We showed previously that 20 min of low-volume anoxia ("ischemia") and 20 min of "reperfusion" preconditions quiescent pediatric myocyte cultures against damage resulting from 90 min of subsequent prolonged ischemia and 30 min of reperfusion. The purpose of this study was to assess the roles of adenosine and protein kinase C (PKC) in this preconditioning model. Our results suggest that 1) preconditioned myocytes secrete a protective mediator(s) into the "ischemic" supernatant that is transferable to other cells, and adenosine is released into the supernatant in quantities sufficient for adenosine-receptor activation (2) preconditioning is inhibited by adenosine-receptor antagonism, and myocyte protection similar to preconditioning can be achieved with exogenously administered adenosine or adenosine-receptor stimulation; (3) brief ischemic and adenosine-induced myocyte preconditioning is mimicked by the phorbol ester 4beta-phorbol 12-myristate 13-acetate (PKC agonist) and inhibited by PKC antagonists; and (4) brief ischemic and adenosine-induced myocyte preconditioning both induce PKC translocation to myocyte membranes and increase the PKC phosphorylation rate. These data suggest that adenosine released from ischemic human pediatric myocytes mediates preconditioning through activation of PKC.

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