Adenosine A1 and A3 receptor agonists inhibit nonadrenergic, noncholinergic relaxations in the guinea pig isolated trachea

Background: Adenosine affects the tone and reactivity of airways by activating specific membrane receptors, named A1, A2a, A2b 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 pKB 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.