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Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan, Republic of China
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Hong, S. J., and C. C. Chang.
Trauma-induced changes of skeletal muscle membrane: decreased
K+ and increased
Na+ permeability.
J. Appl. Physiol. 83(4):
1096-1103, 1997.
Trauma of skeletal muscle causes membrane
depolarization and reduces membrane resistance. The underlying
mechanisms were studied in isolated mouse phrenic nerve diaphragms
subject to sharp transections of muscle. Depolarization was most marked
at the vicinity (~1 mm) of trauma, where the membrane potential
dropped rapidly from about 
80 mV to zero and repolarized to
about 25 mV. At the end-plate region (located ~3 mm away from
the cut end), the membrane gradually attained a plateau potential
around 45 mV. The magnitude of depolarization was not reduced by
inhibition of Na+,
Ca2+, or
Cl
channel, whereas the
progress of depolarization was delayed in low-Na+ medium. Activation of the
K+ channel with lemakalim induced
some hyperpolarization at damaged site but produced a
glybenclamide-sensitive outward current and hyperpolarization of
end-plate region to the levels before trauma, as if there was no
diminution of transmembrane K+
gradient in this area. Appropriate elevation of extracellular K+ to stimulate
K+ conductance also hyperpolarized
the end-plate region. The results suggest that depolarization at
regions remote from trauma is related to decreased
K+ and increased
Na+ permeability. The cytoplasma
compartmentalization and permeability changes may protect muscle fiber
from trauma.
adenosine 5 INTRODUCTION STRIATED SKELETAL MUSCLES are highly ordered structures
arranged for active displacement and force generation (28). The information transmission in skeletal muscle, as in other excitable cells, depends on electrical currents flowing across the cell membrane.
In the resting state, high K+
permeability is responsible for the setting of membrane potential, and
the activities of K+ channels are
regulated by a wide range of cellular mediators, such as ions,
nucleotides, and lipids (5). In the membranes of human and murine
skeletal muscles, a high density of ATP-sensitive K+
(KATP) channels has been
functionally identified that can be selectively activated by lemakalim
and inhibited by glybenclamide (30, 35, 37). Because membrane potential
is dictated by the electrochemical gradient created between intra- and
extracellular conducting solutions, damages of muscle membrane, as
often encountered in clinical medicine, would disrupt the
shielding integrity and would be expected to dissipate the chemical
gradient. Indeed, the depolarization of the membrane of an experimental
nerve cut muscle model (2), a traumatic muscle preparation employed for investigation of neuromuscular transmission, was hypothetically attributed to changes in myoplasmic ion composition (31), particularly the depletion of cytoplasmic K+.
However, in view of the organization of muscle fibers, which are packed
in a cylindrical shape, some millimeters-to-several-decimeters long,
with complex cytoskeleton-to-surface membrane lattice (29), trauma-induced changes of membrane properties might be qualitatively and quantitatively inhomogeneous in different parts of a muscle fiber
with respect to the damaged area. For example, high-resolution image
scanning of the legs of marathoners indicates that abnormalities tended
to be located near the origin/insertion of muscles rather than due
to diffuse injuries throughout whole muscles (12). To
investigate the function of muscle membrane after trauma, we studied
the temporal and spatial changes of the membrane potential of
fast-twitch skeletal muscle in response to muscle transection. The
results show that the acute depolarization at the membrane area away
from the trauma site was related to decreased
K+ and increased
Na+ conductance but not due to a
depletion of intracellular K+.
METHODS Nerve Muscle Preparation
-triphosphate-sensitive potassium channel; cut
muscle; membrane potential
Intracellular Recording
Electrophysiological studies of membrane potential and current were performed with a single-electrode voltage-clamp amplifier (Dagan 8100). Glass microelectrodes were filled with 3 M KCl (resistance of 5-15 M
). Membrane resistance was measured from the ratio of induced
voltage change over the applied hyperpolarizing current (10 nA, 300 ms)
injected through the recording electrode. Potentials and currents were
direct-current coupled and recorded on a thermal recorder (Gould
TA5000-AM800). The frequency bandwidth of the recording unit was 10 kHz.
Solutions and Protocol
To explore the influence of specific ion on trauma-induced change of resting membrane potential, bath solution was switched to modified Tyrode solutions, such as 1) low-Cl (8.6 mM) solution
prepared by substituting 137 mM sodium propionate for NaCl;
2)
high-K+ solution prepared by
increasing KCl to 8.4, 28, or 56 mM; and 3)
low-Na+ (30 mM) solution
containing 240 mM sucrose in place of 120 mM NaCl. We chose sucrose for
substitution of NaCl instead of using choline chloride or
tetraethylammonium chloride because the latter two solutions induced
drastic and sustained depolarization (>20 mV) of muscle membrane that
was not observed when the preparation was incubated in the
sucrose-substituted solution.
Bath solution was renewed every 30 min for effective equilibration of cellular and extracellular constituents around the traumatic zone. When the effect of activation of KATP channel (by lemakalim) was monitored, the duration of exposure to the channel activator was minimized to avoid possible rundown of the KATP channel (21): preparations were incubated with lemakalim for 10 min and were washed thereafter three times with lemakalim-free solution (each wash period lasted 3~5 min). This procedure was repeated at 20- to 30-min intervals. Control tests revealed that this washing protocol did not cause deterioration of membrane potential.
When the effect of lemakalim on end-plate potential was studied in cut muscle preparation, the chemical was introduced locally onto restricted end-plate regions by pressure ejection (5 µl of 30-mM stock solution). This was necessary for stable intracellular recording because bath application of lemakalim induced significant membrane hyperpolarization of end-plate regions of whole diaphragm (described in RESULTS), which removed Na+ channel inactivation and resumed substantial contractions on nerve stimulation to dislodge microelectrode.
Statistics
Data are presented as means ± SE of recordings obtained from three to five preparations. For every specified observation time, membrane potentials of each preparation were pooled from 20-30 muscle fibers sampled within 5 min, and the lowest 10% values were excluded. Differences between means were analyzed by Student's t-test, and a statistical P value of <0.05 was considered significant.Chemicals
Lemakalim (Wellwyn Garden City, UK) and glybenclamide (Sigma Chemical, St. Louis, MO) were dissolved in dimethylsulfoxide. The vehicle concentration was kept below 0.1%, which did not induce significant change of resting membrane potential.RESULTS
Intact Preparation
For untransected diaphragms, the resting membrane potentials at the center of muscle fiber (end-plate region) and those 2-3 mm distal to the nerve-muscle junctional area (fiber terminal) were83.7 ± 2.1 and 81.6 ± 2.3 mV, respectively. There was no difference between the two pooled values (but see
DISCUSSION). Membrane potentials did
not change significantly for the first 3 h and were retained at a level
more negative than 80 mV. Four to five hours after isolation,
the membrane depolarized by 8.1 ± 1.2 mV (Fig.
1). To test whether the depolarization was
caused by a decrease of K+
conductance, the KATP channel was
stimulated pharmacologically with lemakalim. Lemakalim (10-100
µM) produced a slight hyperpolarization (~2 mV) in the first 3 h
and a little more hyperpolarization (~5 mV) 3-4 h after
isolation, when the resting potential started to decline (Fig. 1).
These results suggest that the sarcolemma of intact muscle maintains a
high K+ conductance during
experimental period.
solution on membrane
potential of uncut (intact) diaphragm after isolation. 
, Control;
phrenic nerve diaphragms were incubated in normal Tyrode solution. 
, Preparations were challenged every 20-30 min with lemakalim (100 µM for 10 min). In-between tests, Tyrode solution was renewed 3 times
to wash out lemakalim. Shown are potentials in presence of lemakalim.
, Bath solution was switched to
low-Cl (8.6 mM) Tyrode
solution from time 0.
It is well known that mammalian skeletal muscle has a high
Cl conductance, which
participates in the stabilization of membrane potential (25, 27).
Because trauma might impair the activity of
Cl channel, the effect of
inhibition of Cl
conductance on the membrane potential of intact muscle was studied for
comparison. When sodium propionate was substituted for NaCl, muscles
depolarized promptly by ~15 mV, a process accompanied by spontaneous
firings of action potential. However, the depolarization was transient, and 20-40 min later membrane potential resumed, although it remained slightly more depolarized than that of
preparations incubated in normal Tyrode solution (Fig. 1).
In low-Cl solution,
membrane resistance increased ~50% (986 ± 47 vs. 654 ± 33 k).
Cut Muscle Preparation
Membrane depolarization after physical damage. Because the influx of extracellular ions (especially for Ca2+ and Na+) and the efflux of cellular ingredients (K+ and metabolites) via the damaged membrane would depend on the severity of mechanical injury, diaphragms were cut in varying degree to see whether the change in membrane potential is proportional to the extent of trauma. After transection of both ends of muscle fibers, the resting membrane potential at the vicinity (~1 mm) of the damaged end depolarized rapidly down to more positive than10 mV within 60 min (Fig.
2A).
However, for the subsequent 2-4 h, the membranes spontaneously
repolarized to 24.8 ± 2.1 mV. An enormous decrease of
membrane resistance by 75% (down from ~650 to 157 ± 49 k) was
observed. However, because the membrane was near the damaged leaky end,
the decrease might have been overestimated. The membrane
of the end-plate region, which was located ~3 mm away from the cut
ends, also depolarized after trauma. Compared with the damaged end, the
progress and extent of membrane depolarizations were considerably
slower and smaller, and the initial surge of depolarization was not
observed (Fig. 2A). The membrane
potential leveled off at a value around 45 mV ~1.5 h after
transection and maintained this steady state for another 2-3 h.
The membrane resistance of the cut muscle at end-plate area decreased
by 30% to 478 ± 26 k at 1.5 h and by 50% to 313 ± 24 k
at 4 h. The potential (and resistance) of muscle membranes between the
cut end and end-plate region were in between the two respective values (Fig. 2A).
, Near the vicinity (<1 mm) of cut end; 
, ~2 mm
away from cut end; , around end-plate region (~3 mm away from cut
end). B: membrane potential of
end-plate region in modified Tyrode solution. , In
low-Cl Tyrode solution;
, in low-Na+ (30 mM)-sucrose
(240 mM) Tyrode solution. Shaded area denotes membrane potential (means ± 1 SE) of cut diaphragms incubated in normal Tyrode solution.
When muscle fibers were cut at only one end, the depolarization at end-plate region progressed more slowly than that after transection of both ends, taking ~2.5 h to reach a steady state of
50.3 ± 2.6 mV (membrane resistance 536 ± 31 k). Four hours after the
preparation was cut, the membrane potential of the distal intact end
(~6 mm away from the cut end) was 71.9 ± 3.5 mV, slightly more depolarized than the uncut control. If the site of transection was
close to end-plate region (~1 mm), the membrane of the end-plate area
depolarized rapidly, with a time course similar to those parts of
nonjunctional membrane situated near the vicinity of transection, as
described above. After taking spatial and temporal factors into
consideration, these results suggest that the depolarization response
to physical trauma was not different for junctional and nonjunctional
parts of sarcolemma.
Effect of inhibition of
Na+,
K+ or
Cl conductance.
The above results indicate that muscle trauma produced membrane
depolarization for a limited range depending on the closeness to the
trauma. In addition to the inevitable changes of ionic composition due
to leakages via the cut ends, the ionic mechanisms leading to membrane
depolarization may include increased
Na+-Ca2+
permeability, decreased
K+-Cl
permeability, and/or shutdown of
Na+-K+
pump. Depolarizations at the end-plate region were studied under conditions where the membrane channel was blocked or where the ionic
composition of extracellular solution was manipulated to deliberately
exclude specific ions.
Inhibition of Ca2+ channel with
verapamil (30 µM) or Ni2+ (5 mM), Na+ channel with tetrodotoxin
(3 µM), or ATP-gated ion channel with reactive blue-2 or suramin (30 µM) did not change the progress of depolarization (not shown). When
the preparations were equilibrated in sodium propionate-Tyrode
solution, a low-Cl medium,
the rate and extent of depolarization increased slightly (Fig.
2B), similar to those of intact
muscle bathed in the same low-Cl medium. The
contribution of Na+-influx, via
tetrodotoxin-insensitive pathway, to the depolarization of the
end-plate region was investigated in
low-Na+ solution. Diaphragms were
preequilibrated with
low-Na+-sucrose Tyrode for 40 min
before being cut (to allow readjustment of membrane potential from the
initial transient depolarization, since the
low-Na+-sucrose Tyrode was also a
low-Cl solution; cf. Fig.
1). In the low-Na+ solution, the
onset of membrane depolarization was markedly delayed (Fig.
2B), whereas the steady-state
potential reached ~2.5 h after the cut was close to that in normal
Tyrode solution. Because a simple
low-Cl solution tended to
induce slight membrane depolarization, this delay of depolarization in
the low-Na+-sucrose solution, with
respect to that in the normal Tyrode or simple
low-Cl solution, would be
likely related to a low-Na+
effect.
Participation of decreased K+
permeability in trauma-induced depolarization appeared difficult to
evaluate by way of studying effects under inhibition of
K+-channel activity. First,
removal of extracellular K+ would
impair the operation of the
Na+-K+
pump. Second, for the two K+
channels dominant in the control of the resting membrane potential (inward rectifier and KATP
channel) there was no specific blocker for the former while the latter
was normally suppressed by ATP present in the myoplasm. We thus
approached this issue from the opposite direction, i.e., by
investigating effects produced by stimulation of membrane
K+ conductance, based on the
rationale that if the trauma-induced depolarization is caused by a
decrease in K+ conductance, rather
than transmembrane K+ gradient, an
increase of K+ conductance would
produce substantial membrane hyperpolarization. K+ conductance was stimulated by
lemakalim or by elevation of extracellular K+ as described below.
Membrane hyperpolarization by stimulation of
KATP channel.
Figure 3 illustrates a continuous
monitoring of membrane current (Fig.
3A) or potential (Fig.
3B) in response to a focal puff ejection of lemakalim onto the end-plate region of nerve muscle preparations, which had been cut for 90 min and depolarized. Lemakalim induced the outward current and hyperpolarized the end-plate membrane nearly to control level (80 mV). As resting membrane potential was hyperpolarized, the amplitude of evoked end-plate potentials increased markedly, and miniature end-plate potentials, which were
small (because of membrane depolarization) and often obscured by
noises, became larger and clearly visible. When the membrane hyperpolarized to a level beyond 60 mV, nerve stimulation evoked initially abortive and eventually full action potentials with prominent
overshoot (Fig. 3B,
bottom). At those end-plates
hyperpolarized to beyond 75 mV (78.9 ± 1.7 mV, 23 fibers, n = 4) by lemakalim, the
maximal rate of rise of evoked muscle action potential reached 637 ± 29 V/s, which was only 15% less than that obtained from an uncut
end plate (728 ± 21 V/s, resting membrane potential 82.4 ± 1.9 mV, 20 fibers, n = 4).
48 mV. B:
potential change of an end-plate region (resting membrane potential:
48 mV). Phrenic nerve was stimulated at 0.05 Hz (downward
artifacts). Horizontal line marks zero potential. Note that end-plate
potentials triggered action potentials when membrane was
hyperpolarized. Bottom tracings: nerve
stimulation-evoked end-plate potential or action potential at the
corresponding times.
We examined whether the augmentation of the end-plate potential by lemakalim was due to any effect on presynaptic site. In an intact preparation treated with µ-conotoxin, which immobilizes muscle by a selective blockade of muscle Na+ channel without perturbation of transmembrane electrochemical gradient (15), lemakalim did not augment the amplitude of the end-plate potential (31.9 ± 3.8 vs. 33.6 ± 2.9 mV, n = 4). Furthermore, in the cut muscle preparation, massive stimulation of neurotransmitter release (with 3,4-diaminopyridine) did not restore muscle action potential (16). Obviously, the increase of the end-plate potential as well as the initiation of action potential after lemakalim appear to be related to lemakalim-induced hyperpolarization of postsynaptic membrane, which increased electrical gradient and rendered the previously inactivated Na+ channel available again for generation of action potential. The hyperpolarization of the end-plate membrane caused by lemakalim after intermittent bath application is illustrated in Fig. 4A. For the first 2 h, the membrane of depolarized cut muscles was repolarized almost to precut level. Thereafter, lemakalim gradually lost hyperpolarization capability during the plateau phase of depolarization (2-4 h) and eventually became ineffective 4 h after trauma. If lemakalim was added at 4~5 h posttrauma, instead of being repeatedly challenged, membrane hyperpolarization was not observed (not shown), suggesting that the fade of hyperpolarization was caused by trauma per se but not by repetitive stimulation of KATP channel. The magnitude and time course of lemakalim-induced hyperpolarizations at different locations of muscle membrane are compared in Fig. 4B. The largest membrane hyperpolarization (
33.7 ± 2.9 mV) registered was around the
end-plate region ~90-150 min after muscle cut. It seems that the
low magnitude of hyperpolarization during the initial 90 min was
related both to the lesser extent of depolarization at the early stage
following muscle transection (cf. Fig.
2A) and to a ceiling level of
lemakalim-inducible hyperpolarization. The induced membrane
hyperpolarizations were dependent on the concentration of lemakalim
(Fig. 4C) and were abolished in
preparations pretreated with glybenclamide (not shown). The remarkable
restoration of resting membrane potential by lemakalim during the early
stage also suggests that the depolarization caused by muscle trauma could not be accounted for by an inhibition of
Na+-K+
pump, under which transmembrane K+
gradient would decrease and suppress the effect of lemakalim.
, Control (diaphragms were cut at
time 0, from Fig. 2); ,
preparations were challenged every 20-30 min with lemakalim (100 µM for 10 min) from time 0. B: hyperpolarizations at different
locations of muscle fiber. , <1 mm from cut end; 
, ~2
mm away from cut end; , end-plate region (~3 mm away from cut
end). C: dose-effect curve of
lemakalim in muscles cut for 90-120 min. Hyperpolarizations at
end-plate regions in absence () or presence of glybenclamide (3 µM, ). Pretreatment of glybenclamide alone did not alter resting
membrane potential. 
denotes change.
The gradual loss of response to lemakalim in the late phase of trauma could be due to progressive leakage out of intracellular K+ and/or leakage in of extracellular Na+. We examined the magnitude of end-plate potential, which depends on transmembrane gradient of both univalent cations. The end-plate potential soon after reaching the steady-state of membrane depolarization (~2 h after muscle cut) was 9.8 ± 0.9 mV, and the mean amplitude was still 8.2 ± 0.8 mV (n = 6) 4 h after muscle cut, suggesting that, during this period, Na+ and K+ gradients did not change dramatically. At the membrane's near-cut end, lemakalim produced slight hyperpolarization (5-10 mV) only for the initial 20 min. Afterward, lemakalim did not induce membrane hyperpolarization. Effect of extracellular K+. The activity of inward rectifier K+ channel, which is intimately related to the control of resting membrane potential, can be upregulated by elevation of extracellular K+ concentration (34). For intact muscle fibers, an increase of extracellular K+ produced concentration-dependent monophasic membrane depolarization as expected from the Nernst equation. Interestingly, the membrane potential of cut muscle end plates was affected biphasically: high-K+ Tyrode solution (>28 mM) depolarized muscle membrane as in the case of intact muscle, whereas an intermediate increase of K+ concentration to 8.4 mM produced a significant hyperpolarization (Fig. 5), although the membrane was not fully restored to the potential level of intact muscle incubated in the same K+ medium.
, Uncut diaphragms; , diaphragms
cut for 90-120 min.
[K+]o,
extracellular K+ concentration.
DISCUSSION
No Presynaptic Event Involved
In the neuromuscular junction, there are mutual trophic interactions between motor nerve and muscle for biogenesis and remodeling of synaptic transmission. A nonquantal release of acetylcholine may activate the electrogenic Na+-K+ pump at the subsynaptic membrane resulting in a slight hyperpolarization (~5 mV) around the end-plate zone (26), whereas chronic denervation reduces the activity of the Na+-K+ pump and/or K+ conductance, leading to homogenous large depolarization (20-30 mV) of whole muscle membrane and also to spontaneous generation of action potential (17, 23). In the present experiments, all data were collected within 6 h after transection of phrenic nerve, during which spontaneous and stimulation-evoked quantal releases were not significantly affected. The membrane potential of intact muscles was still more negative than70 mV, and signs of chronic denervation-induced
fluctuation of membrane potential were not observed. Besides, the
trauma-induced depolarization far outreached the limited range
controlled by nonquantal release. It is evident that the depolarization
of cut muscle can be ascribed mostly to changes of postsynaptic
component but not to acute denervation of the muscle. Whether the late
slight depolarization in intact muscle, which occurred at 4-5 h
after isolation, was an early sign of denervation remains to be
elucidated. Before we explore trauma-induced depolarization, several
observations need to be addressed. In intact preparations, which
maintain a high resting membrane potential close to
K+ equilibrium potential,
lemakalim produced negligible membrane hyperpolarization. This could be
due to high-K+ (1, 3, 32) and
high-Cl (4, 27)
permeabilities of skeletal muscles, which tend to clamp membrane
potential against small fluctuation. The reason why lemakalim became
progressively ineffective as the time after cut went on (even though
there were few depressions of the end-plate potential) could be due to
rundown of KATP channel, which
probably is related to altered metabolic activities (phosphorylation
and pH; see Refs. 19, 21).
Self-Limitation of Membrane Depolarization in Traumatized Muscle
Transection of both ends of diaphragm elicited persistent membrane depolarization of entire fiber. Because the membrane potential near the cut end declined rapidly almost to zero, it can be inferred that, at the site of damage, destructions of membrane resulted in unrestrained leakages of myoplasmic constituents and exterior milieu, leading to complete collapse of the electrochemical gradient. This effect apparently extended to the nearest vicinity, within 1 mm, of the cut end. However, the drastic depolarization was not sustained and, at later times, there was a partial yet significant restoration of membrane potential near the cut end, suggesting that the damaged sarcolemma could restore part of membrane integrity. From a thermodynamic point of view, the severed lipid bilayers may favor a resealed configuration, similar to the processes in synaptosome formation, and this would help sequestrate the interior of cut muscle from the detrimental exterior environment (24).Away from the cut end, the rate and extent of membrane depolarization and the decrease of membrane resistance were rather gradual and seemed to depend on how close they were to the locus of trauma. At the end-plate region, which resides farthest from the cut ends, the membrane potential reached a characteristic steady state more negative than those at the damaged site. The membrane maintained the plateau potential for a long time. When only one end of muscle was cut, the membrane potential at the intact end was much less affected. These observations suggest the existence of myoplasmic barriers to prevent rapid intermixing of cellular contents along the cylindrical axis of muscle fiber. As far as the considerable variations of the membrane potential of cut muscles (2, 14, 18, 31) are concerned, it is probable that the fiber size, the manner of trauma, and the incubation medium alter the maintenance of transmembrane potential.
Na+ and
Cl Channels Are not Involved
permeability, which
produced an abrupt but transient membrane depolarization and, in
contrast to the effect by trauma, increased membrane resistance (6).
However, some skeletal muscles might set intracellular
Cl under active control
(8); hence, further studies are needed to evaluate the role of the
Cl channel in muscle
trauma. It is also unlikely that the depolarization is related to
activation of the voltage-gated
Na+ or
Ca2+ channel or to opening of the
ATP-gated nonselective cation channel (which might be stimulated by ATP
exuded from myoplasm), as blockers of these ion channels did not
alleviate or aggravate the cut-induced depolarization. After these
possibilities are excluded from the list of major predisposing factors,
what appear to be the causes of trauma-induced depolarization include
decreases of transmembrane Na+ and
K+ gradients and/or
changes of membrane conductance for these ions.
How About Na+ and K+ Gradients?
The transmembrane Na+ gradient at the end-plate region of cut muscle could be logically estimated from electrophysiological measurement of muscle action potential, the rate of rise of which depends critically on Na+ gradient. In lemakalim-hyperpolarized cut muscle, the maximal rate of rise of action potential decreased only slightly compared with that of intact muscles. Furthermore, in the voltage-clamped end plate (14), muscle trauma did not substantially alter the size of miniature end-plate currents or the reversal potential of acetylcholine-gated channel, which are also dependent on Na+ gradient. It seems that, away from the damaged site, muscle membrane could maintain a rather stable transmembrane Na+ gradient for a prolonged time.The possibility of a decrease of myoplasmic K+ concentration as the cause of trauma-induced depolarization is incompatible with the following results: 1) activation of KATP channel (with lemakalim) produced outward current in the end-plate region and hyperpolarized the membrane nearly to the level of intact muscle; and 2) stimulation of inward rectifier by elevation of extracellular K+ concentration, which simultaneously reduces transmembrane K+ gradient, also drove the membrane potential of the cut muscle (by hyperpolarization or depolarization) to levels approaching those of intact preparations incubated in the corresponding high-K+ medium. These results indicate that the trauma-induced depolarization is not caused by a depletion of myoplasmic K+.
Trauma Altered Permeabilities of Sarcolemma
In excitable membranes, membrane potential is determined not only by transmembrane ionic gradients but also by relative ion permeabilities. In the case of skeletal muscles, large inherent K+ conductance dominates resting membrane potential (3, 7, 20). From the inferences described above, it appears that the depolarization around the end-plate area, at least during the initial stage of muscle trauma, might arise from alterations of sarcolemmal permeability. Compared with lemakalim-induced efficient hyperpolarization (a nearly complete restoration of membrane potential), K+ (at 8.4 mM) produced a lower magnitude of restoration, implying that the inward K+ current may be inhibited by trauma. As lemakalim hyperpolarized the membrane of the cut muscle and low-Na+ medium hindered the process of depolarization, both the decreased K+ and increased Na+ permeabilities seem to underlie the cut-induced depolarization. The messengers-membrane architectures that transduce permeability changes of the area remote from a traumatic site await further investigations.The electrical field sensed by membranes is in general related more closely to ions within the restricted (fuzzy) space near the plasmalemma than to the bulk of cytoplasmic/extracelluar ion concentrations. Many lines of evidence indicate that there might be great alterations of ion fluxes, which are not faithfully reflected by macroscopic changes of intracellular ion concentrations, and vice versa (9, 13, 22, 33). In view of the nonuniform ion distributions in the cytoplasm, it is probable that excitable cells have subtle cellular compartmentalizations (36) and that the Nernst equation predicts functional transmembrane ionic gradient that is sensed by the plasma membrane. Under the conditions of muscle trauma, the changes of membrane permeability are likely to be physiologically relevant: reduced K+ permeability may conserve the myoplasmic K+ level, whereas the leakage out of K+ from the traumatic site elevates the local extracellular concentration and may mobilize sequestered Na+ pump (10, 11), which, in collaboration with the leakage in of Na+, may stimulate pump activity. Before regeneration or degeneration of the damaged muscle fibers, these reactions to trauma would alleviate disturbances of ion composition and promote cellular homeostasis.
ACKNOWLEDGEMENTS
This work was supported by Grants NSC86-2314-B002-120 and 87-2314-B002-006 from the National Science Council of Taiwan.
FOOTNOTES
Address for reprint requests: C. C. Chang, Dept. of Pharmacology, College of Medicine, National Taiwan University, No. 1, Sec. 1, Jen-Ai Rd., Taipei, Taiwan, Republic of China.
Received 25 October 1996; accepted in final form 4 June 1997.
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