The cytosolic phospholipase A2 pathway, a safeguard of beta2-adrenergic cardiac effects in rat.

We have recently demonstrated that in human heart, β(2)-adrenergic receptors (β(2)-ARs) are biochemically coupled not only to the classical adenylyl cyclase (AC) pathway but also to the cytosolic phospholipase A(2) (cPLA(2)) pathway (Pavoine, C., Behforouz, N., Gauthier, C., Le Gouvello, S., Roudot-Thoraval, F., Martin, C. R., Pawlak, A., Feral, C., Defer, N., Houel, R., Magne, S., Amadou, A., Loisance, D., Duvaldestin, P., and Pecker, F. (2003) Mol. Pharmacol. 64, 1117-1125). In this study, using Fura-2-loaded cardiomyocytes isolated from adult rats, we showed that stimulation of β(2)-ARs triggered an increase in the amplitude of electrically stimulated [Ca2+](i) transients and contractions. This effect was abolished with the PKA inhibitor, H89, but greatly enhanced upon addition of the selective cPLA(2) inhibitor, AACOCF(3). The β(2)-AR/cPLA(2) inhibitory pathway involved G(i) and MSK1. Potentiation of β(2)-AR/AC/PKA-induced Ca2+ responses by AACOCF3 did not rely on the enhancement of AC activity but was associated with eNOS phosphorylation (Ser(1177)) and L-NAME-sensitive NO production. This was correlated with PKA-dependent phosphorylation of PLB (Ser(16)). The constraint exerted by the β(2)-AR/cPLA(2) pathway on the β(2)-AR/AC/PKA-induced Ca2+ responses required integrity of caveolar structures and was impaired by Filipin III treatment. Immunoblot analyses demonstrated zinterol-induced translocation of cPLA(2) and its cosedimentation with MSK1, eNOS, PLB, and sarcoplasmic reticulum Ca2+ pump (SERCA) 2a in a low density caveolin-3-enriched membrane fraction. This inferred the gathering of β(2)-AR signaling effectors around caveolae/sarcoplasmic reticulum (SR) functional platforms. Taken together, these data highlight cPLA(2) as a cardiac β(2)-AR signaling pathway that limits β(2)-AR/AC/PKA-induced Ca2+ responses in adult rat cardiomyocytes through the impairment of eNOS activation and PLB phosphorylation.