Distinct Effects of Mitochondrial Na+/Ca2+ Exchanger Inhibition and Ca2+ Uniporter Activation on Ca2+ Sparks and Arrhythmogenesis in Diabetic Rats

Abstract

Background Mitochondrial dysfunction contributes to the cardiac remodeling triggered by type 2 diabetes (T2D). Mitochondrial Ca²⁺ concentration ([Ca²⁺]_m) modulates the oxidative state and cytosolic Ca²⁺ regulation. Thus, we investigated how T2D affects mitochondrial Ca²⁺ fluxes, the downstream consequences on myocyte function, and the effects of normalizing mitochondrial Ca²⁺ transport. Methods and Results We compared myocytes/hearts from transgenic rats with late-onset T2D (rats that develop late-onset T2D due to heterozygous expression of human amylin in the pancreatic β-cells [HIP] model) and their nondiabetic wild-type (WT) littermates. [Ca²⁺]_m was significantly lower in myocytes from diabetic HIP rats compared with WT cells. Ca²⁺ extrusion through the mitochondrial Na⁺/Ca²⁺ exchanger (mitoNCX) was elevated in HIP versus WT myocytes, particularly at moderate and high [Ca²⁺]_m, while mitochondrial Ca²⁺ uptake was diminished. Mitochondrial Na⁺ concentration was comparable in WT and HIP rat myocytes and remained remarkably stable while manipulating mitoNCX activity. Lower [Ca²⁺]_m was associated with oxidative stress, increased sarcoplasmic reticulum Ca²⁺ leak in the form of Ca²⁺ sparks, and mitochondrial dysfunction in T2D hearts. MitoNCX inhibition with CGP-37157 reduced oxidative stress, Ca²⁺ spark frequency, and stress-induced arrhythmias in HIP rat hearts while having no significant effect in WT rats. In contrast, activation of the mitochondrial Ca²⁺ uniporter with SB-202190 enhanced spontaneous sarcoplasmic reticulum Ca²⁺ release and had no significant effect on arrhythmias in both WT and HIP rat hearts. Conclusions [Ca²⁺]_m is reduced in myocytes from rats with T2D due to a combination of exacerbated mitochondrial Ca²⁺ extrusion through mitoNCX and impaired mitochondrial Ca²⁺ uptake. Partial mitoNCX inhibition limits sarcoplasmic reticulum Ca²⁺ leak and arrhythmias in T2D hearts, whereas mitochondrial Ca²⁺ uniporter activation does not.

Keywords: heart; mitochondrial Ca2+ uniporter; mitochondrial Na+/Ca2+ exchanger; type 2 diabetes; ventricular arrhythmias.