β2-adrenoceptor agonist-evoked reactive oxygen species generation in mouse atria: implication in delayed inotropic effect

Abstract Fenoterol, a β 2 -adrenoceptor agonist, has anti-apoptotic action in cardiomyocytes and induces a specific pattern of downstream signaling. We have previously reported that exposure to fenoterol (5 μM) results in a delayed positive inotropic effect which is related to changes in both Ca 2+ transient and NO. Here, the changes in reactive oxygen species (ROS) production in response to the fenoterol administration and the involvement of ROS in effect of this agonist on contractility were investigated in mouse isolated atria. Stimulation of β 2 -adrenoceptor increases a level of extracellular ROS, while intracellular ROS level rises only after removal of fenoterol from the bath. NADPH-oxidase inhibitor (apocynin) prevents the increase in ROS production and the Nox2 isoform is immunofluorescently colocalized with β 2 -adrenoceptor at the atrial myocytes. Treatments with antioxidants (N-acetyl- l -cysteine, NADPH inhibitors, exogenous catalases) significantly inhibit the fenoterol induced increase in the contraction amplitude, probably by attenuating Ca 2+ transient and up-regulating NO production. ROS generated in a β 2 -adrenoceptor-dependent manner can potentiate the activity of some Ca 2+ channels. Indeed, inhibition of ryanodine receptors, TRPV-or l -type Ca 2+- channels shows a similar efficacy in reduction of positive inotropic effect of both fenoterol and H 2 O 2 . In addition, detection of mitochondrial ROS indicates that fenoterol triggers a slow increase in ROS which is prevented by rotenone, but rotenone has no impact on the inotropic effect of fenoterol. We suggest that stimulation of β 2 -adrenoceptor with fenoterol causes the activation of NADPH-oxidase and after the agonist removal extracellularly generated ROS penetrates into the cell, increasing the atrial contractions probably via Ca 2+ channels.