Preserved cardiac performance and adrenergic response in a rabbit model with decreased ryanodine receptor 2 expression.

Ryanodine receptor 2 (RyR2) is an ion channel in the heart responsible for releasing into the cytosol most of the Ca2+ required for contraction. Proper regulation of RyR2 is critical, as highlighted by the association between channel dysfunction and cardiac arrhythmia. Lower RyR2 expression is also observed in some forms of heart disease; however, there is limited information on the impact of this change on excitation-contraction (e-c) coupling, Ca2+-dependent arrhythmias, and cardiac performance. We used a constitutive knock-out of RyR2 in rabbits (RyR2-KO) to assess the extent to which a stable decrease in RyR2 expression modulates Ca2+ handling in the heart. We found that homozygous knock-out of RyR2 in rabbits is embryonic lethal. Remarkably, heterozygotes (KO+/-) show ~50% loss of RyR2 protein without developing an overt phenotype at the intact animal and whole heart levels. Instead, we found that KO+/- myocytes show (1) remodeling of RyR2 clusters, favoring smaller groups in which channels are more densely arranged; (2) lower Ca2+ spark frequency and amplitude; (3) slower rate of Ca2+ release and mild but significant desynchronization of the Ca2+ transient; and (4) a significant decrease in the basal phosphorylation of S2031, likely due to increased association between RyR2 and PP2A. Our data show that RyR2 deficiency, although remarkable at the molecular and subcellular level, has only a modest impact on global Ca2+ release and is fully compensated at the whole-heart level. This highlights the redundancy of RyR2 protein expression and the plasticity of the e-c coupling apparatus.

Overexpression of a Short Sulfonylurea Splice Variant Increases Cardiac Glucose Uptake and Uncouples Mitochondria by Regulating ROMK Activity.

The mitochondrial splice variant of the sulfonylurea receptor (SUR2A-55) is associated with protection from myocardial ischemia-reperfusion (IR) injury, increased mitochondrial ATP sensitive K+ channel activity (mitoKATP) and altered glucose metabolism. While mitoKATP channels composed of CCDC51 and ABCB8 exist, the mitochondrial K+ pore regulated by SUR2A-55 is unknown. We explored whether SUR2A-55 regulates ROMK to form an alternate mitoKATP. We assessed glucose uptake in mice overexpressing SUR2A-55 (TGSUR2A-55) compared with WT mice during IR injury. We then examined the expression level of ROMK and the effect of ROMK modulation on mitochondrial membrane potential (Δψm) in WT and TGSUR2A-55 mice. TGSUR2A-55 had increased glucose uptake compared to WT mice during IR injury. The expression of ROMK was similar in WT compared to TGSUR2A-55 mice. ROMK inhibition hyperpolarized resting cardiomyocyte Δψm from TGSUR2A-55 mice but not from WT mice. In addition, TGSUR2A-55 and ROMK inhibitor treated WT isolated cardiomyocytes had enhanced mitochondrial uncoupling. ROMK inhibition blocked diazoxide induced Δψm depolarization and prevented preservation of Δψm from FCCP perfusion in WT and to a lesser degree TGSUR2A-55 mice. In conclusion, cardio-protection from SUR2A-55 is associated with ROMK regulation, enhanced mitochondrial uncoupling and increased glucose uptake.