The mitochondrial bioenergetic phenotype for protection from cardiac ischemia in SUR2 mutant mice

The sulfonylurea receptor-2 (SUR2) is a subunit of ATP-sensitive potassium channels (KATP) in heart. Mice with the SUR2 gene disrupted (SUR2m) are constitutively protected from ischemia-reperfusion (I/R) cardiac injury. This was surprising because KATP, either sarcolemmal or mitochondrial or both, are thought to be important for cardioprotection. We hypothesized that SUR2m mice have an altered mitochondrial phenotype that protects against I/R. Mitochondrial membrane potential (ΔΨm), tolerance to Ca2+ load, and reactive oxygen species (ROS) generation were studied by fluorescence-based assays, and volumetric changes in response to K+ were measured by light scattering in isolated mitochondria. For resting SUR2m mitochondria compared with wild type, the ΔΨm was less polarized (46.1 ± 0.4 vs. 51.9 ± 0.6%), tolerance to Ca2+ loading was increased (163 ± 2 vs. 116 ± 2 μM), and ROS generation was enhanced with complex I [8.5 ± 1.2 vs. 4.9 ± 0.2 arbitrary fluorescence units (afu)/s] or complex II (351 ± 51.3 vs. 166 ± 36.2 afu/s) substrates. SUR2m mitochondria had greater swelling in K+ medium (30.2 ± 3.1%) compared with wild type (14.5 ± 0.6%), indicating greater K+ influx. Additionally, ΔΨm decreased and swelling increased in the absence of ATP in SUR2m, but the sensitivity to ATP was less compared with wild type. When the mitochondria were subjected to hypoxia-reoxygenation, the decrease in respiration rates and respiratory control index was less in SUR2m. ΔΨm maintenance in the SUR2m intact myocytes was also more tolerant to metabolic inhibition. In conclusion, the cardioprotection observed in the SUR2m mice is associated with a protected mitochondrial phenotype resulting from enhanced K+ conductance that partially dissipated ΔΨm. These results have implications for possible SUR2 participation in mitochondrial KATP.

Cardiomyocyte sulfonylurea receptor 2-KATP channel mediates cardioprotection and ST segment elevation

Sulfonylurea receptor-containing ATP-sensitive potassium (KATP) channels have been implicated in cardioprotection, but the cell type and constitution of channels responsible for this protection have not been clear. Mice deleted for the first nucleotide binding region of sulfonylurea receptor 2 (SUR2) are referred to as SUR2 null since they lack full-length SUR2 and glibenclamide-responsive KATP channels in cardiac, skeletal, and smooth muscle. As previously reported, SUR2 null mice develop electrocardiographic changes of ST segment elevation that were shown to correlate with coronary artery vasospasm. Here we restored expression of the cardiomyocyte SUR2-KATP channel in SUR2 null mice by generating transgenic mice with ventricular cardiomyocyte-restricted expression of SUR2A. Introduction of the cardiomyocyte SUR2A transgene into the SUR2 null background restored functional cardiac KATP channels. Hearts isolated from rescued mice, referred to as MLC2A, had significantly reduced infarct size (27 ± 3% of area at risk) compared with SUR2 null mice (36 ± 3% of area at risk). Compared with SUR2 null hearts, MLC2A hearts exhibited significantly improved cardiac function during the postischemia reperfusion period primarily because of preservation of low diastolic pressures. Additionally, restoration of cardiac SUR2-KATP channels significantly reduced the degree and frequency of ST segment elevation episodes in MLC2A mice. Therefore, cardioprotective mechanisms both dependent and independent of SUR2-KATP channels contribute to cardiac function.