Adenosine A₁ and A₃ Receptor Agonists Inhibit Nonadrenergic, Noncholinergic Relaxations in the Guinea Pig Isolated Trachea

Abstract

Background: Adenosine affects the tone and reactivity of airways by activating specific membrane receptors, named A₁, A_2a, A_2b and A₃. It affects cellular activities either directly by regulating membrane ion exchanges and polarization, or indirectly by modifying neurotransmitter release. Objectives: We assessed the effect of A₁ and A₃ receptor activation on electrically induced nonadrenergic, noncholinergic (NANC) relaxations in the guinea pig isolated trachea and the localization of A₁ and A₃ receptors in tracheal inhibitory neurons. Methods: NANC responses at 3 Hz were evaluat- ed in the presence of 2-chloro-N⁶-cyclopentyladenosine (CCPA), a selective A₁ agonist, and 2-chloro-N⁶-(3-iodobenzyl)-adenosine-5′-N-methyluronamide (Cl-IB-MECA), a selective A₃ agonist, before and after the administration of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A₁ antagonist, or 9-chloro-2-(2-furanyl)-5-((phenylacetyl)amino[1,2,4]triazolo[1,5-c])quinazoline (MRS 1220), a selective A₃ antagonist, respectively. For immunohistochemistry, tissues were exposed to antibodies to HuC/D, a general neuronal marker, neuronal nitric oxide synthase (nNOS), and A₁ or A₃ adenosine receptors and processed by indirect immunofluorescence. Results: CCPA (10 nM–3 μM) inhibited NANC relaxations. DPCPX (10 nM) failed to antagonize the effect of CCPA, but inhibited per se NANC relaxations (range 0.1–100 nM). CCPA (10 nM–10 μM) contracted unstimulated tracheal preparations, an effect antagonized by 10 nM DPCPX, with a pK_B value of 8.43. Cl-IB-MECA (10 nM–3 μM) inhibited NANC relaxations through a mechanism antagonized by MRS 1220 (100 nM). A₁- and A₃-positive neurons containing nNOS were detected in tracheal sections. Conclusions: Enogenous adenosine may induce airway hyperresponsiveness by inhibiting NANC relaxations via A₁ and A₃ receptors.

Purinergic Signaling in the Airways

“…A 1 and A 3 receptor agonists were shown to inhibit NANC relaxation in isolated guinea pig trachea (Dellabianca et al, 2009). Adenosine deaminase reduced the inhibitory response to adenosine and NANC inhibitory nerve stimulation in guinea pig trachea (Satchell, 1984b).…”

Inhaled Ammonium Persulphate Inhibits Non-Adrenergic, Non-Cholinergic Relaxations in the Guinea Pig Isolated Trachea

“…A recent study by our group showed that adenosine, which is a mediator of several inflammatory processes, can impair the non-adrenergic, non-cholinergic (NANC) relaxation in the guinea pig isolated trachea by activating two distinct receptors [9]. The contribution of transmission by vasoactive intestinal peptide (VIP), nitric oxide (NO) and other putative neurotransmitters to nerve-mediated relaxant responses of mammalian airway smooth muscle is not yet established, in both normal and pathological conditions [6].…”