Functional characterization of coronary vascular adenosine receptors in the mouse

Abstract

1. Coronary responses to adenosine agonists were assessed in perfused mouse and rat hearts. The roles of nitric oxide (NO) and ATP-dependent K+ channels (KATP) were studied in the mouse. 2. Resting coronary resistance was lower in mouse vs rat, as was minimal resistance (2.2±0.1 vs 3.8±0.2 mmHg ml-1 min-1 g-1. Peak hyperaemic flow after 20-60 s occlusion was greater in mouse. 3. Adenosine agonists induced coronary dilation in mouse, with pEC50s of 9.4±0.1 for 2-[p-(2-carboxyethyl)phenethylamino]-5′-N-ethyl carboxamidoadenosine (CGS21680, A2A-selective agonist), 9.3±0.1 for 5′-N-ethylcarboxamidoadenosine (NECA, A1/A2 agonist), 8.4±0.1 for 2-chloroadenosine (A1/A2 agonist), 7.7±0.1 for N6-(R)-(phenylisopropyl)adenosine (R-PIA, A1/A2B selective), and 6.8±0.2 for adenosine. The potency order (CGS21680 = NECA > 2-chloroadenosine > R-PIA > adenosine) supports A2A adenosine receptor-mediated dilation in mouse coronary vessels. 0.2-2 μM of the A2B-selective antagonist alloxazine failed to alter CGS21680 or 2-chloroadenosine responses. 4. pEC50s in rat were 6.7±0.2 for CGS21680, 7.3±0.1 for NECA, 7.6±0.1 for 2-chloroadenosine, 7.2±0.1 for R-PIA, and 6.2±0.1 for adenosine (2-chloroadenosine > NECA = R-PIA > CGS21680 > adenosine), supporting an A2B adenosine receptor response. 5. NO-synthase antagonism with 50 μM NG-nitro L-arginine (L-NOARG) increased resistance by ∼25%, and inhibited responses to CGS21680 (pEC50 = 9.0±0.1), 2-chloroadenosine (pEC50 = 7.3±0.2) and endothelial-dependent ADP, but not sodium nitroprusside (SNP). KATP channel blockade with 5 μM glibenclamide increased resistance by ∼80% and inhibited responses to CGS21680 in control (pEC50 = 8.3±0.1) and L-NOARG-treated hearts (pEC50 = 7.3±0.1), and to 2-chloroadenosine in control (pEC50 = 6.7±0.1) and L-NOARG-treated hearts (pEC50 = 5.9±0.2). 6. In summary, mouse coronary vessels are more sensitive to adenosine than rat vessels. A2A adenosine receptors mediate dilation in mouse coronary vessels vs A2B receptors in rat. Responses in the mouse involve a sensitive NO-dependent response and KATP-dependent dilation.