Increased O₂ cost of basal metabolism and excitation-contraction coupling in hearts from type 2 diabetic mice

Abstract

We have reported previously that hearts from type 2 diabetic (db/db) mice show decreased cardiac efficiency due to increased work-independent myocardial O(2) consumption (unloaded MVo(2)), indicating higher O(2) use for nonmechanical processes such as basal metabolism (MVo(2)(BM)) and excitation-contraction coupling (MVo(2)(ECC)). Although alterations in cardiac metabolism and/or Ca(2+) handling may contribute to increased energy expenditure in diabetic hearts, direct measurements of the O(2) cost for these individual processes have not been determined. In this study, we 1) validate a procedure for measuring unloaded MVo(2) directly (MVo(2)(unloaded)) and for determining MVo(2)(BM) and MVo(2)(ECC) separately in isolated perfused mouse hearts and 2) determine O(2) cost for these processes in hearts from db/db mice. Unloaded MVo(2), extrapolated from the relationship between cardiac work (measured as pressure-volume area, PVA) and MVo(2), was found to correspond with MVo(2) measured directly in unloaded retrograde perfused hearts (MVo(2)(unloaded)). MVo(2) in K(+)-arrested hearts was defined as MVo(2)(BM); the difference between MVo(2)(unloaded) and MVo(2)(BM) represented MVo(2)(ECC). This procedure was validated by demonstrating that elevations in perfusate fatty acid (FA) and/or Ca(2+) concentrations resulted in changes in either MVo(2)(BM) and/or MVo(2)(ECC). The higher MVo(2)(unloaded) in db/db mice was due to both a higher MVo(2)(BM) and MVo(2)(ECC). Elevation of glucose and insulin decreased FA oxidation and reduced both MVo(2)(unloaded) and MVo(2)(BM). In conclusion, this study provides direct evidence that MVo(2)(BM) and MVo(2)(ECC) are elevated in diabetes and that acute metabolic interventions can have a therapeutic benefit in diabetic hearts due to a MVo(2)-lowering effect.

Chronic and acute exposure of mouse hearts to fatty acids increases oxygen cost of excitation-contraction coupling

“…As the latter (myocardial substrate utilization) can play an important role determining MV O 2 and cardiac efficiency in diabetic hearts (7,22,26), we suggest that the underlying mechanisms behind the increased MV O 2 following acute and chronic elevation in FA are not the same and clearly need more detailed examination. …”

“…Mice were anesthetized with pentobarbital sodium (10 mg ip), and the hearts were excised and the aortas cannulated with an 18-gauge cannula. The hearts were perfused in the working or retrograde mode using a modified Krebs-Henseleit bicarbonate buffer that was supplemented with glucose (5 mM) and palmitate (for concentrations, see individual protocols) bound to 3% BSA (1,7 MV O2 was calculated based on measurements of the difference in oxygen content in the buffer entering and leaving the heart (using fiber-optic oxygen probes placed in the preload line and in the pulmonary trunk, respectively) and coronary flow (7). Electrodes were placed on the right atrium, and all hearts were paced at a frequency of 7 Hz.…”

“…Accordingly, MV O2 measured under this condition is defined as the oxygen cost of basal metabolism (MV O2BM). The calculated difference between the measured unloaded MV O2 obtained in unloaded retrograde mode (MV O2unloaded) and MV O2BM was used to obtain the oxygen cost of EC coupling (MV O2ECC) (2,7,19,20).…”

Impaired left ventricular mechanical and energetic function in mice after cardiomyocyte-specific excision of Serca2

“…After anesthesia (10 mg ip injection of pentobarbital sodium), hearts were excised, and the aorta was cannulated with an 18-gauge cannula. Hearts were perfused in the working or retrograde mode as previously described (9). Modified Krebs-Henseleit bicarbonate buffer containing 2.5 mM Ca 2ϩ was supplemented with glucose (5 mM) and palmitate (0.7 mM, bound to 3% BSA) (1,9) LV mechanical function was assessed using a 1.4-Fr conductance catheter (Millar Instruments, Houston, TX) inserted in the LV through the apex in electrically paced hearts (7 Hz).…”

“…Hearts were perfused in the working or retrograde mode as previously described (9). Modified Krebs-Henseleit bicarbonate buffer containing 2.5 mM Ca 2ϩ was supplemented with glucose (5 mM) and palmitate (0.7 mM, bound to 3% BSA) (1,9) LV mechanical function was assessed using a 1.4-Fr conductance catheter (Millar Instruments, Houston, TX) inserted in the LV through the apex in electrically paced hearts (7 Hz). Hearts were exposed to different work loads by changing the preload (from 4 to 10 mmHg) and afterload (from 40 to 50 mmHg), and steady-state functional and contractile parameters were analyzed at various work loads (18).…”