Heart failure and arrhythmias occur at 3 to 5 times higher rates among individuals with diabetes mellitus, compared with age-matched, healthy individuals. Studies attribute these defects in part to alterations in the function of cardiac type 2 ryanodine receptors (RyR2s), the principal Ca(2+)-release channels on the internal sarcoplasmic reticulum (SR). To date, mechanisms underlying RyR2 dysregulation in diabetes remain poorly defined. A rat model of type 1 diabetes, in combination with echocardiography, in vivo and ex vivo hemodynamic studies, confocal microscopy, Western blotting, mass spectrometry, site-directed mutagenesis, and [(3)H]ryanodine binding, lipid bilayer, and transfection assays, was used to determine whether post-translational modification by reactive carbonyl species (RCS) represented a contributing cause. After 8 weeks of diabetes, spontaneous Ca(2+) release in ventricular myocytes increased ~5-fold. Evoked Ca(2+) release from the SR was nonuniform (dyssynchronous). Total RyR2 protein levels remained unchanged, but the ability to bind the Ca(2+)-dependent ligand [(3)H]ryanodine was significantly reduced. Western blotting and mass spectrometry revealed RCS adducts on select basic residues. Mutation of residues to delineate the physiochemical impact of carbonylation yielded channels with enhanced or reduced cytoplasmic Ca(2+) responsiveness. The prototype RCS methylglyoxal increased and then decreased the RyR2 open probability. Methylglyoxal also increased spontaneous Ca(2+) release and induced Ca(2+) waves in healthy myocytes. Treatment of diabetic rats with RCS scavengers normalized spontaneous and evoked Ca(2+) release from the SR, reduced carbonylation of RyR2s, and increased binding of [(3)H]ryanodine to RyR2s. From these data, we conclude that post-translational modification by RCS contributes to the heterogeneity in RyR2 activity that is seen in experimental diabetes.