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Am J Physiol Endocrinol Metab 290: E1172-E1179, 2006. First published January 10, 2006; doi:10.1152/ajpendo.00633.2005
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Role of calcineurin in exercise-induced mitochondrial biogenesis

Pablo M. Garcia-Roves, Janice Huss, and John O. Holloszy

Department of Medicine, Washington University School of Medicine, St. Louis, Missouri

Submitted 14 December 2005 ; accepted in final form 9 January 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Raising cytosolic Ca2+ induces an increase in mitochondrial biogenesis in myotubes. This phenomenon mimics the adaptive responses of skeletal muscle to exercise. It has been hypothesized that increases in cytosolic Ca2+ during motor nerve activity stimulate mitochondrial biogenesis by activating calcineurin. Overexpression of constitutively active calcineurin increases expression of peroxisome proliferator-activated receptor-{gamma} coactivator-1{alpha} (PGC-1{alpha}) and induction of genes involved in mitochondrial energy metabolism in muscle cells. The purpose of this study was to determine whether calcineurin plays a role in the stimulation of mitochondrial biogenesis by exercise. Rats were exercised on 5 successive days by means of swimming. Inhibition of calcineurin with cyclosporin did not prevent the exercise-induced increases in PGC-1{alpha} and a range of mitochondrial proteins. In contrast to the other mitochondrial proteins, the increases in cytochrome oxidase (COX)-I and -IV proteins were blocked by cyclosporin treatment. This inhibitory effect of cyclosporin occurred at the posttranscriptional level, as evidenced by normal increases in COX-I and COX-IV mRNAs in response to exercise in the cyclosporin-treated rats. This toxic effect of cyclosporin may account for the decrease in muscle respiratory capacity reported to occur with cyclosporin treatment. In conclusion, inhibition of calcineurin does not prevent the exercise-induced increase in mitochondrial biogenesis in skeletal muscles, providing evidence that the adaptive response is not mediated by activation of calcineurin.

calcium; muscle; peroxisome proliferator-activated receptor-{gamma} coactivator-1{alpha}

THE PEROXISOME PROLIFERATOR-ACTIVATED RECEPTOR-{gamma} (PPAR{gamma}) coactivator-1{alpha} (PGC-1{alpha}) coactivates the transcription factors that regulate expression of genes encoding mitochondrial proteins (37, 50, 66). PGC-1{alpha} is responsible for mediating the coordinated expression of mitochondrial proteins and plays a key role in regulating biogenesis of mitochondria (37, 50). Increasing the PGC-1 content of muscle cells by means of transgenic overexpression (39, 66) or endurance exercise training (2, 34, 61) induces an increase in mitochondrial biogenesis.

It seems well established from studies on myotubes in culture that raising cytosolic Ca2+ induces an increase in mitochondrial biogenesis (20, 47, 48). This phenomenon in muscle cells in culture (47) closely mimics the adaptive response of muscle to endurance exercise (2). It has been hypothesized that increases in intracellular Ca2+, resulting from motor nerve activity, mediate this adaptive response by activation of the calcium/calmodulin-dependent protein phosphatase calcineurin. One line of evidence supporting this hypothesis (65) came from experiments where the adaptive response of mouse skeletal muscle to wheel running or electrical stimulation of the nerve was blocked by the calcineurin inhibitor cyclosporin or the endogenous calcineurin inhibitor modulatory calcineus in interacting protein.

Calcineurin has been shown to increase PGC-1{alpha} gene transcription (26), and overexpression of constitutively active calcineurin in skeletal muscle of transgenic mice results in increased expression of PGC-1 (55). Furthermore, expression of constitutively active calcineurin in cardiac myocytes has been shown to result in increased expression of PGC-1{alpha} and the induction of a wide range of genes involved in mitochondrial energy metabolism (56). Other, less direct, evidence has also been interpreted as evidence that activation of calcineurin is responsible for, or plays a major role in, mediating the increase in mitochondrial biogenesis induced in skeletal muscle by endurance exercise and increases in cystosolic Ca2+ (23, 26, 39, 45). A finding that appears to be in conflict with this possibility is that the specific inhibitor of calcium/calmodulin-dependent protein kinase KN93 completely blocks the increase in mitochondrial biogenesis induced by raising the Ca2+ level in L6 myotubes (47). In this context, the purpose of the present study was to determine whether or not activation of calcineurin plays a role in the stimulation of mitochondrial biogenesis in skeletal muscle by exercise.


   MATERIALS AND METHODS
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 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
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Materials

Cyclosporin A (CsA), was purchased from Novartis (Cambridge, MA). A Biomol Green Cellular Calcineurin Assay Kit Plus was purchased from Biomol International (Plymouth Meeting, PA). Reagents for SDS-PAGE were from Bio-Rad (Hercules, CA). Mouse anti-human monoclonal antibodies against succinate-ubiquinone oxidoreductase (SUO) 70-kDa subunit, ubiquinone-cytochrome c oxidoreductase core 1 subunit (core 1), cytochrome oxidase (COX) subunits I, IV, and VIc, and ATP synthase (ATP synth) subunit-{alpha} were obtained from Molecular Probes (Eugene, OR). A rabbit polyclonal antibody directed against the 19 carboxyl-terminal amino acids of {delta}-aminolevulinate synthase (ALAS) was generated by Alpha Diagnostic International (San Antonio, TX). A mouse anticytochrome c monoclonal antibody was purchased from PharMingen International (San Diego, CA). Rabbit anti-PGC-1 polyclonal antibody was purchased from Calbiochem (San Diego, CA). Polyclonal antibodies specific for medium-chain acyl-CoA dehydrogenase (MCAD) and long-chain acyl-CoA dehydrogenase (LCAD) were generated as described previously (36, 38). Antibody against estrogen-related receptor-{alpha} (ERR{alpha}) was raised in rabbits against an NH2-terminal peptide, corresponding to amino acids 16–33 of the mouse ERR{alpha} protein conjugated to keyhole limpet hemocyanin (KLH) or BSA. This domain is conserved among species; therefore, the antibody recognizes rodent and human ERR{alpha} isoforms. The KLH-conjugated peptide in Freund’s adjuvant was used for the initial and second booster immunizations. Subsequent boosts were performed with the BSA-conjugated peptide. Serum was collected starting at 6 wk after the initial immunization. Peptide synthesis, conjugation, and HPLC purification was performed by the Washington University School of Medicine Protein and Nucleic Acid Chemistry Laboratory. Peptide immunization and serum collection were performed by Biosource (Hopkinton, MA). Horseradish peroxidase-conjugated donkey antirabbit and goat antimouse IgG were purchased from Jackson ImmunoResearch Laboratories (West Grove, PA). Reagents for enhanced chemiluminescence (ECL) were obtained from Amersham (Arlington Heights, IL). TRIzol Reagent, for isolation of RNA, was purchased from Gibco-BRL (Grand Island, NY). Reagents for isolation of mRNA were obtained from Ambion (Austin, TX). Taqman reverse transcriptase and RT-PCR reagents were purchased from Applied Biosystems (Foster City, CA). All other reagents were obtained from Sigma Chemical (St. Louis, MO).

Animal Care

This research was approved by the Animal Studies Committee of the Washington University School of Medicine (Washington University, St. Louis, MO). Male Wistar rats (body mass 180–200 g) were obtained from Charles River Laboratories (Wilmington, MA) and maintained on a diet of Purina chow and water. Rats were randomly assigned to an exercise-plus-CsA treatment group, an exercise-plus-vehicle (V) group, a sedentary-plus-V group, or a sedentary-plus-CsA group. Rats in the exercise groups were accustomed to swimming for 10 min/day for 2 days. They were then exercised on 5 successive days using a swimming protocol, described previously (51), that involves two 3-h-long swimming sessions separated by a 45-min-long rest period, during which the rats are kept warm and given food and water. After completion of the swimming on the 5th day, the animals were fasted overnight. Two days before they started the exercise, animals were injected once daily subcutaneously between the shoulder blades with either CsA (20 mg/kg body mass) or an equal volume of V (650 mg/ml Cremophor EL and 32.9% ethanol) (67). The animals that were not given cyclosporin had their food intake decreased so that they gained weight at the same rate as the animals that were given cyclosporin. During the period of exercise training, the CsA injections were given 2 h before the exercise. Fasted rats were killed 18 h after the last exercise bout and 3 h after the last injection of CsA or V. The animals were anesthetized with an intraperitoneal injection of pentobarbital sodium (5 mg/100 g body mass), the epitrochlearis and triceps muscles were dissected out, and a blood sample was drawn from the aorta. These forelimb muscles were chosen because they are extensively used during, and undergo a major adaptive response to, swimming (2, 21).

Calcineurin Activity

A Biomol Green Cellular Calcineurin Assay Kit Plus was employed to measure calcineurin (PP2B) phosphatase activity in skeletal muscle homogenates.

Cyclosporin Assay

Cyclosporin concentration in blood was measured using the Dade Dimension CSA immunoassay, which was performed using a Flex reagent cartridge. (Dade Behring, Newark, DE) (62).

Western Blot

Epitrochlearis muscles were homogenized in ice-cold 250 mM sucrose containing 10 mM HEPES, 1 mM EDTA, pH 7.4. Parts of the whole muscle homogenates were subjected to three freeze-and-thaw cycles to disrupt the mitochondria. The samples were then centrifuged at 800 g for 15 min at 4°C, and the supernatant was collected. Protein concentration was measured by the method of Lowry et al. (41), and sample volumes were adjusted to give the same protein concentration in homogenates of muscles from different animals. Aliquots of homogenates were solubilized in Laemmli buffer, subjected to SDS PAGE, and transferred to polyvinylidene difluoride membranes. The membranes were blocked overnight at 4°C with 5% nonfat dry milk in phosphate-buffered saline containing 0.1% Tween. The blots were probed with the primary antibodies followed by incubation with the appropriate horseradish peroxidase-conjugated anti-IgG antibody. Antibody-bound protein was detected by using ECL and quantified by densitometry.

Real-Time Quantitative RT-PCR

Real-time PCR was performed as described by Schoenfeld et al. (58). Total RNA isolated from triceps muscle was reverse transcribed with Taqman reverse transcription reagents (Applied Biosystems) using oligo(dT) and random hexamers (1:1 ratio). Reactions were performed in triplicate in a 96-well format, using Taqman core reagents and a Prism 7700 Sequence Detector (Applied Biosystems). Rat-specific primer/probe sets used to detect specific gene expression were selected by using Primer Express (Applied Biosystems): ALAS, forward 5'-AGTGCCAGCAGG TCAAAGAAAC-3', reverse 5'-GACTGCGGCTTTGGCAGT T-3', probe 5'-TCTCTTTCT CATTGGCT-3'; ATP synth, forward 5'-CGTGAGGGCA ATGATTTATACCAT-3', reverse 5'-TCCTGGTCTCTGAAGTATTCAGCAA-3', probe 5'-ACCAACGCTACCTT GGAAGTGGCATCT-3'; COX-IV, forward 5'-CTACCAGGGCACTTAGCCTAAT-3', reverse 5'-TTGACGTGGGCCACATC-3', probe 5'-TAGTCTTCACTCTTCACAACACTCCCATGT-3'; mitochondrial transcription factor A (Tfam), forward 5'-AGGCTTGGAAA AATCTGTCTC-3', reverse 5'-TGCTCTTCCCAAGACTTCATT-3', probe 5'-AAAGCA GGCATATATTCAGCTTGCT-3'; nuclear respiratory factor (NRF)2a, forward 5'-CAAGAGCAACAGATGAA TGAG-3', reverse 5'-ACTTTAATCGTAGTCGGTGTAG-3', probe 5'-TTGACCAGCCT GTGCAGATTATTC-3'; PGC-1{alpha}, forward 5'-GTGCAGCCAAGACTCTGTATGG-3', reverse 5'-GTCCAGGTCATTCACATCAAGTTC-3', probe 5'-AGTGACATAGAGTG TGCTGCC-3'; peroxisome proliferator-activated receptor-{alpha} (PPAR{alpha}), forward 5'-ACTATGGAGTCCACGCATGTG-3', reverse 5'-TTGTC GTACGCCAGCTTTAGC-3', probe 5'-AAGGCTGTAAGGGCTTCTTTCGGCG-3'; PPAR{delta}, forward 5'-TCACTGGCAAGTCCAGCCA-3', reverse 5'-ACACCAGGCCCTTC TCTGCCT-3', probe 5'-AACGCACCCTTCATCA TCCACGA-3'; COX-I was measured by real-time quantitative PCR using SYBR Green detection. COX-I forward 5'-ACCATCATTTCTCCTTCTCCTA-3', reverse 5'-TAGATTTCCGGCTAGAGGTG-3'; and a GAPDH RNA (VIC) probe set was included in all reactions as an internal correction control, and corrected data were normalized to 18S ribosomal expression (Applied Biosystems).

Statistical Analysis

Values are expressed as means ± SE. Statistically significant differences were determined using unpaired Student’s t-tests.


   RESULTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
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 REFERENCES
 
Inhibition of Calcineurin

The cyclosporin treatment resulted in a plasma cyclosporin concentration >4.5 µg/ml measured 2–3 h after the final cyclosporin injection. As previously reported (22), calcineurin activity in triceps muscles averaged 0.95 ± 0.11 nmol phosphate released·mg protein–1·min–1 in the control V-treated group and was undetectable in the cyclosporin-treated group. This finding demonstrates that the cyclosporin dose used was adequate for prevention of any calcineurin-mediated effects.

Responses of PGC-1{alpha} and Transcription Factors Involved in Mitochondrial Biogenesis

Protein levels. The five daily bouts of swimming induced an ~2-fold increase in PGC-1 protein content in triceps muscles (Fig. 1). This adaptive response was not affected by the inhibition of calcineurin by cyclosporin. ERR{alpha} protein was also significantly increased in both groups by ~50% (Fig. 1). PPAR{alpha} protein content of muscle was not increased by exercise and was unaffected by cyclosporin treatment. PPAR{delta} protein level increased by ~25%; although this increase was not statistically significant, we think, in light of the increase in PPAR{delta} mRNA level (see below), that it is probably real.


Figure 1
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  		Fig. 1. Effects of exercise training with or without cyclosporin A (CsA) treatment on triceps muscle protein levels of peroxisome proliferator-activated receptor (PPAR)-{gamma} coactivator-1{alpha} (PGC-1{alpha}) and transcription factors involved in mitochondrial biogenesis. A: representative Western blots; B: average protein values of PGC-1{alpha}, estrogen-related receptor-{alpha} (ERR{alpha}), PPAR{alpha}, and PPAR{delta}. Sed-V, sedentary, vehicle treated; Ex-V, exercised, vehicle-treated; Ex-CsA, exercised, cyclosporin A-treated. Values are means ± SE for 7 to 8 muscles per group. *P < 0.05 vs. Sed-V.

 
mRNA. To obtain insights regarding the mechanisms involved in the adaptive response, and because antibodies against NRF2 and mitochondrial transcription factor A (Tfam) were not available to us, we also quantified mRNA levels of PGC-1 and some of the transcription factors involved in mitochondrial biogenesis. [Because muscles were obtained 18 h after the last exercise bouts, the values shown in Fig. 2 were considerably lower than the peak values that were seen at earlier time points after cessation of exercise (P. M. Garcia-Roves and J. O. Holloszy, unpublished time-course study findings).] As shown in Fig. 2, PGC-1{alpha}, PPAR{delta}, NRF2, and Tfam mRNA levels were all significantly increased in triceps muscles 18 h after the fifth exercise bout. These adaptive responses were unaffected by inhibition of calcineurin by cyclosporin. Like PPAR{alpha} protein, PPAR{alpha} mRNA was not increased in response to the exercise.


Figure 2
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  		Fig. 2. Effects of exercise training with or without CsA treatment on mRNA levels of PGC-1{alpha} and transcription factors involved in mitochondrial biogenesis. Real-time PCR analysis of PGC-1{alpha}, PPAR{alpha}, PPAR{delta}, nuclear respiratory factor (NRF)2, and mitochondrial transcription factor A (Tfam) in triceps muscles. Values are means ± SE for 6 to 9 muscles per group. *P < 0.05 vs. Sed-V group.

 
Responses of Mitochondrial Enzyme Proteins to Daily Bouts of Swimming

As shown in Fig. 3, a range of mitochondrial proteins, used as markers of increased mitochondrial biogenesis, increased significantly in response to the exercise program. These included a number of mitochondrial respiratory chain proteins: SUO (70-kDa subunit), ubiquinol-cytochrome c oxidoreductase core protein 1 (core 1), cytochrome c, and COX-VIc, as well as ATP synth subunit-{alpha}, MCAD, LCAD, and ALAS. The magnitude of these increases ranged from ~35 to 100% and were similar in the control V-treated swimmers and the cyclosporin-treated swimmers. The major reason for the differences in the magnitudes of the increase in these mitochondrial proteins relates to their half-lives, which range from a few hours to ~7 days (7, 9, 32).


Figure 3
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  		Fig. 3. A and C: responses of mitochondrial enzyme proteins to exercise in triceps muscle. Representative Western blots of mitochondrial marker proteins. B and D: protein quantification. Values are means ± SE for 6–9 muscles per group. *P < 0.05 vs. Sed-V.

 
COX Subunits I and IV Protein Levels Do Not Increase in Response to Exercise in Muscles of Cyclosporin-Treated Rats

In contrast to the other mitochondrial proteins measured, COX-I and COX-IV proteins did not increase in response to the exercise program in skeletal muscle of the cyclosporin-treated rats (Fig. 4). The COX-IV gene is nuclear encoded, whereas the COX-I gene is encoded in mitochondrial DNA. The transcription factor NRF2 regulates expression of COX-IV, whereas expression of mitochondrial encoded genes is regulated by Tfam, the expression of which is also regulated by NRF2 in rats (37). Therefore, because NRF2 and Tfam induction by exercise was not prevented by cyclosporin, and in light of the increases in other mitochondrial proteins in the cyclosporin-treated group, we were puzzled by the inhibition of the exercise-induced increase in COX-I and COX-IV.


Figure 4
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  		Fig. 4. Cytochrome oxidase (COX) subunits I and IV protein levels do not increase in response to exercise in muscles of cyclosporin-treated rats. A: representative Western blots. B: protein quantification. Values are means ± SE for 6–9 muscles per group. *P < 0.05 vs. Sed-V group; {dagger}P < 0.05 vs. Ex-V group.

 
To determine whether the inhibitory effect of cyclosporin on the exercise-induced increase in COX-I and COX-IV occurs at the pretranslational or posttranslational level, we measured COX-I and COX-IV mRNA levels. As shown in Fig. 5, COX-I and COX-IV mRNA levels were significantly increased in muscle 18 h after exercise. The magnitude of this increase was similar to that of ATP synth mRNA, which was measured for comparison.


Figure 5
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  		Fig. 5. COX subunits I and IV mRNA levels do increase in response to exercise in muscles of cyclosproin-treated rats. Real-time PCR analysis of {delta}-aminolevulinate synthase (ALAS), ATP synthase (ATP synth), COX-I, and COXIV mRNA levels in triceps muscles. Values are means ± SE for 6–9 muscles per group. *P < 0.05 vs. Sed-V.

 
COX-I and COX-IV Expression in Muscles of Cyclosporin-Treated Sedentary Rats

The finding that the exercise-induced increase in the transcription of COX-I and COX-IV was not blocked by the inhibition of calcineurin provides evidence that prevention of the increases in COX-I and COX-IV proteins is mediated at the translational level by a toxic effect of cyclosporin that is unrelated to calcineurin inhibition. If this interpretation is correct, one would expect that cyclosporin treatment should also result in decreases in COX-I and COX-IV proteins in muscles of sedentary animals. As shown in Fig. 6, COX-I and COX-IV protein levels were significantly reduced in muscle of sedentary rats in response to the cyclosporin. The cyclosporin treatment had no effect on ATP synth and ALAS protein levels, which were used for comparison, in muscles of sedentary rats.


Figure 6
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  		Fig. 6. COX-I and COX-IV protein levels are reduced in muscles of sedentary rats treated with cyclosporin. A: representative Western blot. B: protein quantifications. Values are means ± SE for 7 muscles per group. *P < 0.005 vs. Sed-V group.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Endurance exercise training induces an adaptive increase in skeletal muscle mitochondria (8, 15, 29, 31). Increases in cytosolic Ca2+ stimulate biogenesis of mitochondria in muscle cells (20, 47, 48), and it seems likely that the increases in sarcoplasmic Ca2+ during contractile activity are involved in mediating the stimulation of mitochondrial biogenesis by exercise (47). Considerable evidence suggests that the effects of exercise and increases in sarcoplasmic Ca2+ on mitochondrial biogenesis are mediated by induction of PGC-1 (2, 25, 34, 47, 59). Overexpression of constitutively active calcineurin in skeletal muscle of transgenic mice results in increased expression of PGC-1{alpha} (55). Furthermore, adenoviral-mediated expression of constitutively active calcineurin in cardiomyocytes induces increased expression of PGC-1{alpha} and increased transcription of a wide range of mitochondrial enzymes (56). It has been reported (26) that activation of PGC-1{alpha} gene expression by calcineurin is mediated, at least in part, by activation of myocyte enhancer factor-2 (MEF2), resulting in increased activation of the PGC-1{alpha} promoter by the MEF2.

In this context, the hypothesis that calcineurin activation by Ca2+ is involved in mediating the exercise-induced increase in the capacity of muscle for oxidative metabolism seemed reasonable. Most of the evidence that has been interpreted as support for the hypothesis is indirect (23, 26, 39, 45, 54). However, the results of a study by Wu et al. (65) appeared to provide direct evidence for a key role of calcineurin in the adaptive response of muscle to exercise. Those investigators generated doubly transgenic mice that overexpressed the endogenous inhibitor of calcineurin MCIP1, as well as an MEF2-dependent indicator gene, in skeletal muscle and used activation of MEF2 as evidence for activation of the pathway leading to an increased capacity for oxidative metabolism. They found that wheel running or electrical stimulation failed to activate MEF2 in the MCIP1 transgenic mice (65).

It was previously found that the Ca2+/calmodulin-dependent protein kinase (CaMK) inhibitor KN93 completely blocks the increase in mitochondrial biogenesis induced by raising cytosolic Ca2+ in L6 myotubes by intermittent exposure to caffeine (47). This finding, which seems incompatible with a key role for calcineurin in the adaptive increase in mitochondria induced by Ca2+, motivated us to conduct the present study. Our initial experiment showed that the exercise-induced increase in expression of COX-I and COX-IV proteins was blocked, whereas expression of a range of other mitochondrial proteins and PGC-1 increased normally in muscles of rats where calcineurin was inhibited. The finding that COX-I and COX-IV proteins did not increase was puzzling in light of the normal increase in PGC-1{alpha}, which regulates expression of these proteins in rat skeletal muscle by coactivating NRF2 (37). Our subsequent finding that COX-I and COX-IV mRNA levels increased normally in response to exercise in the cyclosporin-treated group provides evidence that calcineurin is not involved in mediating the exercise-induced increases in COX-I or COX-IV.

The finding that COX-I and COX-IV proteins decreased in muscles of sedentary rats treated with cyclosporin and did not increase in cyclosporin-treated exercised rats despite increases in their mRNAs provides evidence that cyclosporin has a toxic, inhibitory effect on a posttranscriptional step in the expression of these proteins. The decrease in COX-I and COX-IV seems a likely explanation for the reduction in the capacity of muscle for substrate oxidation induced by cyclosporin (43). Our finding that inhibition of calcineurin does not block the stimulation of mitochondrial biogenesis by exercise confirms and extends the findings of Terada et al. (60), who showed that cyclosporin treatment did not prevent the increases in citrate synthase and 3-hydroxyacyl-CoA dehydrogenase enzyme activities in muscles of rats that were exercise-trained for 10 days by means of treadmill running. Similarly, Meissner et al. (42) found that cyclosporin was not able to prevent upregulation of citrate synthase mRNA in response to Ca2+ ionophore treatment in rabbit skeletal muscle cells.

Maintenance of the slow-twitch type I skeletal muscle fiber type and conversion of fast-twitch type II fibers to type I fibers is dependent on calcineurin activity (12, 44, 46). Muscle fiber type is determined by type of motor nerve, and fiber type can be altered by cross-innervation, i.e., switching the motor nerves of fast and slow muscles (53). Fast-twitch fibers can also be, at least partially, converted to slow-twitch fibers by chronic stimulation at the firing frequency of a slow nerve, i.e., ~10 to 20 Hz (49). The misconception that endurance exercise training results in conversion of type II to type I fibers (26, 39, 46) may have contributed to development of the hypothesis that the adaptive response of muscle to exercise is mediated by calcineurin.

Actually, the endurance exercise training programs that have been shown to result in an increase in mitochondria induce increases in the mitochondrial content of all the muscle fiber types without conversion of fast-to-slow fibers (3, 4, 6, 17–19, 24, 30, 33, 52, 64). The only conversion of fiber types in response to endurance exercise that is well documented is conversion of type IIb or IIx to type IIa fibers (19, 24, 65). This phenomenon is much more marked in humans than in rats, with almost complete disappearance of type IIb or IIx fibers in highly trained endurance athletes (1) and a gradual reversal back to type IIb or IIx fibers when training is stopped (14). It is possible that the extreme training programs of professional cyclists, triathletes, etc., that involve many hours of intense exercise daily for years may cause an increase in slow muscle fibers (13, 57), but this remains to be documented. Another potential source of confusion is the equating of "oxidative muscle fibers" with type I slow twitch fibers. Actually, in rats, the species on which much of the research on the adaptive responses of muscle to exercise was done, the type IIa fibers have a considerably higher content of mitochondria and capacity for substrate oxidation than the type I fibers (4, 5). In humans, type I fibers have a greater content of mitochondria than type IIa fibers (28, 40), but this difference largely disappears in response to strenuous endurance exercise training (10, 35).

Sarcoplasmic Ca2+ concentrations increase during contractile activity, calcineurin is activated by Ca2+, and an increase in calcineurin activity results in an increase in mitochondrial biogenesis in cultured muscle cells. In this context, the present results raise the question: Why does calcineurin activation not play a significant role in the exercise-induced increase in muscle mitochondria? Calcineurin activation is mediated specifically by sustained, low-amplitude increases in cytosolic Ca2+ (16). One possible explanation, therefore, is that the firing frequency of slow motor units, of ~10 to 20 Hz, results in sustained elevations of sarcoplasmic Ca2+ into the 100-to-300 nM range needed to activate calcineurin (11, 27). On the other hand, the high-amplitude Ca2+ spikes induced by the phasic firing patterns of fast motor units, into the ~1.0 µM range, may be too transient to activate calcineurin (27, 63). As a consequence, calcineurin may not be activated in muscles during activities such as running or swimming.

In conclusion, our results provide evidence that calcineurin does not play a significant role in mediating the exercise-induced increase in muscle mitochondria. This finding is in keeping with the observation by Terada et al. (60) that cyclosporin treatment did not prevent the increases in muscle citrate synthase and 3-hydroyacyl-CoA dehydrogenase activities induced by exercise. It also fits with the conclusion that the increase in mitochondrial biogenesis induced by raising cytosolic Ca2+ in L6 myotubes is mediated by CaMK, as evidenced by the finding that the CaMK inhibitor KN93 completely prevents the increase in mitochondria (47). Our finding that cyclosporin inhibits a posttranscriptional step in the expression of COX-I and COX-IV provides an explanation for the cyclosporin-induced decrease in the capacity of skeletal muscle for substrate oxidation (43).


   GRANTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This research was supported by National Institutes of Health grants AG-00425, K01-DK-063051, and P60 DK-20579–28. P. M. Garcia-Roves was supported by an American Diabetes Association Mentor-Based Postdoctoral Fellowship.


   ACKNOWLEDGMENTS
 
We are grateful to Victoria Reckamp for expert assistance with preparation of this manuscript.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. O. Holloszy, Washington University School of Medicine, Applied Physiology, Campus Box 8113, 4566 Scott Ave., St. Louis, MO 63110 (e-Mail: jhollosz{at}im.wustl.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
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 RESULTS
 DISCUSSION
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 REFERENCES
 

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