Activation of ecto-5′-nucleotidase by protein kinase C and its role in ischaemic tolerance in the canine heart

1. Ischaemic preconditioning (IP) protects the myocardium against irreversible ischaemic injury by activating protein kinase C (PKC). The mechanism by which PKC protects the myocardium is unknown. We have shown that PKC increases the activity of ecto-5'-nucleotidase (ecto-5'-N) and thereby the production of adenosine in cardiomyocytes which may protect the myocardium against ischaemia-reperfusion injury in vivo. 2. The objective of this study was to elucidate the possible role of PKC-induced activation of ecto-5'-N in the cardioprotection associated with IP in the canine heart. 3. IP increased the activities of both ecto-5'-N and PKC, and minimized ischaemic damage (infarct size: 7.5 +/- 1.8 vs. 42.3 +/- 2.8%, P < 0.01 vs. the control group). Treatment with the PKC activator (4 beta-phorbol 12-myristate-13-acetate) also reduced infarct size (13.5 +/- 2.9%, P < 0.01 vs. the control group). 8-Sulfophenyltheophylline (an antagonist of adenosine receptors) or alpha,beta-methyleneadenosine 5'-diphosphate (an inhibitor of ecto-5'-N) eliminated the cardioprotective effect of the PKC activator (infarct size: 36.6 +/- 3.9 and 34.7 +/- 4.2%, respectively), suggesting that PMA limits infarct size by increasing the activity of ecto-5'-N and the adenosine level. 4. The PMA-induced cardioprotection was blunted by GF109203X (an inhibitor of PKC, infarct size: 36.2 +/- 3.1%), but not by pretreatment with dexamethasone (infarct size, 14.2 +/- 2.6%). 5. We conclude that the PMA- and IP-induced cardioprotection is attributable to phosphorylation and activation of ecto-5'-N.