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 microm) delayed TOC to 6.7 +/- 0.6 min, as did pretreatment with 10 microm
2-chloroadenosine (7.2 +/- 0.5 min) or 50 nm of A(1)
adenosine receptor (AR) agonist
N(6)-cyclohexyladenosine (CHA) (6.7 +/- 0.3 min), but not A(2A)AR or A(3)AR agonists (20 nm 2-[4-(2-carboxyethyl) phenethylamino]-5' N-methylcarboxamidoadenosine (
CGS21680) or 150 nm 2-chloro-N(6)-(3-iodobenzyl)-
adenosine-5'-N-methyluronamide (Cl-
IB-MECA), respectively). Adenosinergic
contracture inhibition was eliminated by A(1)AR gene knockout (KO), mimicked by A(1)AR 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 A(1)AR KO, and non-selective and A(1)AR-selective antagonists (50 microm 8-sulphophenyltheophylline and 150 nm 8-cyclopentyl-1, 3-dipropylxanthine (
DPCPX), respectively), all failed to alter intrinsic
contracture development. Finally, delayed
contracture with A(1)AR agonism/overexpression or ischaemic
2,3-butanedione monoxime (BDM; 5 microm to target Ca(2+) cross-bridge formation) was linked to enhanced postischaemic outcomes. In summary, adenosinergic inhibition of
contracture is solely A(1)AR mediated; the response is 'supraphysiological', evident only with significant periods of pre-ischaemic AR agonism (or increased A(1)AR 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.