Mitochondria are fast Ca²⁺ sinks in rat extraocular muscles: A novel regulatory influence on contractile function and metabolism

PURPOSE. The ultrafast extraocular muscles necessitate tight regulation of free cytosolic Ca²⁺ concentration ([Ca²⁺]_i). Mitochondrial Ca²⁺ influx may be fast enough for this role. In the present study, three hypotheses were tested: (1) Mitochondrial Ca²⁺ uptake regulates [Ca²⁺]_i and production of force in extraocular muscle; (2) mitochondrial content correlates with their use as Ca²⁺ sinks; and (3) mitochondrial content in extraocular muscle is determined by the transcription factors and coactivators that initiate muscle adaptation to aerobic exercise. METHODS. Extraocular and extensor digitorum longus (EDL) muscles from adult Sprague-Dawley rats were used to examine how the Ca²⁺ release agonists caffeine and 4-chloro-3-ethylphenol (CEP), calcimycin (a Ca²⁺ ionophore) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP; a mitochondrial uncoupler) alter [Ca²⁺]_i and force transients. Mitochondrial volume density and capillary density were analyzed by stereology and citrate synthase and cytochrome c oxidase by biochemical assays. Real-time PCR measured mRNAs of genes involved in mitochondrial biogenesis. RESULTS. Caffeine, CEP, and calcimycin increased resting [Ca²⁺]_i to a greater extent in EDL. Peak tetanic [Ca²⁺]_i increased in extraocular muscle with caffeine and CEP. CCCP augmented peak tetanic and submaximum [Ca²⁺]_i and force significantly more in extraocular muscles. Mitochondrial volume density and capillary density were three times greater, and citrate synthase and cytochrome c oxidase were only ∼2-fold higher in extraocular muscle. Calcineurin Aα, calcineurin B, and peroxisome proliferator activated receptor (PPAR)γ were more abundant in extraocular muscle. CONCLUSIONS. These data support the hypothesis that mitochondria serve as Ca²⁺ sinks in extraocular muscles. The high mitochondrial content of these muscles may partly reflect this additional function. It is likely that mitochondrial Ca²⁺ influx increases the dynamic response range of the extraocular muscles and matches metabolic demand to supply.