Extraocular muscle mitochondria have a lower respiratory capacity: evidence for intrinsic differences in mitochondrial composition and function
Samir P Patel 1,
Jorge L Gamboa 1,
Colleen A McMullen 1,
Alexander G Rabchevsky 1,
and
Francisco H. Andrade 2*
1 Physiology, University of Kentucky, Lexington, Kentucky, United States
2 Physiology, University of Kentucky, MS508 UKMC, Lexington, Kentucky, 40536-0298, United States
* To whom correspondence should be addressed. E-mail: paco.andrade{at}uky.edu.
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Abstract |
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Purpose: The constant activity of the extraocular muscles is supported by abundant mitochondria. These organelles could enhance energy production by increasing the content of respiratory complexes. Therefore, we tested the hypothesis that extraocular muscle mitochondria respire faster than those from limb muscles because of higher content of respiratory complexes.
Methods: Inner mitochondrial membrane density was determined by stereological analysis of triceps surae (a limb muscle) and extraocular muscles of adult male Sprague-Dawley rats. We measured respiration rates of isolated mitochondria using a Clark-type electrode. The activity of respiratory complexes I, II and IV was determined by spectrophotometry. The content of respiratory complexes was estimated by western blot.
Results: State 3, 4 and 5 respiration rates in extraocular muscle mitochondria were 40-60% less than in limb muscle mitochondria. Extraocular muscle inner mitochondrial membrane density was similar to other skeletal muscles. Activity of complexes I and IV was less in extraocular muscle mitochondria (~50% the activity in triceps), but their content was ~15-30% higher. There was no difference in complex II content or activity, or complex III content. Finally, complex V was less abundant in extraocular muscle mitochondria.
Conclusions: Our results demonstrate that extraocular muscle mitochondria respire at slower rates than mitochondria from limb muscles, despite similar mitochondrial ultrastructure. Instead, we found differences in the activity (I, IV) and content (I, IV, V) of electron transport chain complexes. The discrepancy between activity and content of some complexes is suggestive of alternative subunit isoform expression in the extraocular muscles compared to limb muscles.
Key Words:
extraocular muscle, muscle fiber, mitochondria, electron microscopy, electron transport chain
