Lower sarcoplasmic reticulum Ca²⁺ threshold for triggering afterdepolarizations in diabetic rat hearts

Type 2 diabetes (T2D) increases arrhythmia risk through incompletely elucidated mechanisms. Ventricular arrhythmias could be initiated by delayed afterdepolarizations (DADs) resulting from elevated spontaneous sarcoplasmic reticulum (SR) Ca²⁺ release (SR Ca²⁺ leak).

The purpose of this study was to test the role of DADs and SR Ca²⁺ leak in triggering arrhythmias in T2D hearts.

We compared rats with late-onset T2D that display pancreatic and cardiac phenotypes similar to those in humans with T2D (HIP rats) and their nondiabetic littermates (wild type [WT]).

HIP rats showed higher propensity for premature ventricular complexes and ventricular tachyarrhythmias, whereas HIP myocytes displayed more frequent DADs and had lower SR Ca²⁺ content than WT. However, the threshold SR Ca²⁺ at which depolarizing transient inward currents (I_tis) are generated was also significantly decreased in HIP myocytes and was below the actual SR Ca²⁺ load, which explains the increased DAD incidence despite reduced Ca²⁺ in SR. In agreement with these findings, Ca²⁺ spark frequency was augmented in myocytes from HIP vs WT rats, which suggests activation of ryanodine receptors (RyRs) in HIP hearts. Indeed, RyR phosphorylation (by CaMKII and protein kinase A) and oxidation are enhanced in HIP hearts, whereas there is no RyR O-GlcNAcylation in either HIP or control hearts. CaMKII inhibition dissipated the difference in Ca²⁺ spark frequency between HIP and WT myocytes.

The threshold SR Ca²⁺ for generating depolarizing I_tis is lower in T2D because of RyR activation after hyperphosphorylation and oxidation, which favors the occurrence of DADs despite low SR Ca²⁺ loads.