Contribution of the Extracellular cAMP-Adenosine Pathway to Dual Coupling of β2-Adrenoceptors to Gs and Gi Proteins in Mouse Skeletal Muscle

beta(2)-Adrenoceptor (beta(2)-AR) agonists increase skeletal muscle contractile force via activation of G(s) protein/adenylyl cyclases (AC) and increased generation of cAMP. Herein, we evaluated the possible dual coupling of beta(2)-AR to G(s) and G(i) proteins and the influence of the beta(2)-AR/G(s)-G(i) /cAMP signaling cascade on skeletal muscle contraction. Assuming that the increment of intracellular cAMP is followed by cAMP efflux and extracellular generation of adenosine, the contribution of the extracellular cAMP-adenosine pathway on the beta(2)-AR inotropic response was also addressed. The effects of clenbuterol/fenoterol (beta(2)-AR agonists), forskolin (AC activator), cAMP/8-bromo-cAMP, and adenosine were evaluated on isometric contractility of mouse diaphragm muscle induced by supramaximal direct electrical stimulation (0.1 Hz, 2 ms duration). Clenbuterol/fenoterol (10-1000 mu M), 1 mu M forskolin, and 20 mu M rolipram induced transient positive inotropic effects that peaked 30 min after stimulation onset, declining to 10 to 20% of peak levels in 30 min. The late descending phase of the beta(2)-AR agonist inotropic effect was mimicked by either cAMP or adenosine and abolished by preincubation of diaphragm with pertussis toxin (PTX) (G(i) signaling inhibitor) or the organic anion transporter inhibitor probenecid, indicating a delayed coupling of beta(2)-AR to G(i) protein which depends on cAMP efflux. Remarkably, the PTX-sensitive beta(2)-AR inotropic effect was inhibited by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and ecto-5'-phosphodiesterase inhibitor alpha,beta-methyleneadenosine 5'-diphosphate sodium salt, indicating that beta(2)-AR coupling to G(i) is indirect and dependent on A(1) receptor activation. The involvement of the extracellular cAMP-adenosine pathway in beta(2)-AR signaling would provide a negative feedback loop that may limit stimulatory G protein-coupled receptor positive inotropism and potential deleterious effects of excessive contractile response.