Adenosine A2Areceptor modulation of juvenile female rat skeletal muscle microvessel permeability

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Abstract
Little is known of the regulation of skeletal muscle microvascular exchange under resting or stimulating conditions. Adenosine (ADO) levels in skeletal muscle increase during physiological (exercise) and pathological (hypoxia, inflammation, and ischemia) conditions. Later stages of these pathologies are characterized by the loss of vascular barrier integrity. This study focused on determining which ADO receptor mediates the robust reduction in microvessel permeability to rat serum albumin ( PsRSA) observed in juvenile female rats. In microvessels isolated from abdominal skeletal muscle, ADO suffusion induced a concentration-dependent reduction in arteriolar [log(IC50) = −9.8 ± 0.2 M] and venular [log(IC50) = −8.4 ± 0.2 M] PsRSA. RT-PCR and immunoblot analysis demonstrated mRNA and protein expression of ADO A1, A2A, A2B, and A3receptors in both vessel types, and immunofluorescence assay revealed expression of the four subtype receptors in the microvascular walls (endothelium and smooth muscle). PsRSAresponses of arterioles and venules to ADO were blocked by 8-( p-sulphophenyl)theophylline, a nonselective A1and A2antagonist. An A2Aagonist, CGS21680 , was more potent than the A1agonist, cyclopentyladenosine, or the most-selective A2Bagonist, 5′-( N-ethylcarboxamido)adenosine. The ability of CGS21680 or ADO to reduce PsRSAwas abolished by the A2Aantagonist, ZM241385. An adenylyl cyclase inhibitor, SQ22536, blocked the permeability response to ADO. In aggregate, these results demonstrate that, in juvenile females (before the production of the reproductive hormones), ADO enhances skeletal muscle arteriole and venule barrier function predominantly via A2Areceptors using activation of adenylyl cyclase-signaling mechanisms.