Evidence for a beta(2)-adrenergic arachidonic acid pathway in ventricular cardiomyocytes - Regulation by the beta(1)-adrenergic/cAMP pathway

The signaling pathway mediating the contractile effect of beta(2)-adrenergi c receptors (beta(2)-AR) in the heart is still matter of debate. By using e mbryonic chick ventricular cardiomyocytes that express both functional beta (1)-and beta(2)-ARs, me show here that the specific beta(2)-AR agonist, zin terol, increases the amplitude of Ca2+ transients and cell contraction of e lectrically stimulated cells. Zinterol, up to 10 mu M, did not stimulate ad enylyl cyclase activity, and its effect on Ca2+ transients was unmodified b y the specific cAMP antagonist, (Rp)-cAMPS. In contrast, zinterol (10-100 n M) triggered arachidonic acid (AA) release from [H-3]AA-loaded cells via th e activation of the cytosolic phospholipase A(2) (cPLA(2)). Stimulation of the Ca2+ transients by zinterol was abolished by the cPLA(2) inhibitor, AAC OCF(3), and was mimicked by AA (0.3-3 mu M). Both stimulations of [H-3]AA r elease and of [Ca2+](i) cycling by zinterol were abolished after treatment of the cardiomyocytes with pertussis toxin. Although cell responses to beta (2)-AR stimulation were mediated by AA,they were under cAMP control as foll ows: (i) the beta(1)-AR stimulation exerted a cAMP-mediated negative constr aint on the beta(2)-AR/cPLA(2) pathway; (ii) cAMP potentiated AA action dow nstream beta-AR stimulation. We conclude that, in cardiomyocytes, beta(2)-A R is coupled to cPLA(2) activation via a pertussis toxin-sensitive G protei n. These results demonstrate the involvement of the cPLA(2)/AA pathway in m ediating positive inotropic effects, which could potentially compensate for a defective cAMP pathway.