Na+ effects on mitochondrial respiration and oxidative phosphorylation in diabetic hearts

Intracellular Na+ is approximately two times higher in diabetic cardiomyocytes than in control. We hypothesized that the increase in Na+i 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+.