Na + Effects on Mitochondrial Respiration and Oxidative Phosphorylation in Diabetic Hearts

Abstract

Intracellular Na⁺ is approximately two times higher in diabetic cardiomyocytes than in control. We hypothesized that the increase in Na⁺₁ activates the mitochondrial membrane Na⁺/Ca²⁺ exchanger, which leads to loss of intramitochondrial Ca²⁺, with a subsequent alteration (generally depression) in bioenergetic function. To further evaluate this hypothesis, mitochondria were isolated from hearts of control and streptozotocin-induced (4 weeks) diabetic rats. Respiratory function and ATP synthesis were studied using routine polarography and ³¹P-NMR methods, respectively. While addition of Na⁺ (1–10 mM) decreased State 3 respiration and rate of oxidative phosphorylation in both diabetic and control mitochondria, the decreases were significantly greater for diabetic than for control. The Na⁺ effect was reversed by providing different levels of extramitochondrial Ca²⁺ (larger Ca²⁺ levels were needed to reverse the Na⁺ depressant effect in diabetes mellitus than in control) and by inhibiting the Na⁺/Ca²⁺exchanger function with diltiazem (a specific blocker of Na⁺/Ca²⁺ exchange that prevents Ca²⁺ from leaving the mitochondrial matrix). On the other hand, the Na⁺ depressant effect was enhanced by Ruthenium Red (RR, a blocker of mitochondrial Ca²⁺ uptake, which decreases intramitochondrial Ca²⁺). The RR effect on Na⁺ depression of mitochondrial bioenergetic function was larger in diabetic than control. These findings suggest that intramitochondrial Ca²⁺ levels could be lower in diabetic than control and that the Na⁺ depressant effect has some relation to lowered intramitochondrial Ca²⁺. Conjoint experiments with ³¹P-NMR in isolated superfused mitochondria embedded in agarose beads showed that Na⁺ (3–30 mM) led to significantly decreased ATP levels in diabetic rats, but produced smaller changes in control. These data support our hypothesis that in diabetic cardiomyocytes, increased Na⁺ leads to abnormalities of oxidative processes and subsequent decrease in ATP levels, and that these changes are related to Na⁺ induced depletion of intramitochondrial Ca²⁺.