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.