Recombinant cardiac ATP-sensitive potassium channels and cardioprotection.

The ATP-dependent potassium channels (KATP channels) were originally identified in isolated membrane patches prepared from guinea pig ventricular myocytes by Noma in 1983. Since their discovery in cardiac cells, KATP channels have also been discovered in many other tissues, such as smooth muscle, skeletal muscle, pancreas, and brain, in which they have been shown to couple cellular metabolism to membrane electrical activity. Primarily on the basis of studies using pharmacological tools, openers of KATP channels have been shown to elicit cardioprotective effects, whereas KATP channel antagonists have been shown to block the cardioprotective effects of KATP channel openers and the powerful protective effect produced by single or multiple brief episodes of ischemia to reduce myocardial infarct size, a phenomenon called ischemic preconditioning. Because the results of these previous studies were obtained indirectly by the use of pharmacological agonists and antagonists, the results of the present study published by Jovanovic and colleagues in this issue of Circulation are particularly exciting and are relevant for helping to clearly define an important role for the endogenous KATP channel protein subunits in conferring the cardioprotective effects of KATP channel openers and ischemic preconditioning. In this elegant study by Jovanovic and coworkers, the authors transfected KATP-deficient COS-7 cells with the Kir 6.2/SUR 2A genes, which Okuyama et al recently showed to form functional KATP channels in HEK 293T cells and to possess the main properties of native KATP channels in terms of activation by pinacidil and nicorandil but not diazoxide, channel rundown, and regulation by intracellular nucleotides such as ADP and UDP. In KATP-deficient COS-7 cells, Jovanovic et al found that when these cells were exposed to 3 minutes of chemical hypoxia (dinitrophenol, DNP) and subsequently reoxygenated, significant calcium loading occurred. In these COS-7 cells deficient in KATP channels, the KATP channel opener pinacidil had no significant effect on calcium loading produced by chemical hypoxiareoxygenation injury. However, when both subunits of the KATP channel Kir 6.2/SUR 2A were cotransfected in COS-7 cells, a phenotype was produced in which pinacidil was capable of markedly attenuating the calcium loading produced by hypoxia-reoxygenation. That this effect was the result of opening KATP channels was confirmed by demonstration that glyburide (1 mmol/L) was capable of abolishing the protective effect of pinacidil. Interestingly, opening of the channel by chemical hypoxia with DNP produced only a marginally protective effect; however, this may have been the result of the short period (3 minutes) of exposure to DNP or the need to sensitize the KATP channel before the main hypoxic insult, such as occurs in ischemic preconditioning. Moreover, in COS-7 cells transfected with either Kir 6.2 or SUR 2A alone, pinacidil had no significant cytoprotective effect. These results suggest that the cardiac KATP channel protein possesses endogenous cytoprotective properties when transfected into a noncardiac cell type. Cardiac myocytes expressing the native endogenous KATP channel were also exposed to the same chemical hypoxia-reoxygenation protocol and demonstrated a marked increase in cellular calcium that was significantly attenuated by pinacidil, an effect that was abolished by glyburide.