β2-Adrenergic Receptor-induced p38 MAPK Activation Is Mediated by Protein Kinase A Rather than by Gi or Gβγ in Adult Mouse Cardiomyocytes

Increasing evidence shows that stimulation of beta-adrenergic receptor (AR) activates mitogen-activated protein kinases (MAPKs), in addition to the classical G(s)-adenylyl cyclase-cAMP-dependent protein kinase (PKA) signaling cascade. In the present study, we demonstrate a novel beta(2)-AR-mediated cross-talk between PKA and p38 MAPK in adult mouse cardiac myocytes expressing beta(2)-AR, with a null background of beta(1)beta(2)-AR double knockout. beta(2)-AR stimulation by isoproterenol increased p38 MAPK activity in a time- and dose-dependent manner. Inhibiting G(i) with pertussis toxin or scavenging Gbetagamma with betaARK-ct overexpression could not prevent beta(2)-AR-induced p38 MAPK activation. In contrast, a specific peptide inhibitor of PKA, PKI (5 microm), completely abolished the stimulatory effect of beta(2)-AR, suggesting that beta(2)-AR-induced p38 MAPK activation is mediated via a PKA-dependent mechanism, rather than by G(i) or Gbetagamma. This conclusion was further supported by the ability of forskolin (10 microm), an adenylyl cyclase activator, to elevate p38 MAPK activity in a PKI-sensitive manner. Furthermore, inhibition of p38 MAPK with SB203580 (10 microm) markedly enhanced the beta(2)-AR-mediated contractile response, without altering base-line contractility. These results provide the first evidence that cardiac beta(2)-AR activates p38 MAPK via a PKA-dependent signaling pathway, rather than by G(i) or Gbetagamma, and reveal a novel role of p38 MAPK in regulating cardiac contractility.