β2-adrenergic receptor agonists increase intracellular free Ca2+ concentration cycling in ventricular cardiomyocytes through p38 and p42/44 MAPK-mediated cytosolic phospholipase A2 activation

We have recently reported that arachidonic acid mediates beta (2)-adrenergic receptor (AR) stimulation of [Ca2+](i) cycling and cell contraction in embryonic chick ventricular cardiomyocytes (Pavoine, C., Magne, S., Sauvadet, A., and Pecker, F. (1999) J. Biol. Chem.. 274, 628-637). In the present work, we demonstrate that beta (2)-AR agonists trigger arachidonic acid release via translocation and activation of cytosolic phospholipase A. (cPLA(2)) and increase caffeine-releasable Ca2+ pools from Fura-2-loaded cells. We also show that beta (2)-AR agonists trigger a rapid and dose-dependent phosphorylation of both p38 and p42/44 MAPKs. Translocation and activation of ePLA(2), as well as Ca2+ accumulation in sarcoplasmic reticulum stores sensitive to caffeine and amplification of [Ca2+](i) cycling in response to beta (2)-AR agonists, were blocked by inhibitors of the p38 or p42/44 MAPK pathway (SB203580 and PD98059, respectively), suggesting a role of both MAPK subtypes in beta (2)-AR stimulation. In contrast, beta (1)-AP. stimulation of [Ca2+](i) cycling was rather limited by the MAPKs, clearly proving the divergence between beta (2)-AR and beta (1)-AR signaling systems. This study presents the first evidence for the coupling of beta (2)-AR to cardiac ePLA(2) and points out the key role of the MAPK pathway in the intracellular signaling elicited by positive inotropic beta (2)-AR agonists in heart.