The adenosine A1 receptor (A1R) inhibits beta-adrenergic-induced contractile effects (antiadrenergic action), and the adenosine A2A receptor (A2AR) both opposes the A1R action and enhances contractility in the heart. This study investigated the A1R and A2AR function in beta-adrenergic-stimulated, isolated wild-type and A2AR knockout murine hearts. Constant flow and pressure perfused preparations were employed, and the maximal rate of left ventricular pressure (LVP) development (+dp/dt(max)) was used as an index of cardiac function. A1R activation with 2-chloro-N6-cyclopentyladenosine (CCPA) resulted in a 27% reduction in contractile response to the beta-adrenergic agonist isoproterenol (ISO). Stimulation of A2AR with 2-P(2-carboxyethyl)phenethyl-amino-5'-N-ethylcarboxyamidoadenosine (CGS-21680) attenuated this antiadrenergic effect, resulting in a partial (constant flow preparation) or complete (constant pressure preparation) restoration of the ISO contractile response. These effects of A2AR were absent in knockout hearts. Up to 63% of the A2AR influence was estimated to be mediated through its inhibition of the A1R antiadrenergic effect, with the remainder being the direct contractile effect. Further experiments examined the effects of A2AR activation and associated vasodilation with low-flow ischemia in the absence of beta-adrenergic stimulation. A2AR activation reduced by 5% the depression of contractile function caused by the flow reduction and also increased contractile performance over a wide range of perfusion flows. This effect was prevented by the A2AR antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385). It is concluded that in the murine heart, A1R and A2AR modulate the response to beta-adrenergic stimulation with A2AR, attenuating the effects of A1R and also increasing contractility directly. In addition, A2AR supports myocardial contractility in a setting of low-flow ischemia.
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