Myocardial protection by insulin is dependent on phospatidylinositol 3-kinase but not protein kinase C or K-ATP channels in the isolated rabbit heart

Because tyrosine kinase blockade prevents protection by ischemic preconditi oning (PC) in several species, activation of tyrosine kinase appears to be critical for cardioprotection. The tyrosine kinase's identity, however, is unknown. The present study tested whether activation of a receptor tyrosine kinase, the insulin receptor, could mimic PC and if the mechanism of prote ction was similar to that of PC. Isolated rabbit hearts were subjected to 3 0 min of regional ischemia and 2 h of reperfusion. Infarct size: was determ ined by triphenyltetrazolium staining and expressed as a percentage of the area at risk. Infarct size in control hearts was 32.6 +/- 2.3 %. A 5-min in fusion of insulin (5 mU/ml) followed by a 10-min washout period prior to is chemia significantly reduced infarction to 14.7 +/- 2.1 % (P < 0.05). The t yrosine kinase inhibitor genistein (50 mu M) given around the insulin infus ion blocked protection (28.9 +/- 2.8 %). However, when present during the o nset of ischemia, genistein had no effect on protection triggered by insuli n (14.0 +/- 2.4 %; P < 0.05). Inhibition of either PKC by polymyxin B (50 m u M) or K-ATP channels by 5-hydroxydecanoate (100 mu M) also failed to prev ent protection by insulin (17.5 +/- 3.2 % and 17.6 +/- 3.0 %; respectively) . However, the reduction. in infarct,size by insulin was significantly atte nuated by wortmannin (100 nM), a selective inhibitor of phosphatidylinosito l 3-kinase (PI3K, 28.3 +/- 2.2 %). Insulin was still able to protect the he art when given only during the reperfusion period (13.2 +/- 3.4 %). PC redu ced infarction to 12.8 +/- 2.0 % (P < 0.05) and still offered significant p rotection in the presence of wortmannin (22.1 +/- 2.4 %; P < 0.05). In conc lusion, activation of the insulin receptor reduces infarct size in the rabb it heart even when instituted upon reperfusion. However, the mechanism of p rotection is quite different from that of PC and involves activation of PI3 K but not PKC or K-ATP channels.