The Role of Sodium in Diabetic Cardiomyopathy

Abstract

Cardiovascular complications are the major cause of mortality and morbidity in diabetic patients. The changes in myocardial structure and function associated with diabetes are collectively called diabetic cardiomyopathy. Numerous molecular mechanisms have been proposed that could contribute to the development of diabetic cardiomyopathy and have been studied in various animal models of type 1 or type 2 diabetes. The current review focuses on the role of sodium (Na⁺) in diabetic cardiomyopathy and provides unique data on the linkage between Na⁺ flux and energy metabolism, studied with non-invasive ²³Na, and ³¹P-NMR spectroscopy, polarography, and mass spectroscopy. ²³Na NMR studies allow determination of the intracellular and extracellular Na⁺ pools by splitting the total Na⁺ peak into two resonances after the addition of a shift reagent to the perfusate. Using this technology, we found that intracellular Na⁺ is approximately two times higher in diabetic cardiomyocytes than in control possibly due to combined changes in the activity of Na⁺-K⁺ pump, Na⁺/H⁺ exchanger 1 (NHE1) and Na⁺-glucose cotransporter. We hypothesized that the increase in Na⁺ activates the mitochondrial membrane Na⁺/Ca²⁺ exchanger, which leads to a loss of intramitochondrial Ca²⁺, with a subsequent alteration in mitochondrial bioenergetics and function. Using isolated mitochondria, we showed that the addition of Na⁺ (1-10 mM) led to a dose-dependent decrease in oxidative phosphorylation and that this effect was reversed by providing extramitochondrial Ca²⁺ or by inhibiting the mitochondrial Na⁺/Ca²⁺ exchanger with diltiazem. Similar experiments with ³¹P-NMR in isolated superfused mitochondria embedded in agarose beads showed that Na⁺ (3-30 mM) led to significantly decreased ATP levels and that this effect was stronger in diabetic rats. These data suggest that in diabetic cardiomyocytes, increased Na⁺ leads to abnormalities in oxidative phosphorylation and a subsequent decrease in ATP levels. In support of these data, using ³¹P-NMR, we showed that the baseline β-ATP and phosphocreatine (PCr) were lower in diabetic cardiomyocytes than in control, suggesting that diabetic cardiomyocytes have depressed bioenergetic function. Thus, both altered intracellular Na⁺ levels and bioenergetics and their interactions may significantly contribute to the pathology of diabetic cardiomyopathy.

Keywords: NMRS; calcium–sodium exchanger; mitochondrial bioenergetics; oxygen consumption; sodium.

FIGURE 1

(A) A typical 23Na spectra obtained from control rat cardiomyocytes showing intra- and extra-cellular sodium during baseline conditions and during administration of 2-deoxyglucose (2-DG, 10 mM); 2,4-dinitrophenol (DNP, 10-4 M); and ouabain (OUA, 100 μM). (B) Effects of 2-DG, DNP, and OUA on ³¹P spectra obtained from control rat cardiomyocytes (typical spectra presented). MDP, methylene diphosphonate standard; PME, phosphomonoester; Pi, inorganic phosphate; PCr, phosphocreatine; ATP, adenosine triphosphate (α, γ, β); Na_i, intracellular sodium; Na₀, extracellular sodium. Data reprinted with permission from Doliba et al. (2000) Translated from Biokhimiya. 2000:65(4) 590-97. Copyright 2000 by MAIK “Nauka/Interperiodica”; DOI 0006-2979/00/6504-0502$25.00; Copyright permission granted by Pleiades Publishing, LLC.

FIGURE 2

ADP and Ca²⁺-stimulated respiration of mitochondria from control and diabetic rats. Mitochondria (2 mg) were added to assay medium supplemented with 3 mM pyruvate plus 2.5 mM malate. ADP (0.3 mM) or CaCl₂ (50 μM) was used to initiate state 3 respiration and Ca-uptake. Ca²⁺ uptake by mitochondria was monitored by using the change in H⁺ flux. Stimulation of oxygen consumption by ADP or Ca²⁺ was approximately 50% lower in DM mitochondria compared to Con. Data reprinted by permission from Nature/Springer/Palgrave: Doliba et al. (1997). Copyright 1997 by Springer Nature; License Number 4385511236859. Originally published by Plenum Press, New York 1997 (DOI 10987654321).

FIGURE 3

¹³C¹⁶O₂ production (Boudina and Abel, 2010) and O₂ consumption (MVO₂) (Veeranki et al., 2016) during oxidation of [1-¹³C] pyruvate by heart mitochondria from control and diabetic rats. (A) ¹³C¹⁶O₂ production and MVO₂ after addition of ADP. (B) ¹³C¹⁶O₂ production and MVO2 after addition of FCCP to uncouple oxidative phosphorylation. Both the ¹³C¹⁶O₂ production and MVO₂ stimulated by ADP FCCP were much less in DM mitochondria compared to Con. Data reprinted by permission from Nature/Springer/Palgrave: Doliba et al. (1997). Copyright 1997 by Springer Nature; License Number 4385511236859. Originally published by Plenum Press, New York 1997 (DOI 10987654321).

FIGURE 4

The effect of Ca²⁺ on CO₂ production in Con (black squares) and DM mitochondria (open squares). Addition of Ca²⁺ caused minimal changes in ¹³C¹⁶O₂ production in DM; whereas Ca²⁺ increased ¹³C¹⁶O₂ production by 33–40% in Con. Data reprinted by permission from Nature/Springer/ Palgrave: Doliba et al. (1997). Copyright 1997 by Springer Nature; License Number 4385511236859. Originally published by Plenum Press, New York 1997 (DOI 10987654321).

FIGURE 5

Effect of different concentrations of Na⁺ on ADP stimulated mitochondrial oxygen consumption (state 3) in control (CON) and diabetic (DM) heart mitochondria (means ± SE, n = 5). ^∗ < 0.05 CON vs DM. Baseline of state 3 (without Na⁺) is assumed to be 100%. Data reprinted with permission from Babsky et al. (2001). Copyright 2001 by the Society for Experimental Biology and Medicine; DOI: 0037-9727/01/2266-0543$15.00.

FIGURE 6

(A) The effect of extramitochondrial Na⁺ on ATP and Pi ratios in CON and DM heart mitochondria. (B) 250 μM DLTZ, an inhibitor of mitochondrial Na⁺–Ca²⁺ exchange, was added to perfusate. Baseline (without Na⁺) is assumed to be 100% (means ± SE. n = 4). Significance: DM vs CON: ^∗P < 0.05. Data reprinted with permission from Babsky et al. (2001). Copyright 2001 by the Society for Experimental Biology and Medicine; DOI: 0037-9727/01/2266-0543$15.00.

FIGURE 7

Relative changes in intracellular sodium (Na_i) resonance areas as a function of time in control (CON, n = 6) and preconditioned (IPC, n = 4) rat hearts. Na_i baseline is normalized to 100. Significance: ^∗P < 0.01 (IPC vs CON), ^#P < 0.05 (IPC group vs end of ischemia), and ^&P < 0.01 (vs pre-ischemic level for each group). Data reprinted with permission from Babsky et al. (2002). Copyright 2002 by the Society for Experimental Biology and Medicine; DOI: 1535-3702/02/2277-0520$15.00.