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Metabolic Control of Sodium Transport in Streptozotocin-Induced Diabetic Rat Hearts

Nat. M. Doliba1, A. M. Babsky1, S. L. Wehrli2, Nic. M. Doliba1, T. M. Ivanics3, M. F. Friedman4, and M. D. Osbakken1,5*

1Biochemistry/Biophysics Department, 352 Anatomy/Chemistry Bldg., University of Pennsylvania, 36th & Spruce Street, Philadelphia, PA 19104, USA; fax: (215) 573-807

2Children's Hospital of Pennsylvania, Philadelphia, PA 19104, USA

3Semmelweis University of Medicine, Budapest, 1082 Hungary

4Monell Chemical Senses Center, Philadelphia, PA 19104, USA; fax: (215) 898-2084

5Covance, Princeton, NJ 08540-6233, USA; fax: (609) 987-9209; E-mail: mary.osbakken@covance.com or andriy10@yahoo.com

* To whom correspondence should be addressed.

Received June 24, 1999; Revision received September 30, 1999
Diabetic and control cardiomyocytes encapsulated in agarose beads and superfused with modified medium 199 were studied with 23Na- and 31P-NMR. Baseline intracellular Na+ was higher in diabetic (0.076 ± 0.01 µmoles/mg protein) than in control (0.04 ± 0.01 µmoles/mg protein) (p < 0.05). Baseline betaATP and phosphocreatine (PCr) (peak area divided by the peak area of the standard, methylene diphosphonate) were lower in diabetic than in control, e.g., betaATP control, 0.70 ± 0.07; betaATP diabetic, 0.49 ± 0.04 (p < 0.027); PCr control, 1.20 ± 0.13; PCr diabetic, 0.83 ± 0.11 (p < 0.03). This suggests that diabetic cardiomyocytes have depressed bioenergetic function, which may contribute to abnormal Na,K-ATPase function, and thus, an increase in intracellular Na+. In the experiments presented herein, three interventions (2-deoxyglucose, dinitrophenol, or ouabain infusions) were used to determine whether, and the extent to which, energy deficits or abnormalities in Na,K-ATPase function contribute to the increase in intracellular Na+. In diabetic cardiomyocytes, 2-deoxyglucose and ouabain had minimal effect on intracellular Na+, suggesting baseline depression of, or resetting of both glycolytic and Na,K-ATPase function, whereas in control both agents caused significant increases in intracellular Na+after 63 min exposure: 2-deoxyglucose control, 32.9 ± 8.1%; 2-deoxyglucose diabetic, -4.6 ± 6% (p < 0.05); ouabain control, 50.5 ± 8.8%; ouabain diabetic, 21.2 ± 9.2% (p < 0.05). In both animal models, dinitrophenol was associated with large increases in intracellular Na+: control, 119.0 ± 26.9%; diabetic, 138.2 ± 12.6%. Except for the dinitrophenol intervention, where betaATP and PCr decreased to levels below 31P-NMR detection, the energetic metabolites were not lowered to levels that would compromise sarcolemmal function (Na,K-ATPase) in either control or diabetic cardiomyocytes. In conclusion, in diabetic cardiomyocytes, even though abnormal glycolytic and Na,K-ATPase function was associated with increases in intracellular Na+, these increases were not directly related to global energy deficit.
KEY WORDS: 23Na-NMR, 31P-NMR, 2-deoxyglucose, 2,4-dinitrophenol, ouabain