The objective of this study was to determine whether a defect in mitochondrial respiratory function accompanies the development of diabetic cardiomyopathy. The hypothesis tested in this study is that a decrease in Ca2+ uptake into mitochondria may prevent the stimulation of Ca(2+)-sensitive matrix dehydrogenases and the rate of ATP synthesis. Streptozotocin (55 mg/kg)-induced diabetic rats were used as a model of insulin-dependent diabetes mellitus. Hearts from 4-wk diabetic rats had basal heart rates and rates of contraction and relaxation similar to control. Isoproterenol caused a similar increase in the rate of contraction in diabetic and control hearts, whereas the peak rate of relaxation was reduced in diabetic hearts. Mitochondrial Ca2+ uptake was reduced in mitochondria from diabetic hearts after 2 wk of diabetes. Na(+)-induced Ca2+ release was unchanged. State 3 respiration rate was depressed in mitochondria from diabetic rats only when the respiration was supported by the substrate of a Ca(2+)-regulated matrix enzyme. The pyruvate dehydrogenase activity was reduced in diabetic mitochondria compared with that of control. It was concluded that mitochondria from diabetic hearts had a decreased capacity to upregulate ATP synthesis via stimulation of Ca(2+)-sensitive matrix dehydrogenases. The impairment in the augmentation of ATP synthesis rate accompanies a decreased rate of relaxation during increased work load.

SUMMARY1. Low concentrations of ouabain which produce a positive inotropic effect on rat ventricular muscle do not inhibit the isolated Na+-K+-ATPase enzyme from this tissue, suggesting that these low-concentration inotropic effects are not related to sodium pump inhibition (Erdmann, Philipp & Scholz, 1980; Adams, Schwartz, Grupp, Grupp, Lee, Wallick, Powell, Twist & Gathiram, 1982).2. We tested this hypothesis by continuously measuring intracellular Na+ activity with Na+-selective micro-electrodes and, separately, twitch tension of rat ventricular muscle during exposure to and wash-out of ouabain.3. Intracellular Na+ activity (a~8) and transmembrane potential of quiescent muscle cells averaged 8-5 + 2-6 mm (mean + S.D., n = 27) and -79-2 + 2-4 mV (n = 34) respectively. 4. Low concentrations of ouabain (0-1, 0-5 and 1-0 #tm) produced concentrationdependent increases in both a'a and twitch tension. At lower concentrations of ouabain (0'01 and 0-05 EM), no detectable changes in aka and twitch tension were observed.5. The data strongly indicate that in rat ventricular muscle sodium pump inhibition is present at low concentrations of ouabain which produce positive inotropy. This is consistent with previous results in canine and sheep cardiac Purkinje fibres.

In the present study, isolated dog and rat hearts were perfused in the Langendorff mode with Krebs bicarbonate buffer in the absence and presence of 10(-5) M oligomycin. The perfusion protocols employed allowed tissue pH to drop during subsequent ischemic incubations essentially as it would in blood-perfused hearts. Tissue pH, ATP, lactate, and mitochondrial respiratory function were measured during the course of subsequent zero-flow ischemic incubations. The adenosinetriphosphatase (ATPase) activities attributable to both mitochondrial and nonmitochondrial ATPases in sonicated heart homogenates and the actomyosin ATPase in isolated cardiac myofibrils were measured in both species. Consistent with earlier results with a different model in which tissue pH was buffered during the ischemic incubations [W. Rouslin, J. L. Erickson, and R. J. Solaro. Am. J. Physiol. 250 (Heart Circ. Physiol. 19): H503-H508, 1986], the inhibition of the mitochondrial ATPase in situ by oligomycin markedly slowed both tissue ATP depletion and the loss of mitochondrial function during ischemia in the dog. However, oligomycin had only a very small and transient effect on ATP depletion and mitochondrial function in the rat. This was apparently so because of the fivefold higher rate of glycolytic ATP production as well as the nearly threefold higher total nonmitochondrial ATPase activity of ischemic rat compared with ischemic dog heart. These results suggest that although the inhibition of the mitochondrial ATPase makes a major contribution to ATP conservation in ischemic dog heart, it makes only a very small contribution in rat.

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