Possible role of JAK-2 in attenuated cardioprotective effect of ischemic preconditioning in hyperhomocysteinemic rat hearts.

The present study investigated the possible role of Janus kinase-2 (JAK-2) in hyperhomocysteinemia-induced attenuation of the cardioprotective effects of ischemic preconditioning (IPC). Rats were administered L-methionine (1.7 g/kg/day, p.o.) for 4 weeks to produce hyperhomocysteinemia. Isolated Langendorff's perfused normal and hyperhomocysteinemic rat hearts were subjected to global ischemia for 30 min, followed by reperfusion for 120 min. Myocardial infarct size was assessed macroscopically using triphenyltetrazolium chloride staining. Coronary effluent was analyzed for lactate dehydrogenase (LDH) and creatine kinase (CK) release to assess the extent of cardiac injury. The oxidative stress in the heart was assessed by measuring thiobarbituric acid-reactive substances (TBARS), superoxide anion generation and the reduced form of glutathione. Ischemia-reperfusion (I/R) induced oxidative stress by increasing TBARS, superoxide anion generation and decreasing reduced form of glutathione in normal and hyperhomocystenemic rat hearts. Moreover, I/R produced myocardial injury, which was assessed in terms of the increase in myocardial infarct size, LDH and CK release in coronary effluent, and decrease in coronary flow rate in normal and hyperhomocysteinemic rat hearts. The hyperhomocysteinemic rat hearts showed enhanced I/R-induced myocardial injury with a high degree of oxidative stress as compared with normal rat hearts subjected to I/R. Four episodes of IPC (5 min each) afforded cardioprotection against I/R-induced myocardial injury in normal rat hearts as assessed in terms of improvement in coronary flow rate and reduction in myocardial infarct size, levels of LDH, CK, and oxidative stress. On the other hand, IPC-mediated myocardial protection against I/R-injury was abolished in hyperhomocysteinemic rat hearts. Tyrphostin AG490 (5 microM), a selective inhibitor of JAK-2, did not affect the cardioprotective effects of IPC in normal rat hearts, but its administration markedly restored the cardioprotective potential of IPC in hyperhomocysteinemic rat hearts. Administration of diazoxide (30 microM), an ATP-sensitive potassium (K(ATP)) channel opener, also restored the cardioprotective effects of IPC in hyperhomocysteinemic rat hearts. In conclusion, it is suggested that the high degree of oxidative stress produced in hyperhomocysteinemic rat hearts during reperfusion and consequent activation of JAK-2 and closure of K(ATP) channels may be responsible for abolishing the cardioprotective potential of IPC against I/R-induced myocardial injury.