Abstract 766: SGLT-Mediated Na + Overload Results in Oxidative Stress and Abnormal SR Ca 2+ Release in Diabetic Hearts

Abstract

Intracellular Na + concentration ([Na + ] i ) regulates Ca 2+ cycling, oxidative state and electrical stability of the heart. [Na + ] i is linked to glucose uptake through the Na + -glucose cotransporter (SGLT). The SGLT1 isoform is present in the heart and its overexpression causes hypertrophy and left-ventricular dysfunction. Here, we hypothesized that cardiac SGLT activity is increased in type-2 diabetes (T2D), which causes myocyte Na + overload and results in oxidative stress and exacerbated sarcoplasmic reticulum (SR) Ca 2+ leak. To test this hypothesis, we analyzed myocardial tissue from humans with/without T2D and compared rats with late-onset T2D that display diabetic cardiomyopathy similar to that seen in humans (HIP rats) with their non-diabetic littermates (WT). SGLT1 expression was increased in hearts from T2D patients compared to non-diabetic individuals and in hearts from HIP vs . WT rats. SGLT inhibition significantly decreased the uptake of both glucose and Na + in myocytes from HIP rats but not in WT, which indicates that SGLT function is increased in T2D hearts. While SGLT1 upregulation may partially compensate for the reduced insulin-dependent glucose uptake, the ensuing raise in Na + influx resulted in elevated [Na + ] i in HIP myocytes (by 3 mM compared to WT). Higher [Na + ] i causes oxidative stress by activating the mitochondrial Na + /Ca 2+ exchanger (mitoNCX), which lowers mitochondrial [Ca 2+ ] and thus slows down regeneration of the antioxidant NADPH. In agreement with a role for elevated [Na + ] i in causing oxidative stress in T2D hearts, H 2 O 2 production was increased in HIP myocytes vs . WT and mitoNCX inhibition, which uncouples mitochondria from [Na + ] i , significantly reduced H 2 O 2 production in HIP hearts. Oxidative stress enhances the SR Ca 2+ leak directly through oxidation of ryanodine receptors (RyRs) and indirectly via CaMKII activation and consequent RyR phosphorylation. SR Ca 2+ leak was augmented in myocytes from HIP vs . WT rats and mitoNCX inhibition significantly reduced the leak in HIP myocytes, but not in the WT. Thus, our data indicate that SGLT activity is enhanced in myocytes from T2D rats, which increases Na + influx and causes Na + overload. Elevated [Na + ] i contributes to oxidative stress and abnormal SR Ca 2+ leak in diabetic hearts.