Skip to main content
PMC home page
The Journal of Physiology logoLink to The Journal of Physiology
. 2006 Jun 22;575(Pt 2):645–656. doi: 10.1113/jphysiol.2006.108472

Activated calcineurin ameliorates contraction-induced injury to skeletal muscles of mdx dystrophic mice

Nicole Stupka 1, David R Plant 1, Jonathan D Schertzer 1, Tennent M Emerson 1, Rhonda Bassel-Duby 2, Eric N Olson 2, Gordon S Lynch 1
PMCID: PMC1819459  PMID: 16793906

Abstract

Utrophin expression is regulated by calcineurin and up-regulating utrophin can decrease the susceptibility of dystrophic skeletal muscle to contraction-induced injury. We overexpressed the constitutively active calcineurin-A α in skeletal muscle of mdx dystrophic mice (mdx CnA*) and examined the tibialis anterior muscle to determine whether the presence of activated calcineurin promotes resistance to muscle damage after lengthening contractions. Two stretches (10 s apart) of 40% strain relative to muscle fibre length were initiated from the plateau of a maximal isometric tetanic contraction. Muscle damage was assessed 1, 5 and 15 min later by the deficit in maximum isometric force and by quantifying the proportion of muscle fibres staining positive for intracytoplasmic albumin. The force deficit at all time points after the lengthening contractions was approximately 80% in mdx muscles and 30% in mdx CnA* muscles. The proportion of albumin-positive fibres was significantly less in control and injured muscles from mdx CnA* mice than from mdx mice. Compared with mdx mice, mean fibre cross-sectional area was 50% less in muscles from mdx CnA* mice. Furthermore, muscles from mdx CnA* mice exhibited a higher proportion of fibres expressing the slow(er) myosin heavy chain (MyHC) I and IIa isoforms, prolonged contraction and relaxation times, lower absolute and normalized maximum forces, and a clear leftward shift of the frequency–force relationship with greater force production at lower stimulation frequencies. These are structural and functional markers of a slower muscle phenotype. Taken together, our findings show that muscles from mdx CnA* mice have a smaller mean fibre cross-sectional area, a greater sarcolemmal to cytoplasmic volume ratio, and an increase in utrophin expression, promoting an attenuated susceptibility to contraction-induced injury. We conclude that increased calcineurin activity may confer functional benefits to dystrophic skeletal muscles.

Duchenne muscular dystrophy (DMD) is the result of a mutation in the dystrophin gene such that the functional protein is not expressed. Dystrophin is essential for maintaining muscle membrane integrity in quiescent and contracting muscle fibres (Petrof et al. 1993; Pasternak et al. 1995; Lynch et al. 2000). The absence of dystrophin results in the loss of all dystrophin-associated proteins and the disruption of the linkage between muscle fibre cytoskeleton and the extracellular matrix (Matsumura & Campbell, 1994). Lack of dystrophin expression can increase the susceptibility of dystrophic muscles to contraction-induced injury (Dellorusso et al. 2002). In boys with DMD, a greater susceptibility to muscle damage and impaired muscle regeneration result in the progressive loss of muscle mass and strength and the replacement of viable muscle fibres with fat and connective tissue (Zammit & Partridge, 2005).

Increased susceptibility to muscle damage can be assessed using a variety of markers including increased plasma creatine kinase levels (Griggs & Rennie, 1983), inflammatory cell infiltration (Kissel et al. 2001) and greater myofibrillar protein catabolism (McKeran et al. 1977). In muscles from mdx mice (the mdx mouse is a model for DMD), susceptibility to contraction-induced injury can be assessed directly. Dellorusso et al. (2002) showed that after two maximal lengthening contractions (LCs) in situ, the force deficit of tibialis anterior (TA) muscles from mdx mice is four- to seven-fold greater than of TA muscles from wild-type (non-dystrophic) mice. Following repeated LCs in vitro, the force deficit of the extensor digitorum longus (EDL) muscles from mdx mice is five-fold greater than from wild-type mice (Deconinck et al. 1997). The magnitude of the force deficit following the LC protocol is indicative of the severity of muscle damage sustained. After the contraction-induced injury protocol, the proportion of TA or EDL muscle fibres with sarcolemmal damage can by assessed by quantifying intrafibre Procion orange or Alizarin red staining, which others have shown to be significantly greater in muscles from mdx mice compared to wild-type mice (Deconinck et al. 1997; Dellorusso et al. 2002).

Currently, the efficacy of pharmacological interventions to treat patients with DMD is limited. There is interest in developing a treatment strategy which would improve the resistance of dystrophic muscle to contraction-induced injury (Zammit & Partridge, 2005). One such strategy focuses on the up-regulation of utrophin. Utrophin has 80% homology with dystrophin and is normally localized at the neuromuscular junction (Deconinck et al. 1997). Utrophin can functionally compensate for dystrophin. Overexpression of full-length or truncated utrophin in muscles from mdx mice improved their muscle morphology, sarcolemmal stability and mechanical resistance to forced LCs (Deconinck et al. 1997; Tinsley et al. 1998).

Given the efficacy of utrophin for ameliorating the pathology of dystrophic muscles, there is interest in elucidating the cellular signalling pathways that regulate utrophin expression. Utrophin A protein levels are greater in slow-twitch muscles than in fast-twitch muscles and its expression is mediated by the calcineurin/NFAT (nuclear factor of activated T Cells) signal transduction pathway (Chakkalakal et al. 2003). Transgenic mice that express a constitutively active form of calcineurin-Aα (CnA*) have four-fold greater utrophin mRNA levels and increased utrophin protein expression at the sarcolemma compared with wild-type mice (Chakkalakal et al. 2003). When these mice were crossed with mdx mice, the mdx CnA* F1 offspring were dystrophin deficient and exhibited the hallmarks of increased calcineurin phosphatase activity including increased nuclear localization of NFAT, a slower muscle phenotype and increased utrophin expression (Chakkalakal et al. 2004). In muscles from mdx CnA* mice, greater utrophin expression was associated with greater expression of all members of the dystrophin-associated protein complex, including β-dystroglycan, syntrophin and neuronal nitric oxide synthase (Chakkalakal et al. 2004). Increased utrophin expression and restoration of the dystrophin-associated protein complex improved the dystrophic pathology. In muscles from mdx CnA* mice, inflammatory cell infiltration was attenuated, sarcolemmal stability was improved, as indicated by reduced cytoplasmic albumin and Evans blue staining, and plasma creatine kinase levels and the proportion of centrally nucleated fibres were also decreased (Chakkalakal et al. 2004).

In this study, we demonstrate an improved resistance to contraction-induced injury in TA muscles from mdx CnA* mice. We examined the force deficit following two maximal LCs in situ and assessed the proportion of albumin-positive fibres as indicators of susceptibility to contraction-induced injury. In addition, we characterized the isometric contractile properties of TA muscles from mdx CnA* mice and assessed the effect of expressing activated calcineurin in skeletal muscle on maximal force producing capacity.

Methods

Transgenic mice

All experiments were approved by the Animal Experimentation Ethics Committee of the University of Melbourne. Female mdx mice were mated with male transgenic mice (MCK (muscle creatine kinase)-CnA*) expressing a constitutively active form of CnA* (O'Keefe et al. 1992). The MCK-CnA* male mice were bred on a B6C3F1 background and the female mdx mice had a C57BL/10 ScSn background. The transgenic F1 male offspring were identified by PCR screening of genomic DNA extracted from tail tissue (Naya et al. 2000). TA muscle function and susceptibility to contraction-induced injury was assessed in situ in 9- to 10-month-old mdx CnA* mice (n = 9) and mdx (n = 10) mice.

In situ muscle function and LC protocol

The in situ TA muscle preparation, the isometric contractile properties of the preparation, and the LC protocol have been previously described (Schertzer et al. 2006). The mice were anaesthetized with 100 mg kg−1 ketamine and 10 mg kg−1 xylazine so that they were unresponsive to tactile stimuli. The mice were carefully monitored throughout the experiment and additional doses of anaesthetic were administered as required depending upon whether there was a reflex response to a toe pinch. Briefly, the distal portion of the TA muscle and its tendon were dissected from anaesthetized mice and the tendon was tied with 4.0 braided surgical silk to the lever arm of a dual-mode servomotor/force transducer (305-LR, Aurora Scientific Inc., Ontario, Canada). The TA muscle was stimulated by directly applying supramaximal square-wave pulses (10 V, 300 ms duration) to the sciatic nerve using a hook electrode to produce a maximum isometric contraction (Lynch et al. 2001a; Gregorevic et al. 2002). All stimulation parameters and contractile responses were controlled and measured using custom-built applications of LabView software (National Instruments, Austin, TX, USA) driving a personal computer with an onboard controller (PCI-MIO-16XE-10, National Instruments) interfaced with the transducer-servomotor control/feedback hardware (305-LR; Aurora Scientific Inc.) (Lynch et al. 2001a; Gregorevic et al. 2002). Optimal muscle length (Lo) was determined from micromanipulations of muscle length and a series of isometric twitch contractions. Maximal isometric tetanic force (Po) was determined from the plateau of the frequency–force relationship. The muscles were stimulated at 10, 30, 50, 100, 150, 200, 250 and 300 Hz. Optimal fibre length (Lf) was determined by multiplying Lo by the TA Lf/Lo ratio of 0.6 (Schertzer et al. 2006).

To assess the susceptibility of TA muscles to LC-induced injury, muscles from mdx CnA* mice and mdx littermates were subjected to two LC 10 s apart as previously described (Dellorusso et al. 2002; Schertzer et al. 2006). The muscles were stimulated at the frequency required to elicit Po (i.e. 150 Hz) for 100 ms and then stretched to 40% of Lf at a velocity of 2 Lf s−1. Stimulation during the LC was 200 ms in duration, for a total stimulation time of 300 ms. The muscle was held unstimulated in the lengthened position for 100 ms and then returned to Lo at a velocity of 2 ×Lf s−1. The muscles were allowed to recover for 1, 5 and 15 min, at which times Po was reassessed (by stimulating the muscle maximally, i.e. at 150 Hz). The relative force deficit (loss of maximal force producing capacity) at 1, 5 and 15 min after the LCs was expressed as a percentage of the maximum Po measured before the LCs. Overall muscle cross-sectional area was calculated by dividing muscle mass by the product of Lf and the density of mammalian skeletal muscle (1.06 mg mm−3). Force per cross-sectional area, specific force (sPo, in kN m−2), was determined by dividing Po by TA muscle cross-sectional area (Lynch et al. 2001b). At the end of the experiment, the still deeply anaesthetized mouse was killed by opening the thoracic cavity and surgically excising the heart.

Histology and immunohistochemistry

Sections (5 μm thick) of each muscle were cryosectioned and stained with haematoxylin and eosin (H&E). Images of cross-sections were captured with a digital camera (Spot, v2.2, Diagnostic Instruments, Sterling Heights, MI, USA) mounted on an upright microscope (BX-51, Olympus, Tokyo, Japan), and fibre circumference and myofibre number were determined using an image analysis system (AIS, v6.0, Imaging Research Inc., St Catherines, Ontario, Canada). The proportion of centrally nucleated fibres and mean fibre cross-sectional area were calculated by analysing ∼150 fibres per section. To assess variability in fibre size, the coefficient of variation (CV) for mean fibre cross-sectional area was calculated for each muscle analysed. To determine whether hyperplasia was a consequence of calcineurin activation, the number of myofibres was counted and divided by the area of muscle section examined. Approximately one-third of each muscle section was examined to quantify myofibre number.

For immunohistochemistry, sections were fixed in cold acetone for 15 min and endogenous peroxidase activity was inhibited with 1.5% H2O2 in 0.5 m Tris-buffered saline (TBS) for 5 min. Sections were incubated with blocking solution containing (%) normal goat serum 2, normal mouse serum 2, bovine serum albumin 1, cold fish gelatin 0.1, Triton X-100 0.1, Tween 20 0.05 and sodium azide 0.05, in 0.01 m phosphate-buffered saline (PBS; pH 7.2) for 30 min. A primary polyclonal antibody specific for utrophin A (sc-15377, Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA), diluted 1:60, a polyclonal antibody specific for albumin (Dako Corporation, Botany, Australia), diluted 1:4000, a monoclonal antibody specific for myosin heavy chain (MyHC) type IIa (Hybridoma Bank), diluted 1:40 or a monoclonal antibody specific for MyHC I (Hybridoma Bank), diluted 1:40, was added for 1 h. A secondary antibody, goat anti-rabbit secondary peroxidase-conjugated antibody (Dako Corporation) or biotinylated rabbit anti-mouse antibody (Dako Corporation) was added for 30 min. Visualization was preformed using a peroxidase detection system (Vector Laboratories) and the 3-amino-9-ethylcarbazole (AEC) + substrate chromogen (Dako Corporation).

To determine utrophin A content, three images of myofibres at 400 × magnification were captured for each uninjured TA muscle section exposed to the anti-utrophin A antibody using a digital imaging system (AIS, v6.0, Imaging Research Inc.). The percentage of the section that reacted with the utrophin antibody (and stained red with AEC) was quantified, averaged for each muscle, and expressed as percentage utrophin staining. To assess the proportion of muscle fibres positive for MyHC I and IIa, the number of muscle fibres that reacted with the respective antibodies were counted and expressed as a proportion of the total number of myofibres; ∼1000 fibres were examined for each muscle. To quantify muscle membrane damage, the number of myofibres that reacted with anti-albumin antibody were counted and expressed as a proportion of the total number of muscle fibres; ∼1600 fibres were examined for each muscle. The cross-sectional area of all albumin-positive fibres was also assessed.

Statistics

Values presented in all figures and tables are expressed as means ± s.e.m. All statistical analyses were performed using a statistics software package (Minitab Inc., State College, PA, USA). Two sample unpaired t tests were used to analyse differences in function and morphology in TA muscles from mdx CnA* and mdx mice. Two-way ANOVA was used to assess differences in albumin staining between control and injured TA muscles from mdx CnA* and mdx mice and to determine differences in the frequency–force curves for TA muscles from mdx CnA* and mdx mice. Tukey's post hoc test was used to locate pair-wise significant differences, where appropriate. For all comparisons, P < 0.05 was considered significant.

Results

Histological analysis of dystrophic skeletal muscles overexpressing activated calcineurin

We generated mdx CnA* mice to examine the effect of activation of the calcineurin signalling pathway in dystrophic muscle. Measurements of body mass and TA muscle mass showed lower values in mdx CnA* mice than in mdx mice, but the ratio of muscle mass to body mass was similar (Table 1). Morphological studies of the muscles showed that the mean fibre cross-sectional area was two-fold greater in TA muscles from mdx than mdx CnA* mice (Table 2 and Fig. 1). Muscle mass and whole muscle cross-sectional area were only ∼15% and 5% smaller, respectively, in muscles from mdx CnA* compared to values in muscles from mdx mice, but the mean fibre size was ∼50% less in muscles from mdx CnA*, suggesting an increase in the number of muscle fibres in TA muscles of mdx CnA* mice. This is supported by our data indicating that the number of muscle fibres per mm2 tissue was almost two-fold higher in TA muscles from mdx CnA* than from mdx mice (Fig. 1).

Table 1.

Contractile properties of TA muscles from mdx and mdx CnA* mice

mdx mdx CnA*
Body mass (BM; g) 38.8 ± 0.6 35.2 ± 0.7*
Muscle mass (MM; g) 107.3 ± 2.8 92.6 ± 4.3*
MM:BM 2.76 ± 0.07 2.34 ± 0.32
Muscle CSA (mm2) 11.8 ± 0.4 10.6 ± 0.6 (P = 0.11)
Twitch Pt (mN) 681.8 ± 25.0 721.4 ± 30.7
TPT (ms) 14.3 ± 0.5 19.0 ± 0.4*
½RT (ms) 10.6 ± 0.6 19.3 ± 0.8*
dPt/dt (mN ms−1) 97.8 ± 3.0 84.5 ± 3.7*
Tetanus Po (mN) 2394.8 ± 52.5 1949.5 ± 84.3*
sPo (kN m−2) 204.5 ± 4.7 179.6 ± 8.3*
Open in a new tab

Values shown are means ± s.e.m. TA, tibialis anterior muscle; Pt, twitch force; TPT, time to peak twitch force; ½RT, half relaxation time of twitch; dPt/dt, rate of twitch force development; Po, absolute maximum isometric force; sPo, specific force (Po normalized for muscle cross-sectional area).

*

Significant differences between mdx and mdx CnA* mice are as indicated (P < 0.05).

Table 2.

Analysis of fibre cross-sectional area in injured and uninjured TA muscles of mdx and mdx CnA* mice

Mean cross-sectional area (μm2)
All fibres Fibres with CN Fibres without CN Fibres positive for Albumin
mdx Control TA 1988.3 ± 109.1 1993.7 ± 116.6 1976.6 ± 98.9 2383.3 ± 137.9
CV 74.6% (n = 1201) (n = 613) (n = 526)
(n = 1779)
Injured TA N/A N/A N/A 3080.6 ± 137.6
(n = 856)
mdx CnA* Control TA 1021.9 ± 57.6 1032.4 ± 62.4 991.4 ± 52.1 1541.0 ± 124.5
CV 69.7% (n = 865) (n = 603) (n = 399)
(n = 1479)
Injured TA N/A N/A N/A 1830.1 ± 240.1
(n = 405)
Open in a new tab

Values shown are means ±s.e.m. TA, tibialis anterior muscle; CN, central nuclei; CV, coefficient of variation; N/A, no analysis completed. CV is reported to illustrate the greater variability in fibre size in TA muscles from mdx and mdx CnA* mice. Differences between specific groups are discussed in the Results.

Figure 1. Proportion of centrally nucleated fibres and myofibre number in TA muscles of mdx and mdx CnA* mice.

Figure 1

Open in a new tab

A, representative H&E sections of uninjured TA muscles of mdx and mdx CnA* mice. Note the variability in fibre size in muscles of mdx and mdx CnA* mice. Scale bar represents 100 μm and applies to all panels. B, the proportion of centrally nucleated fibres was quantified to determine which muscle fibres had recently undergone regeneration. *Significant difference between TA muscles of mdx and mdx CnA* mice (P < 0.05). C, the number of myofibres per mm2 tissue was quantified to assess whether expression of the CnA* transgene activated hyperplasia in TA muscles of mdx mice. *Significant difference between TA muscles of mdx and mdx CnA* mice (P < 0.05).

To assess variability in fibre size, the CV for mean fibre cross-sectional area was calculated for each muscle analysed. The average CV for muscles from mdx CnA* and mdx mice was ∼70%–75% (Table 2). Thus, overexpression of the constitutively active calcineurin transgene did not reduce variability in muscle fibre size.

In healthy muscle, nuclei are located at the periphery of the myofibre and the presence of central nuclei is an indicator of recently regenerated fibres, which is characteristic of myopathies where muscle fibres are more susceptible to on-going bouts of injury and repair. The proportion of myofibres with central nuclei was ∼10% lower in TA muscles from mdx CnA* compared to muscles from mdx mice (Fig. 1). In both mdx CnA* and mdx mice, the mean fibre cross-sectional area was similar for muscle fibres that were centrally nucleated and those that were not (Table 2).

To assess the muscle fibre-type proportions, histological sections were immunoreacted with antibodies recognizing MyHC I and IIa protein. We observed in TA muscles of mdx mice that 10% of myofibres expressed MyHC IIa protein and 3% expressed MyHC I protein, whereas 46% of the myofibres expressed MyHC IIa protein and 12% expressed MyHC I protein in TA muscles of mdx CnA* mice (Fig. 2A). Thus, overexpression of activated calcineurin resulted in a four-fold increase in the number of type I and IIa fibres in dystrophic muscle.

Figure 2. Calcineurin activation increased protein expression of the MyHC IIa isoform and utrophin A.

Figure 2

Open in a new tab

A, data and representative images showing MyHC IIa-positive nuclei in TA sections of mdx and mdx CnA* mice. Fibres expressing the MyHC IIa isoform stained red; ⊕= positive. *Significant difference in the proportion of MyHC IIa fibres between TA muscles of mdx and mdx CnA* mice (P < 0.05). Scale bar represents 100 μm and applies to upper panels. B, data and representative images showing utrophin A staining in TA sections from mdx and mdx CnA* mice. Note the greater level of sarcolemmal utrophin A staining, seen as a red outline, in muscle fibres from mdx CnA* mice. *Significant difference in utrophin A expression between TA muscles of mdx and mdx CnA* mice (P < 0.05). Scale bar represents 100 μm and applies to lower panels.

Endogenous utrophin A expression was measured in TA muscles of mdx and mdx CnA* mice (Fig. 2B). We observed that utrophin A protein expression was four-fold greater in TA muscles of mdx CnA* than in muscles of mdx mice (Fig. 2B). Also, we noted a greater level of sarcolemmal utrophin staining (Fig. 2B) in muscle fibres from mdx CnA* mice.

Functional analysis of dystrophic skeletal muscles overexpressing activated calcineurin

When TA muscle function was assessed, a similar twitch force (Pt) was observed in muscles from mdx CnA* and mdx mice; however, Po was ∼20% greater in muscles from mdx mice. When Po was normalized for muscle cross-sectional area, sPo of muscles from mdx CnA* mice was still ∼10% less than that of mdx mice (Table 1). The decrease in Po and sPo observed in TA muscles from mdx CnA* mice compared with values in mdx mice, can be attributed to a greater proportion of Type IIa and type I fibres and a decrease in mean fibre cross-sectional area and muscle mass.

Characteristic of a slower muscle phenotype and the increase in type I and IIa fibres, we observed that the time-to-peak tension (TPT) and ½ relaxation time (½RT) of muscles from mdx mice expressing the CnA* transgene were prolonged compared to values in muscles from their control littermates (Table 1). Furthermore, at lower frequencies, 30 and 50 Hz, muscles from mdx CnA* mice produced greater force than those from mdx mice; but at higher frequencies, 150 and 200 Hz, their force-producing capacity was lower (Fig. 3A). TA muscles of mdx CnA* mice produced higher forces at the lower stimulation frequencies, as a result of the prolongation of ½RT (Fig. 3A).

Figure 3. Effect of calcineurin activation on TA muscle function in mdx mice.

Figure 3

Open in a new tab

A, frequency–force relationship. Muscles were stimulated at increasing frequencies with 2-min rest between contractions to avoid fatigue. *Significant difference in force output between TA muscles of mdx and mdx CnA* mice (P < 0.05). B, maximum isometric force (Po) before lengthening contractions (LCs), force output for LC1 and LC2, and maximum force after the LCs. The muscles were allowed to recover for 1, 5 and 15 min after the LCs, at which times Po was reassessed (see Methods for details). *Significant difference in force output between TA muscles of mdx and mdx CnA* mice (P < 0.05). C, relative force deficit following LC. *Significant difference in force deficit between TA muscles of mdx and mdx CnA* mice (P < 0.05; strain–stimulation frequency interaction).

Activation of calcineurin reduces contraction-mediated injury in dystrophic muscles

After the first LCs (LC1), Po was greater in muscles from mdx than mdx CnA* mice. However, after the second LCs (LC2), TA muscles from mdx CnA* mice produced more force (Fig. 3B). At all time points during recovery after LC1 and LC2, Po was greater in muscles from mdx CnA* than from mdx mice (Fig. 3B) and the relative force deficit was less (Fig. 3C). Given that the magnitude of the force deficit is related to the severity of muscle damage sustained, these findings show that muscles from mdx CnA* mice are more resistant to contraction-induced injury than muscles from mdx mice.

Another marker of muscle damage is the proportion of albumin-positive fibres located in muscle. Following the two LCs, we observed a greater proportion of albumin-positive fibres in TA muscles from mdx than from mdx CnA* mice (Fig. 4). In uninjured muscles from mdx mice, 4% of the myofibres stained positive for albumin, whereas this value was 7% in injured TA muscles. By comparison, 2% of fibres stained positive for albumin in uninjured muscles isolated from mdx CnA* mice, and only 3% were positive for albumin in injured muscles from mdx CnA* mice. These findings show that overexpression of activated calcineurin in muscles of mdx mice results in 50% fewer muscle fibres sustaining muscle membrane damage than in those muscles without activated calcineurin both in the basal state and after LCs.

Figure 4. Membrane lesions in uninjured and injured TA muscles of mdx and mdx CnA* mice.

Figure 4

Open in a new tab

A, representative images showing cytoplasmic albumin staining in injured and uninjured TA sections of mdx and mdx CnA* mice. Fibres with membrane lesions stained red; ⊕= positive. Note that the larger fibres were more susceptible to membrane damage and hence stained positive for albumin. Scale bar represents 100 μm and applies to all panels. B, the proportion of albumin-positive fibres in injured and uninjured TA muscles of mdx and mdx CnA* mice. +The proportion of albumin-positive fibres was greater in injured than uninjured muscles (P < 0.05; main effect); *proportion of albumin-positive fibres was greater in muscles of mdx than mdx CnA* mice (P < 0.05; main effect).

We observed differences in the cross-sectional area of damaged myofibres isolated from mdx and mdx CnA* mice. In muscles from mdx mice, the cross-sectional area of albumin-positive muscle fibres was greater than in muscle from the mdx CnA* mice (Table 2). Furthermore, the cross-sectional area of albumin-positive fibres from injured TA muscles was greater than the cross-sectional area of albumin-positive fibres from uninjured TA muscles and also greater than the mean fibre cross-sectional area (Table 2). In mdx mice, the cross-sectional area of albumin-positive fibres from injured muscles was 55% greater than the mean fibre cross-sectional area and in mdx CnA* mice it was 80% greater than the mean fibre cross-sectional area. In contrast, the cross-sectional area of albumin-positive muscle fibres from uninjured TA muscles of mdx mice was 20% greater than the mean fibre cross-sectional area and in mdx CnA* mice it was 50% greater than the mean fibre cross-sectional area. These findings indicate that the smaller fibre cross-sectional area may mediate a protective effect in dystrophic skeletal muscle.

Discussion

We have demonstrated that stimulation of the calcineurin signal transduction pathway can improve dystrophic muscle function by decreasing muscle susceptibility to contraction-induced injury. Expression of the CnA* transgene in muscles of mdx mice attenuated the force deficit of TA muscles following two LCs and reduced muscle membrane damage in both injured and uninjured TA muscles. This protective effect was probably mediated by higher utrophin expression, a decrease in fibre size and a shift towards a slower muscle phenotype. Whereas the effect of increased calcineurin phosphatase activity on muscle structure has been well characterized (Talmadge et al. 2004; Naya et al. 2000; Chakkalakal et al. 2004), its effect on function has not. Compared to muscles from mdx mice, TA muscles from mdx CnA* mice had a higher proportion of fibres expressing the slow(er) MyHC I and IIa isoforms, prolonged TPT and ½RT values, lower absolute and normalized maximum forces and a leftward shift of the frequency–force relationship.

Expression of the constitutively active calcineurin transgene has been shown to induce a slower muscle phenotype (Chin et al. 1998; Naya et al. 2000) and increase utrophin expression at the sarcolemma (Chakkalakal et al. 2004), although its effect is muscle specific (Talmadge et al. 2004). In both slow-twitch and fast-twitch muscles from MCK-CnA* mice, MyHC type I isoform expression was increased compared to muscle from wild-type mice. Expression of the type IIa MyHC isoform is highly stimulated (100-fold) by calcineurin activation (Allen et al. 2001; Allen & Leinwand, 2002), yet it is increased in fast-twitch, but not in slow-twitch muscles from MCK-CnA* mice (Talmadge et al. 2004). Others have observed that the increase in type I and IIa MyHC expression in muscles from mdx CnA* mice is associated with a decrease in type IIb MyHC expression (Chakkal et al. 2004).

Our observed differences in utrophin and MyHC I and IIa expression between mdx CnA* and mdx mice were two-fold higher than in previous reports (Chakkalakal et al. 2004). This may be attributed to the fact that the basal calcineurin activity levels in the transgenic mice used by Chakkalakal et al. (2004) were significantly lower (∼10-fold) than in the mice used in the present study, as reported by Wu et al. (2001) and Talmadge et al. (2004). In the strain of MCK-CnA* mice used in the present study, the transgene has been reported to represent 21–34% of the endogenous calcineurin pool, although its presence does suppress the expression of endogenous calcineurin (Ryder et al. 2004). Furthermore, its gene and protein expression is greater in fast-twitch compared to slow-twitch muscles (Talmadge et al. 2004).

The effect of CnA* transgene expression on muscle mass and fibre cross-sectional area in mdx mice has not been reported previously; however, in non-dystrophic (wild-type) fast twitch muscles, expression of the CnA* transgene decreased both mean fibre cross-sectional area and muscle mass (Talmadge et al. 2004). In mammalian skeletal muscle, a strong inverse correlation exists between oxidative capacity and fibre cross-sectional area (Nakatani et al. 1999; Talmadge et al. 2004). As the decrease in fibre cross-sectional area was four-fold greater than the decrease in muscle mass, we hypothesized that expression of the CnA* transgene may increase myofibre number in TA muscles of mdx mice. Our data support this contention because the number of fibres per mm2 muscle tissue was almost two-fold greater in TA muscles of mdx CnA* than in muscles of mdx mice. Although our method of evaluating myofibre number has its technical limitations, a similar approach has been used by others to assess changes in myofibre number in response to genetic manipulation of the calcineurin signal transduction pathway (Parsons et al. 2003). The mechanism for the increase in myofibre number in TA muscles of mdx CnA* mice is unknown. However, muscle fibre number is reduced in soleus muscles of CnA*knockout mice (Parsons et al. 2003), as well as in NFATc3 knockout mice (Kegley et al. 2001). It is possible that the constitutively active CnA*transgene increased NFATc3 activation in TA muscles of mdx CnA* mice and stimulated hyperplasia.

A large variability in fibre size is characteristic of dystrophic muscles (Bulfield et al. 1984; Wehling et al. 2001). In this study, expression of the CnA* transgene did not reduce fibre size variability. We assessed fibre size variability by calculating the CV for each muscle examined; the average CV being ∼70% for mdx CnA* mice and ∼75% for mdx mice. By comparing differences in standard deviation, others have reported a reduction in fibre size variability in muscles of from mdx CnA* mdx mice compared to those from mdx mice (Chakkalakal et al. 2004). However, relying on standard deviation to assess fibre size variability can be misleading. In the present study, the mean standard deviation was 707 μm2 for mdx CnA* mice and 1471 μm2 for mdx mice, yet the CV was similar for the two groups.

Expression of the CnA* transgene improved muscle structure. The proportion of centrally nucleated fibres was reduced by ∼10% in TA muscles from mdx CnA* mice compared to muscles from mdx mice, and this may reflect better myofibre viability (Deconinck et al. 1997; Wehling et al. 2001; Chakkalakal et al. 2004). We postulate that when muscle fibres are less susceptible to damage, there should be a reduced need for regeneration and the proportion of centrally nucleated fibres should decrease. A reduction in the proportion of fibres staining for albumin in uninjured TA muscles in mdx CnA* mice compared to muscles in mdx mice, supports the contention that muscle fibres from mdx CnA* mice are less susceptible to damage and have improved sarcolemmal integrity. Others have also reported reduced intramuscle fibre staining for Evans Blue, albumin and IgM, as well as lower plasma creatine kinase levels in mdx CnA* than in mdx mice (Chakkalakal et al. 2004).

TA muscles from mdx CnA* mice are more resistant to muscle damage induced by two LCs in situ. The force deficit following the injury protocol was ∼30% in muscles from mdxCnA* mice, but ∼80% in muscles from mdx mice. The expected force deficit of TA muscles from adult C57BL/10 (non-dystrophic) mice following the two-LCs protocol has been reported to be ∼15%–25% (Dellorusso et al. 2002). The injury protocol increased the number of albumin-positive fibres in TA muscles, but in injured muscles from mdx mice, twice as many fibres stained positive for albumin compared to injured muscles from mdxCnA* mice. Thus, the magnitude of the force deficit and albumin staining suggests that the susceptibility of TA muscles from mdxCnA* mice to LCs resembles that previously reported for TA muscles of non-dystrophic mice (Dellorusso et al. 2002).

Increased utrophin expression can improve the resistance of TA muscles to contraction-induced injury. Expression of truncated (Deconinck et al. 1997) or full-length utrophin transgenes (Tinsley et al. 1998) or adenoviral utrophin gene transfer (Satoru et al. 2000) has been shown to improve dystrophic muscle structure and function. Up-regulation of utrophin was associated with a decrease in central nucleation, indicative of reduced muscle necrosis and regeneration and improved resistance to contraction-induced damage (Deconinck et al. 1997; Tinsley et al. 1998; Satoru et al. 2000). The force deficit and muscle membrane damage after five LCs in vitro were similar in EDL muscles from mdx mice expressing the full-length utrophin transgene and in C57BL/10 mice, and less than in mdx mice (Tinsley et al. 1998). The magnitude of muscle membrane damage sustained was also similar in EDL muscles from mdx mice expressing full-length utrophin transgene and in C57BL/10 mice, as the proportion of Procion Orange-stained fibres was not different between the two strains. Furthermore, staining was ∼11-fold greater in EDL muscles from mdx mice than from mdx mice expressing full-length utrophin transgene and from C57BL/10 mice (Tinsley et al. 1998). Increased utrophin expression can improve sPo in muscles from mdx mice (Deconinck et al. 1997; Tinsley et al. 1998; Satoru et al. 2000). In contrast, our observation of a decrease in sPo and Po of TA muscles from mdx CnA* mice could be attributed to a reduction in fibre cross-sectional area and a slower muscle phenotype.

The effect of muscle fibre size on muscle susceptibility to contraction-induced injury is controversial. Small fibres may be more resistant to contraction-induced injury than large fibres, because of their greater surface area to volume ratio (Zammit & Partridge, 2005). Extra-ocular muscles have very small fibres and do not exhibit the typical dystrophic pathology (Karpati & Carpenter, 1986). Compared with control soleus muscles of mdx mice, fibres from γ-irradiated soleus muscles were smaller and the number of fibres that had not undergone necrosis or regeneration was significantly greater (Granata et al. 1998). However, others have suggested that larger muscle fibres may be more resistant to damage during normal activity, because the force required to perform activities of daily living would be a lower proportion of the maximum force-producing capacity (Zammit & Partridge, 2005). Inhibition of myostatin or overexpression of muscle-specific insulin-like growth factor-I (IGF-I) in muscles of mdx mice increased fibre size and force producing capacity. Both interventions decreased muscle membrane damage in mdx mice, but did not reduce susceptibility to contraction-induced injury (Granata et al. 1998; Barton et al. 2002; Bogdanovich et al. 2002) nor the incidence of centrally nucleated fibres (Bogdanovich et al. 2002). It is difficult to elucidate whether the benefits of myostatin blockade and IGF-I over-expression were due to an increase in fibre size or an improvement in muscle regeneration. However, even in muscles from mdx CnA* mice, where utrophin expression was up-regulated four-fold, it was the larger calibre fibres that were prone to membrane lesions after LCs or in the basal state. The mean cross-sectional area of these fibres was 50%–80% greater than average.

Increased utrophin expression and decreased fibre size may not be the only factors mediating the reduction in muscle damage in the basal state and following LCs in muscles of mdx CnA* compared with mdx mice. Other factors associated with a shift to a slower phenotype may also contribute to the enhanced resistance to muscle damage in mdx CnA* mice, including greater sarcomere homogeneity and expression of more compliant isoforms of structural proteins (e.g. titin) in slow-twitch compared to fast-twitch fibres (Macpherson et al. 1996). Whether such factors are influenced by calcineurin activation remains to be determined; however, rat slow-twitch fibres are less susceptible to contraction-induced injury than fast-twitch fibres at a given percentage of strain beyond optimum length, and slow fibres require a greater strain beyond optimum length for the same force deficit as fast fibres (Macpherson et al. 1996). In another study, we found that exogenous administration of IGF-I to mdx mice increased succinate dehydrogenase activity, shifted the overall MyHC isoform composition towards a slower phenotype, and of most importance, reduced contraction-induced damage in TA muscles (Schertzer et al. 2006).

We have shown that expression of the constitutively active CnA*transgene in muscles from mdx mice not only improves dystrophic muscle structure, as indicated by greater utrophin expression, reduced cytoplasmic albumin staining and fewer centrally nucleated fibres, but also improves function as evident from the lower force deficit after contraction-mediated damage. Although pharmacological interventions that stimulate the calcineurin signal transduction pathway in skeletal muscles may have therapeutic potential for DMD, it is important to note that some studies have implicated increased calcineurin activity in cardiac hypertrophy (Olson & Molkentin, 1999; Zhang, 2002) and aggravated cardiac pathology in mdx mice (Nakamura et al. 2002). Thus, any therapeutic application that stimulates the calcineurin signal transduction pathway in skeletal muscles would also need to be monitored for cardiac effects very closely.

Acknowledgments

This work was supported by Research Grants from the Muscular Dystrophy Association (USA) MDA 3595 and the National Health and Medical Research Council (Australia) 350439.

References

  1. Allen DL, Leinwand LA. Intracellular calcium and myosin isoform transitions. Calcineurin and CAM kinase pathways regulate preferential activation of the IIa myosin heavy chain promoter. J Biol Chem. 2002;277:45323–45330. doi: 10.1074/jbc.M208302200. [DOI] [PubMed] [Google Scholar]
  2. Allen DL, Sartorius CA, Sycuro LK, Leinwand LA. Different pathways regulate expression of the skeletal myosin heavy chain genes. J Biol Chem. 2001;276:43524–43533. doi: 10.1074/jbc.M108017200. [DOI] [PubMed] [Google Scholar]
  3. Barton ER, Morris LD, Musaro A, Rosenthal N, Sweeney LH. Muscle-specific expression of insulin-like growth factor I counters muscle decline in mdx mice. J Cell Biol. 2002;157:137–147. doi: 10.1083/jcb.200108071. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Bogdanovich S, Krag TOB, Barton-Davis ER, Morris LD, Whittlemore L-A, Ahima RS, Khurana TS. Functional improvement of dystrophic muscle by myostatin blockade. Nature. 2002;420:418–421. doi: 10.1038/nature01154. [DOI] [PubMed] [Google Scholar]
  5. Bulfield G, Siller WG, Wight PAL, Moore KJ. X chromosone linked muscular dystrophy (mdx) in the mouse. Proc Natl Acad Sci U S A. 1984;81:1189–1192. doi: 10.1073/pnas.81.4.1189. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Chakkalakal JV, Harrison M-A, Carbonette S, Chin ER, Michel RN, Jasmin BJ. Stimulation of calcineurin signaling attenuates the dystrophic pathology in mdx mice. Hum Mol Genet. 2004;13:379–388. doi: 10.1093/hmg/ddh037. [DOI] [PubMed] [Google Scholar]
  7. Chakkalakal JV, Stocksley MA, Harrison M-A, Angus LM, Deschenes-Furry J, St-Pierre SJG, Chin ER, Michel RN, Jasmin BJ. Expression of utrophin A mRNA correlates with the oxidative capacity of skeletal muscle fiber types and is regulated by calcineurin/NFAT signaling. Proc Natl Acad Sci U S A. 2003;100:7791–7796. doi: 10.1073/pnas.0932671100. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Chin ER, Olson EN, Richardson JA, Yang Q, Humphries C, Shelton JM, Wu H, Zhu W, Bassel-Duby R, Williams SR. A calcineurin-dependent transcriptional pathway controls skeletal muscle fiber type. Genes Dev. 1998;12:2499–2509. doi: 10.1101/gad.12.16.2499. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Deconinck N, Tinsley JM, De Backer F, Fisher R, Kahn D, Phelps S, Davies KE, Gillis JM. Expression of truncated utrophin leads to major functional improvements in dystrophin-deficient muscle mice. Nat Med. 1997;3:1216–1221. doi: 10.1038/nm1197-1216. [DOI] [PubMed] [Google Scholar]
  10. Dellorusso C, Crawford RW, Chamberlain JS, Brooks SV. Tibialis anterior muscles in mdx mice are highly susceptible to contraction-induced injury. J Muscle Res Cell Motil. 2002;22:467–475. doi: 10.1023/a:1014587918367. [DOI] [PubMed] [Google Scholar]
  11. Granata AL, Vecchi C, Graciotti L, Fulgenzi G, Maggi S, Corsi A. Gamma irradiation can reduce muscle damage in mdx dystrophic mice. Acta Neuropathol. 1998;96:564–568. doi: 10.1007/s004010050936. [DOI] [PubMed] [Google Scholar]
  12. Gregorevic P, Plant DR, Leeding KS, Bach LA, Lynch GS. Improved contractile function of the mdx dystrophic mouse diaphragm muscle following IGF-I administration. Am J Pathol. 2002;161:2263–2272. doi: 10.1016/S0002-9440(10)64502-6. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Griggs RC, Rennie MJ. Muscle wasting in muscular dystrophy: decreased protein synthesis or increased degradation. Ann Neurol. 1983;13:125–132. doi: 10.1002/ana.410130204. [DOI] [PubMed] [Google Scholar]
  14. Karpati G, Carpenter S. Small-caliber skeletal muscle fibers do not suffer deleterious consequences of dystrophic gene expression. Am J Med Genet. 1986;25:653–658. doi: 10.1002/ajmg.1320250407. [DOI] [PubMed] [Google Scholar]
  15. Kegley KM, Gephart J, Warren GL, Pavlath GK. Altered primary myogenesis in NFATc3−/– mice leads to decreased muscle size in the adult. Dev Biol. 2001;232:115–126. doi: 10.1006/dbio.2001.0179. [DOI] [PubMed] [Google Scholar]
  16. Kissel JT, Burrow KL, Rammohan KW, Mendell JR. Mononuclear cell analysis of muscle biopsies in prednisone-treated and untreated Duchenne muscular dystrophy. Neurology. 2001;41:667–672. doi: 10.1212/wnl.41.5.667. [DOI] [PubMed] [Google Scholar]
  17. Lynch GS, Cuffe SA, Plant DR, Gregorevic P. IGF-I treatment improves the functional and contractile properties of fast- and slow-twitch skeletal muscle from dystrophic mice. Neuromuscul Disord. 2001a;11:260–268. doi: 10.1016/s0960-8966(00)00192-9. [DOI] [PubMed] [Google Scholar]
  18. Lynch GS, Hinkle RT, Chamberlain JS, Brooks SV, Faulkner JA. Force and power output of fast and slow skeletal muscles from mdx mice 6–28 months old. J Physiol. 2001b;535:591–600. doi: 10.1111/j.1469-7793.2001.00591.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Lynch GS, Rafael JA, Chamberlain JS, Faulkner JA. Contraction-induced injury to single permeabilized muscle fibers from mdx, transgenic mdx, and control mice. Am J Physiol Cell Physiol. 2000;279:C1290–C1294. doi: 10.1152/ajpcell.2000.279.4.C1290. [DOI] [PubMed] [Google Scholar]
  20. Macpherson PC, Schork MA, Faulkner JA. Contraction-induced injury to single fiber segments from fast and slow muscles of rats by single stretches. Am J Physiol Cell Physiol. 1996;271:1438–1446. doi: 10.1152/ajpcell.1996.271.5.C1438. [DOI] [PubMed] [Google Scholar]
  21. McKeran RO, Halliday D, Purkiss P. Increased myofibrillar protein catabolism in Duchenne muscular dystrophy measured by 3-methylhistidine excretion in urine. J Neurol Neurosurg Psychiatr. 1977;40:979–981. doi: 10.1136/jnnp.40.10.979. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Matsumura K, Campbell KP. Dystrophin-glycoprotein complex: its role in the pathogenesis of muscular dystrophies. Muscle Nerve. 1994;17:2–15. doi: 10.1002/mus.880170103. [DOI] [PubMed] [Google Scholar]
  23. Nakamura A, Yoshida K, Takeda S, Dohi N, Ikeda S. Progression of dystrophic features and activation of mitogen-activated protein kinases and calcineurin by physical exercise, in heart of mdx mice. FEBS Lett. 2002;520:18–24. doi: 10.1016/s0014-5793(02)02739-4. [DOI] [PubMed] [Google Scholar]
  24. Nakatani T, Nakashima T, Kita T, Hirofuji C, Itoh K, Itoh M, Ishihara A. Succinate dehydrogenase activities of fibers in the rat extensor digitorum longus, soleus, and cardiac muscles. Arch Histol Cytol. 1999;62:393–399. doi: 10.1679/aohc.62.393. [DOI] [PubMed] [Google Scholar]
  25. Naya FJ, Mercer B, Shelton JM, Richardson JA, Williams SR, Olson EN. Stimulation of slow skeletal muscle fiber gene expression by calcineurin in vivo. J Biol Chem. 2000;275:4545–4548. doi: 10.1074/jbc.275.7.4545. [DOI] [PubMed] [Google Scholar]
  26. O'Keefe S, Tamura J, Kincaid RL, Tocci MJ, O'Neill EA. FK-506- and CsA-sensitive activation of the interleukin-2 promotor by calcineurin. Nature. 1992;357:692–694. doi: 10.1038/357692a0. [DOI] [PubMed] [Google Scholar]
  27. Olson EN, Molkentin JD. Prevention of cardiac hypertrophy by calcineurin inhibition: hope or hype? Circ Res. 1999;84:623–632. doi: 10.1161/01.res.84.6.623. [DOI] [PubMed] [Google Scholar]
  28. Parsons SA, Wilkins BJ, Bueno OF, Molkentin JD. Altered skeletal muscle phenotypes in calcineurin Aα and Aβ gene-targeted mice. Mol Cel Biol. 2003;23:4331–4343. doi: 10.1128/MCB.23.12.4331-4343.2003. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Pasternak C, Wong S, Elson EL. Mechanical function of dystrophin in muscle cells. J Cell Biol. 1995;128:355–361. doi: 10.1083/jcb.128.3.355. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Petrof BJ, Shrager JB, Stedman HH, Kelly AM, Sweeney HL. Dystrophin protects the sarcolemma from stresses developed during muscle contraction. Proc Natl Acad Sci U S A. 1993;90:3710–3714. doi: 10.1073/pnas.90.8.3710. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Ryder JW, Bassel-Duby R, Olson EN, Zinrath JR. Skeletal muscle reprogramming by activation of calcineurin improve insulin action on metabolic pathways. J Bioc Chem. 2004;278:44298–44304. doi: 10.1074/jbc.M304510200. [DOI] [PubMed] [Google Scholar]
  32. Satoru E, Guibinga G-H, Gilbert R, Nalbantoglu J, Petrof BJ. Differential effects of dystrophin and utrophin gene transfer in immunocompetent muscular dystrophy (mdx) mice. Physiol Genomics. 2000;3:133–144. [Google Scholar]
  33. Schertzer JD, Ryall JG, Lynch GS. Systemic administration of IGF-I enhances oxidative status and reduces contraction-induced injury in skeletal muscles of mdx dystrophic mice. Am J Physiol Endocrinol Metab. 2006 doi: 10.1152/ajpendo.00101.2006. in press. [DOI] [PubMed] [Google Scholar]
  34. Talmadge RJ, Otis JS, Rittler MR, Garcia ND, Spencer SR, Lees SJ, Naya FJ. Calcineurin activation influences muscle phenotype in a muscle-specific fashion. BMC Cell Biol. 2004;5:28. doi: 10.1186/1471-2121-5-28. [DOI] [PMC free article] [PubMed] [Google Scholar]
  35. Tinsley J, Deconinck N, Fischer R, Kahn D, Phelps S, Gillis J-M, Davies KE. Expression of full-length utrophin prevents muscular dystrophy in mdx mice. Nat Med. 1998;4:1441–1444. doi: 10.1038/4033. [DOI] [PubMed] [Google Scholar]
  36. Wehling M, Spencer MJ, Tidball JG. A nitric oxide synthase transgene ameliorates muscular dystrophy in mdx mice. J Cell Biol. 2001;155:123–131. doi: 10.1083/jcb.200105110. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Wu H, Rothermel BA, Kanatous S, Rosenberg P, Naya FJ, Shelton JM, Hutcheson KA, DiMaio MJ, Olson EN, Bassel-Duby R, Williams SR. Activation of MEF2 by muscle activity is mediated through a calcineurin-dependent pathway. EMBO J. 2001;20:6414–6423. doi: 10.1093/emboj/20.22.6414. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Zammit PS, Partridge TA. Sizing up muscular dystrophy. Nat Med. 2005;8:1355–1356. doi: 10.1038/nm1202-1355. [DOI] [PubMed] [Google Scholar]
  39. Zhang W. Old and new tools to dissect calcineurin's role in pressure-overload cardiac hypertrophy. Cardiovasc Res. 2002;53:294–303. doi: 10.1016/s0008-6363(01)00451-5. [DOI] [PubMed] [Google Scholar]

Articles from The Journal of Physiology are provided here courtesy of The Physiological Society

RESOURCES