Mitochondrial function is not the same in rat diaphragm and limb muscles

Abstract

The mitochondrial content of skeletal muscles is proportional to their activity levels, supposedly to match ATP supply to demand. This view excludes the existence of intrinsic differences in mitochondrial function. The diaphragm is a constantly active respiratory muscle with high mitochondrial content. This study tested the hypothesis that mitochondrial function differs in rat diaphragm and triceps surae (TS, limb muscle). Mitochondria isolated from diaphragm and TS muscles of adult male Sprague Dawley rats were used to measure O2 consumption with polarography, the content of respiratory complexes, uncoupling protein 2 and 3 (UCP2 and UCP3) and voltage dependent anion channel 1 (VDAC1) by western blot, and complex IV activity by spectrophotometry. Respiration states 3 (substrate‐ and ADP‐driven) and 5 (uncoupled) were 27±8% and 24±10% lower in diaphragm than in TS mitochondria (p<0.05 for both). However, the contents of respiratory complexes III, IV and V and VDAC1 were higher in the diaphragm than in TS (23±6%, 30±8%, 25±8% and 18±8% respectively, p≤0.02 for all comparisons). Likewise, complex IV activity was 64±16% higher in diaphragm mitochondria (p=0.01). UCP2 and UCP3 content was not different. These data indicate that diaphragm mitochondria respire at lower rates, despite a higher content of respiratory complexes. The results support our initial hypothesis and suggest that in the diaphragm mitochondrial content is not the only determinant of aerobic capacity. We propose that VDAC1 influences ATP production in the diaphragm by regulating mitochondrial respiration.Research supported by R01 EY012998‐08A1 to FHA.