Evidence for a beta2-adrenergic/arachidonic acid pathway in ventricular cardiomyocytes. Regulation by the beta1-adrenergic/camp pathway.

The signaling pathway mediating the contractile effect of beta2-adrenergic receptors (beta2-AR) in the heart is still matter of debate. By using embryonic chick ventricular cardiomyocytes that express both functional beta1-and beta2-ARs, we show here that the specific beta2-AR agonist, zinterol, increases the amplitude of Ca2+ transients and cell contraction of electrically stimulated cells. Zinterol, up to 10 microM, did not stimulate adenylyl cyclase activity, and its effect on Ca2+ transients was unmodified by the specific cAMP antagonist, (Rp)-cAMPS. In contrast, zinterol (10-100 nM) triggered arachidonic acid (AA) release from [3H]AA-loaded cells via the activation of the cytosolic phospholipase A2 (cPLA2). Stimulation of the Ca2+ transients by zinterol was abolished by the cPLA2 inhibitor, AACOCF3, and was mimicked by AA (0.3-3 microM). Both stimulations of [3H]AA release and of [Ca2+]i cycling by zinterol were abolished after treatment of the cardiomyocytes with pertussis toxin. Although cell responses to beta2-AR stimulation were mediated by AA, they were under cAMP control as follows: (i) the beta1-AR stimulation exerted a cAMP-mediated negative constraint on the beta2-AR/cPLA2 pathway; (ii) cAMP potentiated AA action downstream beta-AR stimulation. We conclude that, in cardiomyocytes, beta2-AR is coupled to cPLA2 activation via a pertussis toxin-sensitive G protein. These results demonstrate the involvement of the cPLA2/AA pathway in mediating positive inotropic effects, which could potentially compensate for a defective cAMP pathway.