Role of adenosine in delayed preconditioning of myocardium.

Myocardial protection conferred by ischemic preconditioning occurs in a bimodal time course. The early cardioprotection wanes rapidly and is succeeded by a delayed phase of protection reducing infarct development, myocardial stunning and arrhythmias. This 'second window' of preconditioning may be evident for up to 72 h. The current mechanistic paradigm for delayed preconditioning against infarction invokes roles for several freely-diffusible molecules, generated during the preconditioning period, that act in autocrine and/or paracrine fashion as triggers of cellular adaptation. These include adenosine, nitric oxide, reactive oxygen species and bradykinin. A role for adenosine receptor activation as a proximal molecular mechanism leading to delayed preconditioning against infarction was established in 1994. Pharmacological adenosine receptor blockade during preconditioning abolishes the acquisition of delayed protection, while transient adenosine A(1) or A(3) receptor activation fully recapitulates protection against infarction (but not against stunning or arrhythmias) 24 h later. Although nitric oxide is a co-trigger of delayed preconditioning, A(1) agonist-induced delayed protection is independent of nitric oxide production. Adenosine receptor activation causes the activation of a complex protein kinase signalling cascade and, putatively, the subsequent activation of gene transcription. The induction or post-translational regulation of several proteins is associated with A(1) agonist-induced delayed protection. These include the mitochondrial manganese-conjugated superoxide dismutase, and the 27-kDa heat shock protein. Opening of K(ATP) channels during the index ischaemic event is an obligatory downstream event mediating A(1) and A(3) agonist induced delayed protection. However, the mechanism of sub-acute regulation of K(ATP) channels following adenosine receptor activation is unknown. Evidence for induction of inducible nitric oxide synthase as a distal mechanism of A(1) agonist-induced delayed protection is equivocal.