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Department of Biology, California State University, Bakersfield, California 93311
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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This study was undertaken to measure the response of stretch-activated ion channels (SAC) and transcript levels of the oncogene c-fos to separate bouts of eccentric contractions (EC). It was hypothesized that SAC in rat skeletal muscle would contribute to resting membrane potential depolarization after separate repeated bouts of EC. Blockage of SAC during an EC training regime also tested the necessity of SAC for a training response. It was also hypothesized that transcript levels of c-fos would be maximally elevated after the first exposure to EC and diminish with repeated exposures. The results indicate less depolarization after multiple bouts of EC, which could be reversed by blocking the SAC. Transcript levels of c-fos were elevated to a similar degree after either a single or multiple exposures to EC. EC training resulted in significant increases in contractile force and muscle wet and dry weights in nontreated animals. Training in the presence of the SAC-blocker streptomycin produced similar changes in contractile force without changes in muscle weight. SAC and c-fos are activated after several exposures to EC and therefore remain as possible signals in EC training responses.
skeletal muscle; lengthening contractions; oncogenes; repeated bouts
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE CONSEQUENCES OF A NOVEL EXPOSURE to eccentric contractions (EC), such as muscle cell damage and loss of contractile force, are well documented. Several mechanisms for the damage and loss of force have been reviewed previously (1, 32). Experiments have also demonstrated that, after repeated exposures, muscle adaptation to EC results in resistance to damage (12, 29, 48). Training with repeated bouts of EC is an effective means of overloading a muscle to produce hypertrophy and gains in muscle strength in the absence of continued damage (3, 27, 28, 47). It is not known whether the same cellular mechanisms are responsible for adaptation and hypertrophy, although overlap is likely.
Recent experiments utilizing established methods of muscle stretching, including EC, have identified potential signals leading to hypertrophy. Phosphorylation of mitogen-activated protein kinases (MAPK) has been directly linked to muscle passive stretch and EC. MAPK may represent a pathway for regulating protein synthesis necessary for both adaptation and hypertrophy (5, 8, 25, 34). EC also results in a prolonged phosphorylation of the 70-kDa ribosomal protein S6 kinase, which may play an important role in the early events of EC-induced hypertrophy (3, 34). The hypertrophy would occur through posttranscriptional mechanisms that stimulate protein synthesis (3). Activated 70-kDa ribosomal protein S6 kinase is an important regulator of cell size and can provide a rapid signal for an increase in protein synthesis before changes in cellular RNA and DNA content (3, 31). For a more complete description of translational control and second messengers related to skeletal muscle hypertrophy, excellent reviews are available (35, 43, 44).
Stretch-activated ion channels (SAC) (reviewed in Refs. 33, 39) may provide a mechano-chemical link necessary for the activation of intracellular signaling pathways involved in the adaptive and hypertrophic responses to EC. McBride et al. (28) previously reported a SAC-related depolarization of the resting membrane potential (RMP) by an increase in Na+ conductance after EC. The opening of SAC has also been shown to increase K+ conductance as have, to a lesser extent, divalent cations such as Ca2+ and Mg2+ (7, 20, 23). A conformational change in the SAC protein or the transient alteration in ionic influx could activate one or more signaling pathways identified after muscle stretch (43, 45). SAC would thus provide an early signal in the mechano-transduction pathway occurring upstream from MAPK or other second messenger pathways (25).
Upregulation of the activator protein-1 transcription factor c-fos is thought to be an early signal in the hypertrophy process and has been correlated with several models of cardiac and skeletal muscle hypertrophy (9, 10, 17, 22-24, 36, 40, 46). Most of these models demonstrate a very rapid and transient increase in c-fos transcript levels after a single stimulus. Elevated c-fos transcript levels may be more indicative of an acute injury response to an unaccustomed stretch rather than one for hypertrophy. It remains to be seen whether c-fos is elevated after multiple stimuli over an extended period of time. If elevated after repeated exposures, c-fos would represent a downstream signal from the MAPK cascade or other second messenger pathways measured in conjunction with muscle stretch (6, 11, 34, 40, 41). Although there is a strong correlation between hypertrophy-stimulating protocols and c-fos expression, no direct link has been established. Dawes et al. (9) hypothesized that c-fos may function to control transcription of other growth-related genes, such as insulin-like growth factor. Control would occur by dimerization of c-fos with another early response gene c-jun to form an activator protein-1 transcriptional factor (9, 38).
The present study measures transcript levels of c-fos and the contribution of SAC to membrane depolarization after multiple bouts of EC. It was hypothesized that transcript levels of c-fos would be maximally expressed after an acute bout, with diminished levels after repeated bouts. Experiments were also conducted to determine whether c-fos transcript levels depend on functional SAC during EC. The contribution of SAC to membrane depolarization after repeated bouts of EC was also investigated. It was hypothesized that SAC would continue to be activated after repeated bouts, resulting in membrane depolarization each time. The gain in muscle mass and strength associated with EC due to SAC was tested by a pharmacological blockade of SAC during an EC training regime.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals
Female Sprague-Dawley rats, 3 mo of age, with body weights ranging from 240 to 260 g, were used. All animal care and use protocols were approved by the Institutional Animal Care and Use Committee of California State University, Bakersfield, and were consistent with National Institutes of Health guidelines. Animals were housed in a temperature-controlled room (19-21°C) with a 12:12-h light-dark cycle. Rats were provided unlimited access to standard rat chow and water.Experimental Groups
Single bout of EC.
Animals in this group were subjected to a single acute bout of EC with
no additional treatment (n = 4) or with streptomycin treatment in their drinking water (n = 4). Tibialis
anterior (TA) muscles were harvested for RNA analysis from the
anesthetized rats 2 h postexercise. The muscles were weighed and
immediately freeze clamped in metal tongs cooled in liquid
N2. Frozen muscles were stored at 
80 C until processed
for RNA extraction.
Four bouts of EC. Animals in this group were subjected to four bouts of EC with 7 days of recovery between each bout. TA muscles were removed from the anesthetized rats after the final bout of EC for either RMP recordings or RNA isolation. TA muscles used for RMP recordings (n = 6) were removed immediately after the fourth exposure to EC and tested. TA muscles used for RNA extraction (n = 4) were harvested 2 h after the fourth exposure to EC and frozen in the same manner as the single EC group. A separate population of animals in this group underwent the same procedure while receiving streptomycin in their drinking water. Streptomycin treatment was discontinued immediately after the third exposure to EC. This protocol was designed to block SAC during the first three bouts of EC and then allow activation during the fourth exposure. Muscles from the streptomycin-treated animals were then utilized for RMP recordings (n = 6) or RNA extraction (n = 4) in an identical manner to the non-streptomycin-treated muscles.
EC training. Animals in this group were trained with eight exposures to EC over a 4-wk period, training on Tuesdays and Fridays each week. Each training bout was equivalent to the EC protocol described in Muscle Stimulation Protocol. Muscle contractile function was tested 3 days after the final exposure to EC before tissue removal. Animals trained either in the presence of streptomycin in their drinking water (n = 7) or as untreated exercise controls (n = 9).
Muscle Stimulation Protocol
Animals were anesthetized with 60 mg/kg ketamine and 12 mg/kg Rompum and performed EC on a pulley device similar to the one described by Wong and Booth (47). The rat was placed in the prone position on the supporting platform of a pulley apparatus designed to stabilize the leg and allow full ankle rotation. The hind foot was attached directly to a plate connected to the lever arm of the pulley system, and the ankle was stabilized with the foot at 90° with respect to the lower leg (neutral position). Two monopolar stainless steel needle electrodes were inserted percutaneously near the sciatic notch to stimulate the sciatic nerve. Supramaximal stimulation of the sciatic nerve above the branch point of the tibial and peroneal nerves caused the plantarflexors (triceps surae) to contract concentrically, which stretched the dorsiflexors because they were also maximally activated. The dorsiflexors thus contracted eccentrically and lengthened, in opposition to the stronger ankle extensors. Supramaximal stimulation consisted of 100-Hz stimulus trains with a train duration of 2.5 s. The exercise paradigm consisted of four sets of six repetitions with a 20-s rest between repetitions and a 5-min rest between sets. During each procedure, only the right leg was stimulated to produce EC of the TA muscle. The left leg served as a nonexercised contralateral control.Streptomycin Treatment to Block SAC
Animals were treated with streptomycin in their drinking water (4 g/l) to provide continuous in vivo blockade of SAC. Treatment was initiated 6 days before the first exposure to EC and then continued until the evaluation was completed, unless otherwise indicated. This protocol has been shown to be effective at blocking SAC by reducing RMP depolarization after a single exposure to EC (26, 28, 42).Electrophysiology
RMP from control and exercised TA were measured in vitro immediately after the final exposure to EC. The isolated TA was pinned at rest length to the silastic covered bottom of a Lucite chamber filled with 75 ml of a HEPES-buffered physiological saline [(in mM) 150 NaCl, 5 KCl, 4 CaCl2, 1 MgCl2, 11 glucose, and 1.24 HEPES]. The saline solution was maintained at room temperature (21°C) and bubbled continuously with 100% O2. Fresh solutions were added after draining the chamber through a vacuum line inserted at the base. Recordings were obtained by using standard glass microelectrode techniques (26, 28). Electrodes were filled with 3 M KCl with tip resistances of ~20-30 M
. A platinum reference electrode was placed in the
proximal end of the TA. Recordings were obtained from muscle cells to a
depth of no more than five cells from the surface to avoid recording
from possible hypoxic areas of the muscle. The proximal end of the TA
was avoided to prevent recording from fibers that might be damaged
during removal of the muscle from the animal. The SAC blocker
Gd3+ was added at a concentration of 10 µM for in vitro
blockade of SAC (14).
RNA Extraction
Portions (50-100 mg) of the freeze-clamped muscles were removed and weighed. The samples were homogenized in TRIZOL reagent (1 ml/100 mg tissue; GIBCO Life Technologies) and incubated at room temperature for 5 min. Chloroform (0.4 ml/ml) was added, and the samples were centrifuged. The clear supernatant was removed and placed in a separate tube. Samples were then ethanol precipitated. Precipitated samples were resuspended with 0.02 ml formamide. RNA was quantified by measuring the A260/A280 and stored at80°C until needed.
Northern Gel and Transfer
RNA samples (15 µg) were brought up to a 12-µl volume with formamide and combined with 12 µl of 2× RNA gel loading buffer that was prepared fresh. Samples were run on a 0.7% agarose, formaldehyde gel until the bromophenol blue indicator was two-thirds of the way down the gel. Gels were presoaked in 20× saline-sodium citrate for 15 min before Northern transfer. Northern transfer took place overnight in 10× saline-sodium citrate buffer onto nitrocellulose membrane.Labeling and Hybridization
DNA probes for c-fos and 18S were cut from two separate plasmid preparations. The probes were isolated on a low melting agarose gel and purified with a Magic Prep kit (Promega). Probes were labeled for blot hybridization per manufacturer instructions with RadPrime DNA labeling system (GIBCO Life Technologies). Hybridization and washes were performed following manufacturer instructions with PerfectHyb Plus (Sigma Chemical). Blots were incubated for 12 h at 68°C. Hybridization was followed by two low-stringency washes at 25°C, two high-stringency washes at 68°C, and one ultra-high wash at 68°C.Quantification
Transcript content of 18S and c-fos were quantified by exposure to film for 24 and 72 h, respectively. Image analyses was performed on an Epson 1600 with a transparency adapter and quantified by computer with Scion Image Beta 4.0.2 software (Scion). Identical areas were measured on each band to determine the average pixel density. Background radioactivity was subtracted from each area to determine final value. To control for any deviations in loading total RNA samples, c-fos values were normalized to 18S. Individual bands for each time point and treatment protocol were normalized to contralateral, nonexercised controls. It was necessary to normalize to a mean of the control values for each group since control c-fos values were extremely low.Muscle Contractile Function
TA contractile function in the trained animals was measured in situ 3 days after the final bout of EC. The animals were anesthetized with 60 mg/kg ketamine and 12 mg/kg Rompum, and the TA was exposed. The rat was placed on a warming pad to maintain body temperature, and the animal and pad were placed on a metal frame. The distal tendon of the TA was isolated and attached to a force transducer (Grass-FT-03) with silk suture (2-0). The TA was stimulated directly by a platinum plate electrode at supramaximal voltage with ~0.05-ms duration at optimal length. Maximum isometric twitch tension (Pt), time to peak twitch tension, twitch half-relaxation time, rate of twitch relaxation, maximum isometric tetanic tension (Po), and the maximum rate of force development during a tetanus at 330 Hz were recorded at 35 ± 0.5°C. Output voltages from the force transducer were amplified and recorded on an analog-to-digital acquisition system (Powerlab, AD Instruments). Muscle temperature was monitored and maintained at 35 ± 0.5°C by radiant heat.Noncollagenous Protein
Noncollagenous protein (NCP) content was measured by using a detergent compatible protein assay kit (Bio-Rad). Dried muscle samples were powdered in a mortar at room temperature, and 5-6 mg of powder from each sample was removed and weighed. The powdered samples were then resolubulized in 1 ml of 0.05 M NaOH. Samples were stirred several times and allowed to incubate overnight at room temperature. The next day, samples were restirred and then centrifuged to separate the nonsolubulized pellet. A 50-µl sample of the supernatant was removed and diluted 1:4 in distilled H2O. All samples were prepared in duplicate. The samples and protein standards were prepared for colormetric assay in 96-well microplates as per manufacture's instructions. Absorbance was determined in duplicate at 750 nm with a Bio-Tek Instruments micoplate reader. The percentage of NCP was then calculated on the basis of the original wet weight of the muscle determined immediately after removal from the animals.Statistics
Results are expressed as means ± SE of the indicated number of measurements for each group. Each treatment group is compared with its contralateral control by a t-test comparison, with comparisons between treatment groups performed by ANOVA. A Fisher's t-test was used to further analyze any differences indicated by the ANOVA. Differences were accepted as statistically significant at P < 0.05, but actual P values are as indicated (see Tables 1-2 and Figs. 1-2). Analyses were performed by using the Statview software program (Abacus Concepts, Berkeley, CA).
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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c-fos Transcript Levels
Figure 1 shows the increase in c-fos transcripts relative to the pooled control values 2 h after the final bout of EC. The expression of c-fos increased 8.7 ± 3.3-fold after a single bout of EC. With streptomycin pretreatment, there was a 6.4 ± 3.0-fold increase in c-fos expression after a single bout of EC. The expression of c-fos increased 8.3 ± 3.0-fold after a fourth bout of EC. When treated with streptomycin during the first three exposures to EC, the expression of c-fos was limited to a 2.1 ± 0.5-fold increase after the fourth bout of EC.RMP
Figure 2 compares the RMP values immediately after a fourth bout of EC. Nontreated TA muscles were significantly depolarized (76.9 ± 0.8 mV) compared with control
muscles (82.3 ± 1.2 mV). The addition of Gd3+ to
the bathing solution resulted in a significant repolarization to a
value that was not different from control (81.3 ± 1.0 mV). TA
muscles treated with streptomycin in vivo during the first three bouts
of EC were more depolarized after the fourth exposure to EC
(74.2 ± 1.8 mV) compared with nontreated exercised muscles. The
addition of Gd3+ to the bathing solution of the
streptomycin-treated muscles resulted in a significant repolarization
(78.2 ± 1.2 mV), but the mean of the RMP values remained
significantly depolarized compared with control.
Body and Muscle Weights and NCP Content
Values for body weights, muscle weights, and NCP are presented in Table 1. Four weeks of EC training resulted in a significant increase in the muscle wet and dry weights of the non-streptomycin-treated group compared with their contralateral controls. Training did not result in a similar increase in the wet and dry weights of the streptomycin-treated group. Note that the wet and dry weights of the nonexercised control muscles from the streptomycin-treated group were significantly greater than the nonexercised control muscles from the non-streptomycin-treated group. The final body weights between the two groups were not different. Training did not result in a significant increase in total muscle NCP in either the nontreated or streptomycin-treated groups when compared with their contralateral controls. Total NCP was significantly greater in streptomycin-treated control muscles compared with the nontreated control muscles and in streptomycin-treated trained muscles compared with nontreated trained muscles.Contractile
The values for muscle contractile force after 4 wk of eccentric training are presented in Table 2. Training resulted in a significant increase in both the Pt and Po of TA muscles in the non-streptomycin-treated group. Training also resulted in a significant increase in the Po of the streptomycin-treated group. The Pt values were increased in the streptomycin-treated group after training, but they did not reach statistical significance (P < 0.05). There was no significant increase in Pt or Po when the force was normalized to muscle dry weight in either the nontreated or streptomycin-treated groups compared with their contralateral, nonexercised controls. No significant differences in the time to peak twitch tension, twitch half-relaxation time, rate of twitch relaxation, and maximum rate of force development during a tetanus at 330 Hz were measured in either group after training.| |
DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A single novel exposure to EC resulted in a rapid, eightfold increase in c-fos transcript levels as shown in Fig. 1. Osbaldeston et al. (36) reported a maximum 20-fold increase in c-fos 1 h after static stretch of skeletal muscle in vivo. When they measured c-fos at the same time point as in this study, the levels were similar between static stretch and EC (36). The different models of stretch used for stimulating hypertrophy most likely result in some degree of membrane damage. It is possible that c-fos provides a signal necessary for a damage response rather than a signal for hypertrophy as generally accepted. Grembowicz et al. (13) found a correlation between the level of injury to cell membranes and the expression of c-fos in a variety of cell types, including muscle cells. Dawes et al. (10) also found that as the degree of stretch increased so did the level of c-fos expression. Maximal expression was measured at 20% stretch, which may result in muscle cell damage (10, 21). After repeated exposures to the various loading protocols, one would expect the muscle cells to adapt and become resistant to damage. For c-fos to contribute in muscle hypertrophy, its expression would need to reoccur after the adaptive process. In this study, muscle exposure to multiple bouts of EC, which has been shown previously to result in adaptation (12, 29, 48), minimized damage. Figure 1 shows an eightfold increase in c-fos transcript levels after the fourth bout of EC. This is similar to the value measured after the initial bout of EC. The RMP data recorded at the same time point indicate that membrane damage is much less of a factor (Fig. 2). Contrary to the original hypothesis, transcript levels of c-fos were maximally expressed after repeated bouts of EC in the absence of the acute injury response. Therefore, c-fos cannot be ruled out as a signal in the long-term hypertrophy response to EC.
RMP were measured after a fourth bout of EC to establish a correlation
between SAC activation and c-fos expression after multiple bouts of EC. Figure 2 demonstrates an adaptive response in which RMP
were significantly less depolarized after a fourth bout of EC compared
with previous measurements after an acute bout (72.7 ± 0.5 mV)
(28). Addition of the SAC blocker Gd3+ in
vitro after the fourth bout of EC restored RMP to control values.
Repeated bouts of EC, therefore, continue to open SAC, but damage to
the cell membrane no longer occurs. Transcript levels of
c-fos continue to demonstrate a maximal increase after the fourth bout of EC. The transcript increase occurs in conjunction with
SAC activation and depolarization in the absence of damage-induced depolarization. Although no direct link for the correlation of SAC
activation and c-fos expression is shown, it is interesting that concentric contractions, which do not open SAC (28),
also do not stimulate c-fos expression (data not shown). The
absence of a c-fos response after concentric contractions is
consistent with the work of Nader and Esser (34), who
showed that the growth-related second messenger pathways were
specifically stimulated by exercise protocols that result in
hypertrophy. A direct link has been established between membrane
stretch of cardiomyocytes in response to a hypotonic medium and
c-fos expression via increased tyrosine kinase activity (41). Although SAC activity was not measured in this
study, hypotonic stress has been shown to open SAC (33).
SAC could be acting as the mechano-chemical coupler in a hypotonic
membrane stretch leading to tyrosine kinase activation, followed by
c-fos expression.
A further attempt was made to associate SAC activation with c-fos expression by streptomycin treatment to block SAC. Peake et al. (37) were able to block the c-fos response to loading with in vitro treatment of bone cells with the SAC-channel blocker Gd3+. Their experiments demonstrate that load-induced c-fos expression required a Ca2+- and SAC-dependent pathway (37). The results in Fig. 1 show a drop in c-fos transcript levels when SAC were blocked in vivo before EC. Although reduced by SAC blockade, c-fos still demonstrated a large increase (Fig. 1). These data support the hypothesis that multiple pathways converge to have a synergistic effect on c-fos expression after stretch (9). When SAC was blocked during the first three bouts of EC, it resulted in a greater depolarization after the fourth bout that could not be accounted for by SAC (Fig. 2). It is assumed that adaptation was prevented and that membrane damage is responsible for the additional depolarization. At the same time point, however, c-fos expression is lower in these muscles. This would indicate the lack of a correlation between c-fos expression and membrane damage as originally hypothesized (Fig. 1). Thus, if changes in ion conductance are important in activating second messengers related to c-fos expression, it most likely occurs through SAC.
This study also sought to determine what effect SAC blockade would have on muscle strength and hypertrophy gains during EC training. EC training in the non-streptomycin-treated group resulted in a significant increase in TA muscle mass and contractile force, as shown in Tables 1 and 2, respectively. Training with streptomycin treatment to block SAC did not result in a significant increase in TA mass (Table 1); however, there was a significant increase in muscle contractile strength (Po) (Table 2). Although the streptomycin-treated group did not demonstrate an exercise-induced hypertrophy, an interesting and unexpected trend did occur. Both the EC-trained and control muscles from the streptomycin-treated group had significantly greater wet and dry weights and NCP compared with their nontreated counterparts (Table 1). The changes in muscle weight and NCP occur despite no difference in body weight between the two groups (Table 1). Multiple pathways are most likely responsible for hypertrophy, and streptomycin treatment may affect protein synthesis independent of EC training. The binding of streptomycin to SAC or a different regulatory protein may activate one of the intracellular cascades involved in translational control of skeletal muscle hypertrophy reviewed previously (35). Streptomycin treatment potentially stimulating muscle protein accumulation was not anticipated, and it makes it hard to determine the role of SAC in EC training-induced hypertrophy. SAC blockade prevented some of the adaptation to EC, as demonstrated by the RMP data, but its effect on EC training is impossible to separate from a possible effect on control muscle. Other investigators have demonstrated the effects of streptomycin on protein synthesis in muscle independent of additional stimuli (4, 16).
Several conclusions can be derived from the data in this project. SAC clearly play a role in the continued depolarization of muscle cells after repeated bouts of EC. SAC also appears necessary for muscle cell membranes to become resistant to damage after lengthening contractions. With SAC blocked, the adaptive response as defined by the level of muscle cell depolarization not accounted for by SAC was inhibited. The relationship between SAC and expression of the proto-oncogene c-fos was less conclusive. It is not clear whether transcript levels of c-fos are sensitive to the activity of functional SAC, although a trend in that direction is evident. It was clear that c-fos continues to be elevated with repeated bouts of EC, and this phenomenon is not limited to an acute injury response as originally hypothesized. Both SAC and c-fos are activated after several exposures to EC. Therefore, they remain as possible contributors to muscle adaptation and hypertrophy after EC training.
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ACKNOWLEDGEMENTS |
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The author thanks Dr. Richard C. Carlsen for critical review of the manuscript and Melinda A. McBride for editorial assistance.
This project was supported by a Beginning Grant-in-Aid from the American Heart Association, Western States Affiliate.
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FOOTNOTES |
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Address for reprint requests and other correspondence: T. A. McBride, California State Univ. at Bakersfield, Bakersfield, CA 93311 (E-mail: tmcbride{at}csub.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.
First published February 28, 2003;10.1152/japplphysiol.00876.2002
Received 23 September 2002; accepted in final form 24 February 2003.
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TOP
ABSTRACT
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METHODS
RESULTS
DISCUSSION
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