Contribution of the Extracellular cAMP-Adenosine Pathway to Dual Coupling of β₂-Adrenoceptors to G_sand G_iProteins in Mouse Skeletal Muscle

Abstract

␤ 2 -Adrenoceptor (␤ 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 ␤ 2 -AR to G s and G i proteins and the influence of the ␤ 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 ␤ 2 -AR inotropic response was also addressed. The effects of clenbuterol/fenoterol (␤ 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 M), 1 M forskolin, and 20 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 ␤ 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 ␤ 2 -AR to G i protein which depends on cAMP efflux. Remarkably, the PTX-sensitive ␤ 2 -AR inotropic effect was inhibited by the A 1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and ecto-5Ј-phosphodiesterase inhibitor ␣,␤-methyleneadenosine 5Ј-diphosphate sodium salt, indicating that ␤ 2 -AR coupling to G i is indirect and dependent on A 1 receptor activation. The involvement of the extracellular cAMP-adenosine pathway in ␤ 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.

Adenosine-A1 receptor agonist induced hyperalgesic priming type II

“…As do most Gi-coupled receptors, the A1-adenosine receptor normally couples to the inhibition of adenylyl cyclase, attenuating intracellular cAMP levels [18; 25; 65; 67], and can reverse nociceptor sensitization and mechanical hyperalgesia induced by pronociceptive inflammatory mediators that act at stimulatory G-protein (Gs) coupled GPCRs [1; 3; 27; 39; 53; 56; 57; 68], the latter stimulate adenylyl cyclase to increase cAMP levels in the cell [37; 47; 50; 64]. Since the repeated administration of the A1-adenosine receptor agonist CPA to the peripheral receptive field of the primary afferent nociceptor produces tolerance and dependence [1; 3], similar to the effect of MOR agonists [1-3; 12; 15; 34; 46], which induce Type II hyperalgesic priming [7], a proposed model of opioid-induced hyperalgesia (OIH) [6; 40; 43], we determined if the A1-adenosine receptor agonist also produced Type II hyperalgesic priming, and whether priming-induced by repeated activation of this receptor shared mechanisms with Type II hyperalgesic priming induced by a MOR agonist.…”

Cyclic nucleotide permeability through unopposed connexin hemichannels

“…Two examples focus on cyclic adenosine monophosphate (cAMP) as a signal molecule candidate. The extracellular release of cAMP is known to exert effects such as receptor expression in renal cells (Kuzhikandathil et al, 2011) and inhibition of skeletal muscle inotropism (Duarte et al, 2012). Before addressing further questions, such as if hemichannels are involved in a paracrine-like delivery of cAMP, it is essential to understand the characteristics of hemichannel permeability.…”

Extracellular Cyclic AMP/Adenosine Signaling Pathway: A Potential Pharmacological Target for Therapeutic Intervention in Chronic Lymphocytic Leukemia

“…Adenosine contributes to the survival of tumor cells by inhibiting the process of apoptosis-mediated tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and multiresistant drug phenotype of leukemic cells [18], with a high expression of catalytically active ectonucleotidases (such as CD73) being described in CLL [19]. In addition, considering the existence of a cAMP efflux system mediated by multiresistant protein transporters: the blockade of adenosine receptors reduces the negative inotropic effect promoted by extracellular adenosine due to cAMP efflux system, altering important intracellular signaling pathways [20,21]. Thus, we have proposed that the pharmacological modulation of the intra and extracellular signaling mediated by cAMP and adenosine could be a new antitumoral strategy in CLL therapy [22][23][24][25][26].…”