higher agonist concentrations. Dose-dependent cAMP accumulation is tightly controlled by negative regulator phosphodiesterase 4 (PDE4) that hydrolyzes cAMP. At 1 nM isoproterenol, cAMP accumulation is minimal because of the hydrolysis of cAMP by PDE4, which leads to a 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 microM 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. It is interesting that activities of both PKA and protein phosphatase 2A (PP2A) are enhanced under beta(1)AR activation with 10 microM isoproterenol, and PP2A is recruited to PKA/A kinase-anchoring protein complex. Inhibition of PP2A with okadaic acid further enhances the phosphorylation of phospholamban and TnI as well as contraction responses induced by 10 microM isoproterenol. Therefore, PP2A plays a key role in limiting PKA phosphorylation of phospholamban and TnI for myocyte contraction responses under beta(1)AR stimulation.
β肾上腺素能受体（βAR）的激活会诱导严格调控的环磷酸腺苷/蛋白激酶A（PKA）活性，以确保动物心脏中激动剂剂量依赖性和饱和性的收缩反应。我们发现，异丙肾上腺素对β1AR的刺激在心肌细胞中1微摩尔剂量时诱导最大收缩反应；然而，随着激动剂浓度升高，环磷酸腺苷积累仍持续增加。剂量依赖性的环磷酸腺苷积累受到水解环磷酸腺苷的负调节因子磷酸二酯酶4（PDE4）的严格控制。在1纳摩尔异丙肾上腺素作用下，由于PDE4对环磷酸腺苷的水解作用，环磷酸腺苷积累极少，这导致受磷蛋白和肌钙蛋白I（TnI）的PKA磷酸化及收缩反应仅有小幅增加。用咯利普兰抑制PDE4活性可增强环磷酸腺苷积累，使受磷蛋白和TnI的PKA磷酸化达到最大值，并增强心肌细胞收缩反应。相比之下，在10微摩尔异丙肾上腺素作用下，尽管有PDE4的负性作用，环磷酸腺苷积累足以使受磷蛋白和TnI的PKA磷酸化达到最大值。用咯利普兰抑制PDE4可增强环磷酸腺苷积累，但不会增强PKA磷酸化及收缩反应。有趣的是，在10微摩尔异丙肾上腺素激活β1AR的情况下，PKA和蛋白磷酸酶2A（PP2A）的活性均增强，且PP2A被招募到PKA/A激酶锚定蛋白复合物中。用冈田酸抑制PP2A可进一步增强受磷蛋白和TnI的磷酸化以及10微摩尔异丙肾上腺素诱导的收缩反应。因此，在β1AR刺激下，PP2A在限制受磷蛋白和TnI的PKA磷酸化以产生心肌细胞收缩反应中起关键作用。
