Abstract 14255: Roles of Mitochondrial Ca ²⁺ in Contractile Properties in Diabetic Rat Myocardium

Introduction: Diabetes mellitus is an important risk factor for heart failure with reduced relaxation. It has not yet been established how mitochondrial Ca ²⁺ is involved in the reduced relaxation in diabetic myocardium. Thus, using trabeculae from diabetic hearts, we examined whether mitochondrial Ca ²⁺ affects contractile properties in diabetic myocardium.

Methods: Rats were given a subcutaneous injection of 55 mg/kg streptozotocin (STZ-rats) or solvent (Ctr-rats). Eight weeks after the injection, trabeculae were dissected from rat hearts. Force was measured with a strain gauge, intracellular Ca ²⁺ (Ca _i ) with fura-2, mitochondrial Ca ²⁺ with rhod-2, and mitochondrial ROS with MitoSox Red (24°C). To evaluate contractile properties of cardiac muscle, the velocities of contraction (dF/dt) and relaxation (-dF/dt) were calculated at 0.7 and 2.0 mM extracellular Ca ²⁺ . To eliminate the effect of developed force (F _dev ), the dF/dt and -dF/dt were normalized by dividing them by their F _dev .

Results: Blood glucose of STZ-rats was higher than that of Ctr-rats (p<0.01). STZ-rats showed smaller F _dev , smaller dF/dt _max /F _dev , and smaller -dF/dt _min /F _dev (n=9, p<0.05), suggesting that muscle contraction and relaxation are impaired in STZ-rats. STZ-rats showed lower peak and slower decline of Ca _i transients compared to Ctr-rats (n=9, p<0.01). In addition, Ru360 (5 μM), a mitochondrial calcium uniporter (MCU) inhibitor, decreased rhod-2 fluorescence and increased dF/dt _max /F _dev and -dF/dt _min /F _dev in STZ-rats (n=7, p<0.05), suggesting that a decrease in mitochondrial Ca ²⁺ improves muscle contraction and relaxation. On the contrary, spermine (150 μM), a MCU activator, increased rhod-2 fluorescence and decreased dF/dt _max /F _dev and -dF/dt _min /F _dev (n=5, p<0.05). In STZ-rats, Ru360 increased MitoSox Red fluorescence (n=5, p<0.05). The spatial distribution of rhod-2 and MitoSox Red imaged with confocal microscopy was similar to that of MitoTracker Green within trabeculae, meaning that rhod-2 and MitoSoxRed are loaded within mitochondria.

Conclusions: These results suggest that in diabetic hearts, mitochondrial Ca ²⁺ is involved in the improvement of muscle contraction and relaxation, probably through changes in ROS production.