Modulation of ischaemic contracture in mouse hearts: A 'supraphysiological' response to adenosine

While inhibition of ischaemic contracture was one of the first documented cardioprotective actions of exogenously applied adenosine, it is not known whether this is a normal function of endogenous adenosine generated during ischaemic stress. Additionally, the relevance of delayed contracture to postischaemic outcome is unclear. We tested the ability of endogenous versus exogenous adenosine to modify contracture (and postischaemic outcomes) in C57/Bl6 mouse hearts. During ischaemia, untreated hearts developed peak contracture (PC) of 85 ± 5 mmHg at 8.9 ± 0.8 min, with time to reach 20 mmHg (time to onset of contracture; TOC) of 4.4 ± 0.3 min. Adenosine (50 μm) delayed TOC to 6.7 ± 0.6 min, as did pretreatment with 10 μm 2-chloroadenosine (7.2 ± 0.5 min) or 50 nm of A₁ adenosine receptor (AR) agonist N⁶-cyclohexyladenosine (CHA) (6.7 ± 0.3 min), but not A_2AAR or A₃AR agonists (20 nm 2-[4-(2-carboxyethyl) phenethylamino]-5′ N-methylcarboxamidoadenosine (CGS21680) or 150 nm 2-chloro-N⁶-(3-iodobenzyl)-adenosine-5′-N- methyluronamide (Cl-IB-MECA), respectively). Adenosinergic contracture inhibition was eliminated by A₁AR gene knockout (KO), mimicked by A₁AR overexpression, and was associated with preservation of myocardial [ATP]. This adenosine-mediated inhibition of contracture was, however, only evident after prolonged (10 or 15 min) and not brief (3 min) pretreatment. Ischaemic contracture was also insensitive to endogenously generated adenosine, since A₁AR KO, and non-selective and A ₁AR-selective antagonists (50 μm 8-sulphophenyltheophylline and 150 nm 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), respectively), all failed to alter intrinsic contracture development. Finally, delayed contracture with A₁AR agonism/overexpression or ischaemic 2,3-butanedione monoxime (BDM; 5 μm to target Ca²⁺ cross-bridge formation) was linked to enhanced postischaemic outcomes. In summary, adenosinergic inhibition of contracture is solely A₁AR mediated; the response is 'supraphysiological', evident only with significant periods of pre-ischaemic AR agonism (or increased A₁AR density); and ischaemic contracture appears insensitive to locally generated adenosine, potentially owing to the rapidity of contracture development versus the finite time necessary for expression of AR-mediated cardioprotection.