183 - Mitochondria-localized lactate dehydrogenase is not a biologically significant contributor to bioenergetic function in striated muscle

Abstract

Previous studies indicate that mitochondrial lactate dehydrogenase (mLDH) might be a significant contributor to metabolism. The presence of mLDH could provide mitochondria with a higher capacity to generate reducing equivalents for respiration, especially during exercise when circulating lactate levels are high. Furthermore, mitochondrial lactate oxidation could support cardiac output or skeletal muscle function, both of which contribute to exercise capacity. We measured lactate-, pyruvate-, and glutamate-supported respiration in mitochondria isolated from heart and skeletal muscle of sedentary, acutely exercised, and exercise-adapted mice. We modeled the acute effects of exercise by subjecting male FVB/NJ mice to one, 60 min bout of treadmill running. Exercise adaptation was modeled with progressive treadmill training at 40-60 min/d for 2 weeks, with adaptation determined by measuring exercise capacity and indices of cardiac growth. We performed immunoblotting to assess relative abundance of LDH isoforms in isolated mitochondria. Two weeks of treadmill running increased running distance by 1.3-fold. Compared with sedentary mice, exercise training increased cardiac mass by 15% (n=5/group, p<0.01). Cardiac mitochondria energized with 5 mM pyruvate+2.5 mM malate, and skeletal muscle mitochondria provided with 5 mM glutamate+2.5 mM malate, showed approximately 10-fold higher respiration rates compared with mitochondria provided 5 mM lactate+2.5 mM malate. Exercise training did not significantly affect respiration on either substrate. We performed similar studies on mitochondria isolated immediately after one intense bout of exercise. In both sedentary and exercised conditions, cardiac and skeletal muscle mitochondria provided with lactate showed 10-fold lower respiration rates than those supplied with pyruvate or glutamate (n=3/group). Western blotting indicated low levels of LDHB in mitochondrial fractions from heart and low levels of LDHA in mitochondrial fractions from skeletal muscle. Lactate metabolism in striated muscle is primarily a cytosolic phenomenon; mLDH is of minimal biological significance.