The role of membrane cholesterol in effects of β2 adrenoreceptors activation of the mouse atrium

The role of cytoplasmic calcium in the regulation of the resting potential in the pulmonary veins myocardium in rats and mice

We have demonstrated the phenomenon of Са(2+)-induced hyperpolarization in the myocardium of pulmonary veins (PVs) in rats. An increase in cytoplasmic calcium [Са(2+)] i was shown to shift the resting potential (RP) in the PVs towards more negative values. The compounds inducing an increase in [Са(2+)] i , such as isoproterenol (10 μM), caffeine (5 mM), and ryanodine (0.01 μM), caused hyperpolarization of 10 ± 2, 9 ± 1.3, and 4.1 ± 2 mV, respectively. The inhibition of calcium-dependent potassium currents (IKCa) did not change RP of PVs under the control conditions and did not affect the Са(2+)-induced hyperpolarization.

Cholesterol regulates contractility and inotropic response to β2-adrenoceptor agonist in the mouse atria: Involvement of Gi-protein-Akt-NO-pathway

Majority of cardiac β2-adrenoceptors is located in cholesterol-rich microdomains. Here, we have investigated the underlying mechanisms by which a slight to moderate cholesterol depletion with methyl-β-cyclodextrin (MβCD, 1 and 5mM) interferes with contractility and inotropic effect of β2-adrenergic agonist (fenoterol, 50μM) in the mouse atria. Treatment with MβCD itself increased amplitude of Ca²⁺ transient but did not change the contraction amplitude due to a clamping action of elevated NO. Cholesterol depletion significantly attenuated the positive inotropic response to fenoterol which is accompanied by increase in NO generation and decrease in Ca²⁺ transient. Influence of 1mM MβCD on the fenoterol-driven changes in both contractility and NO level was strongly attenuated by inhibition of G_i-protein (pertussis toxin), Akt (Akt 1/2 kinase inhibitor) or NO-synthase (L-NAME). After exposure to 5mM MβCD, pertussis toxin or Akt inhibitor could recover the β2-agonist effects on contractility, NO production and Ca²⁺ transient, while L-NAME only reduced NO level. An adenylyl cyclase activator (forskolin, 50nM) had no influence on the MβCD-induced changes in the β2-agonist effects. Obtained results suggest that slight cholesterol depletion upregulates G_i-protein/Akt/NO-synthase signaling that attenuates the positive inotropic response to β2-adrenergic stimulation without altering the Ca²⁺ transient. Whilst moderate cholesterol depletion additionally could suppress the enhancement of the Ca²⁺ transient amplitude caused by the β2-adrenergic agonist administration in G_i-protein/Akt-dependent but NO-independent manner.

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

Fenoterol, a β2-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 Ca2+ 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 β2-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 β2-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 Ca2+ transient and up-regulating NO production. ROS generated in a β2-adrenoceptor-dependent manner can potentiate the activity of some Ca2+ channels. Indeed, inhibition of ryanodine receptors, TRPV-or L-type Ca2+- channels shows a similar efficacy in reduction of positive inotropic effect of both fenoterol and H2O2. 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 β2-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 Ca2+ channels.