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1 Department of Oral Biology, 2 Department of Exercise Science, and 3 Department of Medical Biochemistry, The Ohio State University, Columbus, Ohio 43210-1218
Reiser, Peter J., William O. Kline, and Pal L. Vaghy.
Induction of neuronal type nitric oxide synthase in skeletal muscle by chronic electrical stimulation in vivo. J. Appl. Physiol. 82(4): 1250-1255, 1997.
Fast-twitch skeletal muscles contain more neuronal-type nitric
oxide synthase (nNOS) than slow-twitch muscles because nNOS is present
only in fast (type II) muscle fibers. Chronic in vivo electrical
stimulation of tibialis anterior and extensor digitorum longus muscles
of rabbits was used as a method of inducing fast-to-slow fiber type
transformation. We have studied whether an increase in muscle
contractile activity induced by electrical stimulation alters nNOS
expression, and if so, whether the nNOS expression decreases to the
levels present in slow muscles. Changes in the expression of myosin
heavy chain isoforms and maximum velocity of shortening of skinned
fibers indicated characteristic fast-to-slow fiber type transformation
after 3 wk of stimulation. At the same time, activity of NOS doubled in
the stimulated muscles, and this correlated with an increase in the
expression of nNOS shown by immunoblot analysis. These data suggest
that nNOS expression in skeletal muscle is regulated by muscle activity
and that this regulation does not necessarily follow the fast-twitch
and slow-twitch pattern during the dynamic phase of phenotype
transformation.
nitric oxide; enzyme induction; striated muscles; membranes
NEURONAL-TYPE NITRIC OXIDE SYNTHASE (nNOS) is
present in all striated muscles of mammals (11). Some skeletal
muscles express more nNOS than any other tissue, including brain (19).
Because skeletal muscle represents ~40% of the total body mass (12), this tissue may be one of the richest sources of nNOS and nitric oxide
(NO) in mammals. The nNOS is associated with the sarcolemma of fast
(type II) skeletal muscle fibers (15). Consistent with this finding is
that expression and activity of nNOS are high in muscles rich in type
II fibers (fast muscles) and low in muscles rich in type I fibers (slow
muscles) (6, 15). In addition to nNOS, skeletal muscle also contains
endothelial nitric oxide synthase (eNOS), albeit at much lower levels
(16). The nNOS is associated with the dystrophin complex by binding to
Although originally thought of as a constitutive enzyme, recent data
indicate that expression and activity of nNOS are altered under
different developmental and hormonal conditions (20, 30). Whether nNOS
expression in type II skeletal muscle fibers can be changed by altering
the muscle activity has not been determined. Chronic electrical
stimulation of fast skeletal muscles is a strong enough stimulus to
produce fast-to-slow fiber type transformation. We tested whether such
electrical stimulation of fast skeletal muscles is able to alter the
expression and activity of nNOS. The data suggest that chronic
electrical stimulation of tibialis anterior (TA) and extensor digitorum
longus (EDL) muscles increases the activity of nitric oxide synthase
(NOS) as well as the level of nNOS expression. A brief report of this
study was presented at the Second International Conference on
Biochemistry and Molecular Biology of Nitric Oxide, Los Angeles,
California, July 13-17, 1996.
1-syntrophin in fast-twitch muscle fibers (5, 6). In
mdx mice, an animal model of muscular
dystrophy, and in humans with Duchenne muscular dystrophy, where the
expression of dystrophin is altered or missing, the nNOS dissociates
from the sarcolemma (5, 6). It has been suggested that such
displacement from the sarcolemma may result in an aberrant regulation
of nNOS and muscle degeneration in Duchenne muscular dystrophy (6).
Examination of nNOS under pathological conditions may lead to a better
understanding of the role of this enzyme in skeletal muscle.
Stimulation protocol.
Three adult female New Zealand White rabbits, weighing 2.5-3.5 kg,
were used in this study. The rabbits were anesthetized with 60 mg/kg
ketamine and 5 mg/kg xylazine before surgical implantation of
electrodes. Continuous stimulation at 10 Hz with 0.1-ms biphasic pulses
of the right peroneal nerve, which innervates the fast-twitch EDL and
TA muscles, was started 3 days later and lasted 3 wk (2). The
contralateral muscles were used as controls. The animals were killed at
the end of the stimulation period with pentobarbital sodium (150 mg/kg
iv), and the muscles were removed and processed as described below. The
care and use of animals in this study were approved by the
institutional laboratory animal care and use committee.
22°C (23) to render the
sarcolemma hyperpermeable. The portions were dissected from the same
region of the control and stimulated muscles. Single skinned fibers
were dissected from the bundles, and
Vmax was measured by utilizing the slack-test method of Edman (9). Only those fibers that
did not have morphological irregularities, when viewed under a
microscope at ×40 magnification, were selected for measurements.
Membrane isolation.
A sample of each muscle was weighed, minced, and diluted 10 times with
isolation buffer [(in mM) 50 Tris · HCl, pH 7.4 at 4°C, 1 EDTA, 0.5 phenylmethylsulfonyl fluoride, and 1 iodoacetamide, as well as 1 µM leupeptin and 1 µM pepstatin
A]. The minced tissue was homogenized with a polytron twice for
15 s at a setting of eight by using a PTA 10S generator. In between
each 15-s homogenization, the tissue was kept at melting-ice
temperature for 60 s. The homogenate was filtered through one layer of
cheesecloth into an ultracentrifuge tube and centrifuged at 105,000 g for 1 h. The pellet was resuspended in isolation buffer.
NOS activity measurement.
The citrulline assay (13), which directly measures one of the NOS
products (citrulline) and can be performed with crude membrane
preparations without the interference of other cellular components, was
used. The assay was performed in 1.5-ml polypropylene bullet tubes
incubated in a 37°C water bath. The final reaction mixture (120 µl) contained 55 mM
N-2-hydroxyethylpiperazine-N
-2-ethanesulfonic acid (HEPES)-NaOH, 20 mM Tris · HCl, pH 7.4, various
concentrations of
L-[14C(U)]arginine
monohydrochloride (Amersham, Arlington Heights, IL), 25 µl skeletal
muscle membrane, 0.8 mM 1,4-dithiothreitol, 0.4 mM EDTA, cofactors
[0.8 µM NADPH, 1 mM CaCl2,
0.5 µM bovine brain calmodulin, 6 µM sapropterin
(BH4), 0.8 µM flavin-adenine dinucleotide,
0.8 µM riboflavin monophosphate], and protease inhibitors (0.2 mM phenylmethylsulfonyl fluoride, 0.4 mM iodoacetamide, 0.4 µM
leupeptin, 0.4 µM pepstatin A). The reaction was initiated by the
addition of protein. The samples were incubated at 37°C for 20 min,
unless otherwise indicated. The reaction was stopped by the addition of
1 ml of ice-cold stop solution (20 mM HEPES, pH 6.8, containing 2 mM
EDTA). The citrulline produced by NOS was separated by cation-exchange
chromatography by using 2-ml AG 50W-X8 columns (Bio-Rad) converted to
the Na+ form and preequilibrated
with the stop solution. The radioactivity was counted in a Beckman
LS7000 liquid scintillation counter at 95% efficiency. The exact
concentration of
L-[14C(U)]arginine
used in the assays was determined by measuring radioactivity in 20-ml
aliquots taken from the reaction mixture not used in the assays.
Control experiments included measurement of the NOS activity in the
presence of stop solution and/or stereoselective NOS inhibitors
such as N
-monomethyl-L-arginine
(L-NMMA). The same control
(blank) values were obtained if the membranes were incubated with stop
solution, if the membranes were incubated with 500 µM
L-NMMA, or if membranes were
omitted from the samples. The protein concentration was determined by
using the bicinchoninic acid protein assay reagent (Pierce, Rockford,
IL).
Statistical analysis.
Differences in MHC isoform composition,
Vmax, and muscle
mass were evaluated statistically with the one-tailed
Mann-Whitney-Wilcoxon test after an analysis of variance. The level of
significance was set at P < 0.05.
The mean mass of electrically stimulated TA and EDL muscles decreased by 40 ± 3 and 30 ± 7%, respectively, after 3 wk of electrical stimulation in vivo (Table 1). To determine whether fast-to-slow fiber type transformation took place, the expression of MHC isoforms and Vmax of isolated skinned fibers were analyzed. MHC isoforms were identified on the basis of their electrophoretic mobilities and relative abundance in control soleus, diaphragm, and adductor magnus muscles (1). Four MHC isoforms, IIA, IID, IIB, and I, were identified in order of their electrophoretic mobilities (Fig. 1A). The soleus expressed primarily MHC type I isoform. The diaphragm contained type IIA, type IID, and type I MHC isoforms. The adductor magnus primarily expressed the type IIB isoform. The four different MHC isoforms were best shown by coelectrophoresis of diaphragm and adductor magnus muscle samples (Fig. 1A).
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The MHC isoforms present in control and stimulated TA and EDL muscles are shown in Fig. 1B. Control TA and EDL muscles contained predominantly MHC types IIA and IID and relatively small amounts of type I MHC. The expression of type IID MHC was reduced and the expressions of type I and IIA MHC were increased in both muscles on chronic electrical stimulation (Fig. 1B). The percentages of total MHC corresponding to type I, IIA, and IID isoforms in control and stimulated TA and EDL muscles are shown in Table 1.
The mean Vmax of stimulated TA and EDL muscle fibers was significantly lower than that of the respective control muscles (Table 1). However, the Vmax of both stimulated TA and EDL muscle fibers was still greater after 3 wk of stimulation than that of soleus fibers (Table 1).
To validate the usefulness of the citrulline assay for quantitation of
NOS activity of rabbit skeletal muscle fractions, the NOS activity was
first measured in skeletal muscle membranes isolated from mixed rabbit
leg and back muscles. Under our assay conditions, the rate of NOS
activity increased linearly during the first 20 min of incubation (Fig.
2A). The
NOS activity also increased linearly with the protein concentration up
to at least 4 mg protein/ml sample (Fig.
2B). In agreement with the findings
of others (15, 19), almost all NOS activity was associated with the
membrane fraction (Fig. 2C). The
supernatant obtained after sedimentation of insoluble materials by
centrifugation at 105,000 g for 60 min contained <15% of the total activity present in rabbit skeletal muscle (Fig. 2C).
-monomethyl-L-arginine
(L-NMMA) and
N
-monomethyl-D-arginine
(D-NMMA)] and by
calmodulin inhibitors (RS-20 and W-7) in presence of 3.1 mM
L-[14C(U)]arginine.
Effects of calmodulin inhibitors are shown in absence of external
calmodulin. E: NOS activity at various
L-arginine concentrations.
Vmax, maximal
velocity of enzyme activity;
Km, dissociation
constant. Inset, double reciprocal
plot of same data.
The NOS activity was completely inhibited by EDTA (Fig.
2C), by arginine analogs, and by
calmodulin inhibitors (Fig. 2D). L-NMMA was about 10 times more
effective (50% inhibitory concentration = 1.17 µM) than
N
-monomethyl-D-arginine
(50% inhibitory concentration = 11.3 µM). The NOS activity increased
with an increase in the
L-arginine concentration in a
hyperbolic manner, and double reciprocal plot of the rate vs. substrate
concentration resulted in a straight line (Fig.
2E). Irrespective of the method of
data analysis (linear or nonlinear curve fitting), essentially the same
Vmax and
dissociation constant
(Km) values
were obtained for the same set of data. However, the calculated
Vmax varied
between 18 and 20 pmol · mg
1 · min
1,
and the Km for
L-arginine varied between 4 and
5 mM in different experiments (n = 4)
when membranes from rabbit back and leg muscles were used.
NOS activity in membranes isolated from control and stimulated muscles
was measured in the presence of 20 mM
L-[14C(U)]-arginine,
a substrate concentration that is four to five times higher than the
Km for
L-arginine. The NOS activity in
control, unstimulated TA and EDL muscles was 21.2 ± 3.8 (n = 3) and 18.7 ± 3.5 pmol · mg
1 · min
1
(n = 3), respectively. In contrast,
the NOS activity approximately doubled in the contralateral, in vivo
stimulated TA and EDL muscles (Fig. 3).
Immunoblot analysis revealed an increase in skeletal muscle nNOS
expression by chronic low-frequency electrical stimulation of TA and
EDL muscles (Fig. 4). An antibody directed
against the nNOS recognized a 160-kDa protein band in isolated skeletal
muscle membranes. Probing the same blots with an anti-eNOS antibody did not reveal the presence of eNOS in the same skeletal muscle samples. This negative result could be due to the low eNOS levels relative to
nNOS in these preparations (16).
-galactosidase (
-gal), are shown in lane
6. Similar data were obtained from 2 other rabbits (not
shown).
Because nNOS is expressed in type II, fast-twitch skeletal muscle fibers but not in type I, slow-twitch fibers (15), one would expect that a fast-to-slow fiber transformation will result in a decreased expression of nNOS. However, under our experimental conditions an increased level of nNOS expression occurred. This was shown by both activity measurements and immunoblot analysis. The chronic low-frequency electrical stimulation employed in this study represents a well-established method for studying the effects of increased function on expression of specific genes in muscle (21). Many different structural and metabolic changes occur under these conditions (17, 21). A consistent finding has been the fast-to-slow muscle fiber transformation (7, 14, 17, 24, 26, 27). In our experiments this was indicated by increased expression of type I and IIA MHC isoforms, by decreased expression of the type IID MHC isoform, and by the reduction of Vmax in single fibers. Taken together, these data suggest that nNOS expression in skeletal muscle is regulated by changes in muscle activity. However, this regulation does not necessarily follow the fast-twitch and slow-twitch pattern during the dynamic phase of phenotype transformation. There is no evidence to show that fiber transformation and increased NOS activity are functionally related.
Recent data suggest that expression of nNOS in the central nervous
system and other tissues such as skeletal muscle is not invariable. In
the central nervous system a transient induction of nNOS occurs during
synaptogenesis (20). The expression of nNOS is also induced during
pregnancy and 17
-estradiol treatment in the uterus, brain, and the
skeletal muscles (30). Our data, by showing that an increase in muscle
activity regulates the expression of the nNOS gene in skeletal muscle,
further support the notion that nNOS is an inducible enzyme. It is
suggested that the nNOS expression in skeletal muscle is under hormonal
and metabolic control and the activity of this enzyme in muscle, just
like in the central nervous system, may be different under different
physiological and pathological conditions.
NO generated in skeletal muscle may have important physiological functions. For example, by inhibition of the ryanodine receptor calcium-release channel (18), NO may decrease calcium release from sarcoplasmic reticulum, depress the contractile force, and decrease the efficiency of excitation contraction coupling (15, 22). NO also increases muscle metabolism by regulating glucose uptake (3) and, as we have shown here, an increase in contractile and metabolic activities during forced stimulation induces the nNOS that produces NO. In addition to these autocrine effects, a portion of the NO produced by muscle fibers may diffuse to neighboring vascular smooth muscle cells and affect them in a paracrine fashion. This is feasible considering that NO is released from contracting muscle and that this release is increased by stimulation of the muscle (3, 15).
On the other hand, excess NO production can have deleterious effects on muscle. It may further decrease the efficiency of excitation-contraction coupling in fast-twitch fibers, in which this coupling is already less efficient than in slow muscles (15, 22). Just like in the nervous system, increased production of NO and reactive oxygen radicals may favor peroxynitrite formation and cell destruction (6, 8). Recent data showing that NO can induce apoptosis in skeletal muscle myoblasts (25) further support the notion that excess NO in the absence of efficient trapping mechanisms may be deleterious to skeletal muscle. However, evidence indicating that NO produced by a constitutive NOS has harmful effects in muscle has not been presented.
We are grateful to Dr. J. David Johnson for the calmodulin inhibitors and many helpful suggestions and comments. The technical assistance of Sherri Mazetis, Wenrong Wu, and Dr. Laszlo P. Vaghy is appreciated.
Address for reprint requests: P. L. Vaghy, Dept. of Medical Biochemistry, The Ohio State Univ., 466 Hamilton Hall, 1645 Neil Ave., Columbus, OH 43210-1218 (E-mail: pvaghy{at}magnus.acs.ohio-state.edu).
Received 15 March 1996; accepted in final form 20 November 1996.
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