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1 Program in Integrative Cardiac Metabolism, University of Illinois at Chicago, College of Medicine, Chicago, IL, USA; Center for Cardiovascular Research, University of Illinois at Chicago, College of Medicine, Chicago, IL, USA
2 Department of Radiology, Massachusetts General Hospital, Boston, MA, USA
3 Center for Cardiovascular Research, University of Illinois at Chicago, College of Medicine, Chicago, IL, USA
* To whom correspondence should be addressed. E-mail: dougl{at}uic.edu.
Triacylglycerol (TAG) storage and turnover rates in the intact, beating rat heart were determined for the first time using dynamic-mode 13C NMR spectroscopy to elucidate profound differences between hearts from diabetic rats (DR, 18 days streptozotocin treatment) and normal rats (NR). The incorporation of [2,4,6,8,10,12,14,16-13C8] palmitate into the TAG pool was monitored in isolated hearts perfused with physiological (0.5 mM palmitate, 5 mM glucose) and elevated substrates levels (1.2 mM palmitate, 11 mM glucose) characteristic of the diabetic condition. Surprisingly, while the normal hearts were enriched at a near-linear profile for at least 2 hrs prior to exponential characterization, the exponential enrichment of TAG in diabetic hearts reached steady state after only 45 min. Consequently, TAG turnover rate was determined by fitting an exponential model to the enrichment data, rather than the conventional two-point linear analysis. In the high substrate group, both turnover rate (DR 820±330 nmol/min/gm dw; NR 190±150, p<0.001) and [TAG] content (DR 78±10 mmol/g dw, NR 32±4, p<0.001) were greater in the diabetic group. At lower substrate concentrations, turnover was greater in diabetics (DR: 530±300; NR: 160±30, p<0.05). However, this could not be explained by simple mass action, because [TAG] content was similar between groups (DR 34±7 mmol/g dw, NR 39 ±9, NS). Consistent with the exponential enrichment data, 13C fractional enrichment of TAG was lower in the diabetics (low substrate groups: DR 4 ±1%, NR 10±4%, p<0.05; high substrate groups: DR 8±3%, 14±9%, NS), thereby supporting earlier speculation that TAG is compartmentalized in diabetic heart.
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