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1 Laboratorio di Fisiologia Generale, Dipartimento di Biologia Animale e dell'Uomo, 2 Istituto Nazionale per la Fisica della Materia and 3 Dipartimento di Medicina Interna, Università degli Studi di Torino, 10123 Torino, Italy
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The role of platelet-activating factor
(PAF) and nitric oxide (NO) as mediators of the effects of tumor
necrosis factor-
(TNF-) on skeletal muscle contractility was
studied in guinea pig extensor digitorum longus (EDL) muscle. TNF-
(5-10 ng/ml) reduced contractility at every stimulation frequency
(1-200 Hz) and shifted the force-frequency relationship to the
right. The role of NO and PAF as mediators of TNF- was
suggested by the protective effect of
NG-nitro-L-arginine methyl ester
(L-NAME; 1 mM), but not of
NG-nitro-D-arginine methyl ester
(D-NAME; 1 mM), and by the inhibitory effect of the
PAF-receptor antagonist WEB-2170 (3 µM). TNF- increased the
production of PAF and NO. Similar to TNF-, both
S-nitroso-N-acetylpenicillamine (0.5-1 µM), an
NO-generating compound, and PAF (10-20 nM) reduced EDL
contractility. L-NAME, but not D-NAME, blocked
the negative effect of PAF. Blockade of phospholipase A2,
which is required for PAF synthesis, significantly reduced the effects
of TNF-. WEB-2170 inhibited NO synthesis induced by TNF- and
PAF-stimulated NO production. These results suggest that both PAF and
NO contribute to the development of the mechanical alterations induced
by TNF- and that NO production is downstream to the synthesis of PAF.
extensor digitorum longus; nitric oxide; platelet-activating
factor; tumor necrosis factor-
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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TUMOR NECROSIS
FACTOR- (TNF-) is a multifunctional cytokine
(31) implicated in several pathophysiological events, in
which the release of TNF- and other cytokines is considered to have a prominent role in the induction of skeletal muscle contractile dysfunction (28). Endotoxic/septic shock has been shown to
impair diaphragmatic function, leading to reduced contractility and
fatigue resistance (3). The potential role of TNF- in
these alterations has been recently confirmed by in vitro findings
showing that, at least at high concentrations, TNF- reduces
diaphragmatic contractility (32). However, to our
knowledge, no data are at present available on the effect of low
concentrations of TNF- comparable to those measured in vivo in
pathophysiological situations and on the mechanism(s) of action of this
cytokine on skeletal muscle. Studies on the mechanisms of
TNF--induced cardiac dysfunction showed that the reduction of
contractile force induced by this cytokine depends on nitric oxide (NO)
(9, 11). We recently showed that, in cardiac muscle, the
negative effect of TNF- is mediated by platelet-activating factor
(PAF), which promotes the synthesis of NO (1). Indeed, it
has been shown that a number of biological activities of TNF- is
mediated by PAF. TNF- induces synthesis and release of PAF from
macrophages, polymorphonuclear neutrophils, and vascular endothelial
cells (5). Similar to TNF-, PAF levels are increased in
the course of pathophysiological events such as septic shock, ischemia
and reperfusion, and rejection of transplanted organs (4, 17, 27,
28). The aim of the present study was to investigate whether
TNF-, at pathophysiological concentrations, has a depressant effect
on contractility of the isolated guinea pig extensor digitorum longus
(EDL) muscle and whether PAF and NO act as secondary mediators for this
cytokine. The major finding of this study is that, at relatively low
concentrations, comparable to those reported in some pathophysiological
conditions (17, 26), TNF- induces a negative effect on
skeletal muscle contractility, triggered by a cascade that involves the
synthesis of PAF as an intermediate step leading to the production of
NO, which acts as final mediator.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Isolated EDL muscle. Studies were carried out on 45 adult guinea pigs weighing from 200 to 250 g. All animals resided in an animal care facility for a minimum of 1 wk before study and were randomly allocated to each experimental group. Animals were anesthetized with ether and stunned and killed by cervical dislocation. EDL muscles were removed and immersed in Tyrode solution of the following composition (in mM): 154 NaCl, 4 KCl, 2 CaCl2, 1 MgCl2, 5.5 D-glucose, 5 HEPES; pH was adjusted to 7.38 with NaOH. EDL muscles were mounted on an apparatus for in vitro physiological studies on muscle preparations, which was detailed in a previous study (1) and continuously perfused with oxygenated (100% O2) Tyrode solution at 37°C with D-tubocurarine (10 µM) added. Transmembrane potentials were recorded by means of standard glass microelectrodes. To study the force-frequency relationship, muscles were stimulated at different rates (1, 15, 30, 50, 67, 80, 100, 150, 200 Hz) with a pair of electrodes connected to a 302 T Anapulse Stimulator via a 305-R Stimulus Isolator (WP Instruments, New Haven, CT) operating in constant current mode. Tetanic stimulations lasted 500 ms and were separated by 120 s; in selected experiments, muscles were stimulated at 67 Hz for 20 s to perform fatigue tests. At the beginning of the experiments, muscles were lengthened incrementally, until maximal twitch tension was obtained. Stimulation voltage (30% higher than that producing maximal twitch tension) and stimulus duration (0.5 ms) were maintained constant during all the experiments. Baseline control isometric force, recorded during maximal activation after stabilization in Tyrode solution, was 16.8 ± 1.6 N/cm2. Baseline control values for isometric twitches were time-to-peak tension 11.2 ± 1.2 ms; half-relaxation time 7.5± 0.8 ms. Fatigue half time, measured during stimulation at 67 Hz for 20 s, was 15.7 ± 2.2 s. No significant differences were present in baseline control parameters among the groups. The electrical and mechanical activities of EDL muscles were recorded onto magnetic tape by a 3964 A Hewlett-Packard recorder (Palo Alto, CA), visualized on a Tektronix 2211 digital storage oscilloscope, and reproduced for data analysis by means of a Hewlett-Packard 7470A plotter.
Experimental protocol.
EDL muscles were equilibrated in Tyrode solution for at least 30 min
before each challenge. All solutions containing TNF- (5 and 10 ng/ml; Sigma Chemical, St. Louis, MO) or the other drugs were prepared
immediately before the experiments and were not recirculated.
NG-nitro-L-arginine methyl ester
(L-NAME, 1 mM; Sigma) was applied for 2 h before
challenge with TNF- (10 ng/ml) or PAF (20 nM) to block the synthesis
of NO (19). The biologically inactive enantiomer of
L-NAME,
NG-nitro-D-arginine methyl ester
(D-NAME; 1 mM; Sigma), was used as control. WEB-2170 (3 µM; Boehringer Ingelheim), a PAF-receptor antagonist
(12), was used to block PAF receptor; WEB-2170 was administered to EDL muscles starting 15 min before and during the entire period of treatment with TNF- (10 ng/ml). PAF (Bachem Feinchemikalien, Bubendorf, Switzerland) was first dissolved in physiological solution containing 0.25% bovine serum albumin (Sigma), and then the appropriate aliquots of the stock solution were added to
the Tyrode solution to reach the concentrations of 10 and 20 nM.
S-nitroso-N-acetylpenicillamine (SNAP; 0.5-1 µM;
Sigma) was used as a donor of NO. Two main pathways of PAF synthesis
are known: the so-called de novo pathway, which is responsible for the
basal production of PAF, is not inducible and is mainly involved in the
synthesis of PAF in the nervous system and in the renal medulla, and a
remodeling pathway of membrane phospholipids, which is inducible and
represents the main pathway of PAF synthesis from inflammatory cells
(34). The activation of phospholipase A2
(PLA2) is a key step in the synthesis of PAF in this latter pathway (34). We used 4-bromophenacyl bromide (4-BPB; 20 µM; Sigma), a PLA2 inhibitor (22), to study
the role of PLA2 in the synthesis of PAF induced by
TNF-; 4-BPB was administered to EDL muscles starting 15 min before
and during all the period of treatment with TNF- (10 ng/ml) or PAF
(20 nM). Treatment with TNF-, PAF, or SNAP lasted 30 min, and then
the perfusion was switched to control, drug-free Tyrode solution, to
study the reversibility of the effect.
Nitrite assay.
To measure nitrite production by isolated EDL muscles, small aliquots
(150 µl) of perfusate were mixed with an equal volume of 1%
sulfanilamide-1% N-(1-naphtyl) ethylenediamine
dihydrochloride in 2% phosphoric free acid (Greiss acid; Sigma) at
room temperature for 10 min (24). Nitrite concentrations
were calculated by comparison with optical density of standard solution
of sodium nitrite prepared in Tyrode solution. All values were
background corrected for nitrite values obtained in nonconditioned
Tyrode solution. Optical density at 550 nm was measured using a
Microplate Reader model 450 (BioRad Laboratories, Hercules, CA). To
assess the effects of TNF- and the role of PAF on the synthesis of
NO, we measured nitrite production from EDL muscles challenged with 10 ng/ml TNF-, without or after pretreatment with L-NAME (1 mM), WEB-2170 (3 µM), or 4-BPB (20 µM); further experiments were
performed in EDL muscles challenged with 20 nM PAF.
PAF assay. PAF was extracted and purified from EDL muscles as previously described in detail (1, 18). PAF bioactivity was tested by bioassay on washed rabbit platelets (4, 18) after extraction and purification by thin layer chromatography (TLC) and HPLC and was characterized by comparison with synthetic PAF according to the following criteria: induction of platelet aggregation by a pathway independent of both ADP and arachidonic acid/thromboxane A2; specificity of platelet aggregation as inferred from the inhibitory effect of PAF receptor antagonist WEB-2170 (5 µM); and TLC and HPLC chromatographic behavior and physicochemical characteristics such as inactivation by strong bases and 5 min heating in boiling water.
Statistical analysis. Data are expressed as the means ± SE. The experimental groups were compared using two-way analysis of variance. If a significant F resulted from the analysis of variance (P < 0.05), the Newman-Keuls multiple-range test was applied to determine where differences were located among the groups.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Effect of TNF- on EDL mechanical performance.
In preliminary experiments, after the equilibration period of 30 min,
EDL muscles were further maintained in Tyrode solution for 60 min to
study the stability of the preparation. In these conditions, the
amplitude of contractions recorded after tetanic stimulation at 67 Hz
declined only to 93.8 ± 1.1% of the control value. As shown in
Fig. 1, perfusion of EDL with Tyrode
solution containing TNF- induced concentration-dependent effects on
mechanical properties of EDL. One nanogram per milliliter TNF- had
no effect on EDL (not shown), whereas the higher concentrations (5 and
10 ng/ml) shifted the force-frequency relationship to the right and reduced developed force. Ten nanograms per milliliter TNF- reduced the maximal twitch tension at 1 Hz to 28.2 ± 6.1% of the control value, whereas no significant difference was observed for the time-to-peak tension (105.5 ± 1.9%) and the half-relaxation time (108.0 ± 0.8%). The reduction of contractility induced by
TNF- was not accompanied by significant changes of the resting
membrane potential (
83.8 ± 1.6 and 83.3 ± 1.7 mV before
and after treatment with TNF-; 5 experiments, at least 10 measurements for each), the overshoot (+23.8 ± 3.4 and +25.0 ± 2.7 mV), the maximum rate of depolarization (635 ± 39 and
622 ± 47 V/s), and the action potential duration (at 50% of
repolarization, 1.6 ± 0.3 and 1.5 ± 0.2 ms), as well as of
excitability. Fatigue half time, recorded during stimulations at 67 Hz
for 20 s, was reduced to 86.0 ± 5.0% of the control value.
In muscles treated with 5 or 10 ng/ml TNF- and subsequently washed
for 20-30 min, mechanical tension recorded during tetani
recovered, respectively, to 88.6 ± 7.7 or 80.4 ± 15.6% of
the control and the force-frequency relationships were highly
significantly different (P < 0.01) from those recorded after TNF- treatment (Fig. 1). However, after 10 ng/ml TNF-, the
recovery was not complete and the force-frequency relationship remained
significantly lower (P < 0.05) than control,
pretreatment values.
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Role of NO in contractile failure induced by TNF-.
Several studies indicate that the effects of TNF- on other muscle
types, such as cardiac (1, 9, 11) and smooth muscle (25), are, at least in part, mediated by NO. To study
whether NO plays a similar role also in skeletal muscle, the effects of TNF- (10 ng/ml) were studied after 2 h pretreatment with the NO-synthase inhibitor L-NAME (1 mM) or with its
biologically inactive enantiomer D-NAME (1 mM). In
accordance with other studies in which NO-synthase inhibitors were used
(10, 14), the incubation of EDL with L-NAME
shifted the force-frequency relationship to the left and increased
contractile force. Pretreatment of EDL muscles with L-NAME
abolished the shift of the force-frequency relationship and reduced the
contractile failure induced by TNF- at every tested frequency (Fig.
2). In contrast, pretreatment with
D-NAME, which was ineffective per se, did not alter the
mechanical responses of EDL to TNF- (Fig. 2).
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. As shown in Fig.
3, SNAP (0.5-1 µM), used to
produce exogenous NO, exerted dose-dependent effects on the
force-frequency relationship that resembled those induced by TNF-.
Moreover, similar to TNF-, SNAP reduced the contractile force in a
dose-dependent manner. The effects of SNAP were completely reversed
after 15- to 20-min washout. The role of NO as mediator of TNF- was
further studied by measuring nitrite production from EDL muscles
stimulated with this cytokine. TNF- (10 ng/ml) enhanced the basal
nitrite production to 183.7 ± 9.3% of the control value. This
effect was completely abrogated by pretreatment of EDL muscles with the
NO synthase inhibitor L-NAME, but not by D-NAME
(Fig. 4).
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Role of PAF in contractile alterations induced by TNF-.
Several studies indicate that TNF- stimulates the synthesis and
release of PAF by various cell types (5, 18, 28), including cardiac muscle (1). It was suggested that in
cardiac muscle, the negative inotropic effect of TNF- is mediated by PAF (1). Therefore, we tested the possibility that PAF
mediates some of the effects of TNF- in skeletal muscle. For this
purpose, EDL muscles were pretreated with WEB-2170 (3 µM), a
PAF-receptor antagonist, before the challenge with TNF- (10 ng/ml).
As shown in Fig. 5, treatment of EDL
muscles with WEB-2170 shifted the force-frequency relationship to the
left; moreover, WEB-2170 significantly increased contractile force. In
the presence of the PAF receptor antagonist, the effects of TNF- on
the force-frequency relationship and on the contractile force of EDL
muscles were completely abrogated. Moreover, the synthesis of PAF by
EDL muscles was evaluated. Small amounts of PAF were present in EDL
homogenates in basal conditions (PAF concentration = 2.9 ± 0.5 pg/g tissue; n = 4), whereas PAF was not detectable
in the perfusate. After stimulation with TNF- (10 ng/ml), an
increased amount of PAF was recovered both in the EDL homogenate
(PAF = 19.2 ± 4.4 pg/g tissue; P < 0.01 vs.
control) and in the perfusate (1.7 ± 0.8 pg/ml, corresponding to
3.1 ± 1.5 pM; n = 4).
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was further studied in experiments in which exogenous PAF (10 and 20 nM, corresponding to 5.5 and 11 ng/ml, respectively) was added to the
bathing solution. Similar to TNF-, PAF induced a dose-dependent
shift in the force-frequency relationship and reduced contractile
force. These effects were completely reversed after a 15- to 20-min
washout of PAF (Fig. 6).
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Relationship between PAF and NO.
Our results indicate that both the generation of NO and production of
PAF contribute to the development of mechanical alterations induced by
TNF-. To evaluate whether a relationship exists between PAF and NO
production, we compared the effects of PAF administration to control
EDL muscles with those caused by PAF in muscles pretreated with
L-NAME (1 mM) or D-NAME (1 mM). When PAF (20 nM) was administered to EDL muscles pretreated for 2 h with
L-NAME, both the reduction of contractility and the shift
of the force-frequency relationship induced by PAF in control muscles
were abrogated. However, D-NAME completely failed to
protect EDL muscles against the negative effect of PAF (Fig.
7).
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was confirmed by the observations that pretreatment of EDL muscles with WEB-2170 completely blocked the enhancement of NO production induced by TNF- (Fig. 4) and that PAF
(20 nM) stimulates NO production (Fig.
8).
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Role of PLA2 in mechanical alterations induced by
TNF-.
Several studies indicate that stimulation of TNF- receptors leads to
activation of PLA2 (31). Activation of this
enzyme represents a primary step in the biosynthesis of PAF
(34), suggesting that PLA2 plays an important
role in the synthesis of PAF induced by TNF-. To test this
hypothesis, EDL muscles were pretreated with the PLA2
blocker 4-bromophenacyl bromide (4-BPB) (20 µM) before the challenge
with TNF- (10 ng/ml). Pretreatment with 4-BPB, which had no
significant effect per se (Figs. 4 and 9), completely blocked the
effects of TNF- on both contractile force and the force-frequency
relationship (Fig. 9). Moreover,
treatment with 4-BPB significantly (P < 0.01) reduced
the synthesis of PAF (1.4 ± 0.3 vs. 19.2 ± 4.4 pg/g tissue
in muscle homogenates; n = 4) as well as the
stimulation of NO production induced by TNF- in EDL muscle (Fig. 4).
PLA2 blockade, however, had no significant effect
on the alterations of contractile activity (Fig. 9) or on the
enhancement of NO synthesis induced by PAF (Fig. 8).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Our study demonstrates that, in the isolated guinea pig EDL
muscle, 1) TNF- markedly alters the contractile activity,
2) the effects induced by TNF- are mediated both by PAF
and NO, and 3) the production of NO induced by TNF- is
consequent to the synthesis and release of PAF.
Effect of TNF- on skeletal muscle contractility.
A deleterious effect of TNF- on skeletal muscle contractility was
previously reported in vivo by Wilcox et al. (33) as well
as in vitro in an isolated hamster diaphragm preparation (32). The latter effect, however, was evident only at very
high doses (500 ng/ml) of TNF-, whereas, at 0.1 ng/ml, a
concentration comparable to those measured in serum during sepsis
(15), TNF- had no effect on this preparation. In our
experiments, 5 ng/ml TNF- induced a significant reduction of
contractility, suggesting that guinea pig EDL muscle is more sensitive
than the hamster diaphragm to this cytokine. Differences in sensitivity
to TNF- observed in EDL muscle, compared with the experiments by
Wilcox et al. (32) on hamster diaphragm, which are
composed of 98 versus 60% of type II muscle fibers (14),
respectively, suggest that fast-twitch muscles have much higher
sensitivity to TNF-. This may also depend on differences of NOS
activity between diaphragm and EDL, which contains a significantly
higher amount of NOS activity (14). The concentrations of
TNF- used in our experiments (1-10 ng/ml) are higher than serum
levels of TNF- detected in patients with septic shock
(15) or cardiac failure (30), but comparable with those measured in other pathophysiological conditions, such as
acute rejection and viral/bacterial infection after renal transplant (26), or acute peritonitis (17). However, the
study by Torre-Amione et al. (30) shows that measurement
of TNF- in serum underestimates the local concentration of the
cytokine present in the interstitium among cells. Moreover, it should
be considered that the use of an in vitro preparation, in which the
perfusion occurs via the external bathing solution instead of the
capillary network, reduces the delivery of substances to muscle cells
(32). This is particularly important for chemical
messengers with a high molecular weight, such as TNF-.
in different
pathophysiological events, including septic shock, tissue injury,
allograft rejection, reperfusion injury, chronic renal failure, cancer,
and human immunodeficiency virus infection (28). The
impairment of skeletal muscle contractility, a commonly reported event
in these pathophysiological conditions, was attributed to muscle
wasting due to muscle protein breakdown (6) and
alterations in vascular tone (13). The use of an in vitro
isolated preparation suggests that an acute administration of TNF-
exerts direct effects on skeletal muscle, independent from impairment
in neuromuscular impulse propagation or alterations in vascular tone
and blood supply. Tracey and colleagues (29) reported
decreases in muscle transmembrane potential after TNF- treatment of
isolated rat EDL and soleus muscles. In the present experiments,
however, TNF- had no significant effect on membrane potential,
suggesting that the negative inotropic effect of this cytokine is
independent of alterations of the electrical activity. Indeed, in
cardiac muscle, TNF- exerts a marked negative inotropic effect
without altering the resting membrane potential (1). This
discrepancy may depend on the concentration of TNF- used by Tracey
and colleagues (29), which was significantly higher
(~25-fold) than that employed in our experiments.
Effects of PAF on skeletal muscle and its role as mediator of
TNF-.
The present study supports the hypothesis that, as previously shown in
cardiac muscle (1), in skeletal muscle PAF acts as a
mediator of the negative inotropic effect of TNF-. Indeed, pretreatment of EDL muscle with both a PAF-receptor antagonist or an
inhibitor of PAF synthesis completely blocked the effects of TNF-.
PAF was found to act as a secondary mediator of TNF- in several
other experimental conditions (4, 18). Recent studies in
our laboratory indicate that in cardiac muscle, the negative inotropic
effect of TNF- is due to the synthesis of PAF (1). In
the present report, we show for the first time that TNF- induces PAF
synthesis in isolated EDL muscle and, similar to TNF-, PAF impairs
skeletal muscle contractility. The concentrations of PAF used in our
experiments are comparable to those released by inflammatory cells
after stimulation with TNF- (5); at these
concentrations, PAF significantly reduces contractility of isolated
cardiac preparations (16). Moreover, the observations that
in control conditions detectable amounts of PAF are present within
skeletal muscle and that treatment with a PAF receptor blocker
increases contractile force, strongly suggest that PAF synthesized may
modulate contractile properties of skeletal muscle also under basal conditions.
Role of NO as secondary mediator of TNF- and PAF.
Previous experiments indicate that in the heart, the negative inotropic
effects of TNF- and PAF depend on the generation of NO (1, 9,
11). Skeletal muscle cells express both the inducible NOS (iNOS)
and constitutive NOS (cNOS) isoforms (8, 10, 14). The
results of the present study indicate that TNF- and PAF induce early
production of NO. This evidence suggests an involvement of cNOS, rather
than the iNOS, in NO production triggered by TNF- and PAF. However,
in pathological conditions such as septic shock, a persistent
production of TNF- and PAF occurs; therefore, it is possible that in
these conditions, iNOS contributes to the NO generation. Indeed, in
experimentally induced endotoxin septic shock, an activation of iNOS
within skeletal muscle was observed in guinea pigs (10)
and rats (8). Moreover, in the latter animal species, the
induction of iNOS caused by infusion of endotoxin was accompanied by
upregulation of both the endothelial and neuronal NOS (8).
NO generation is critical in mediating the negative inotropic effect of
TNF- and PAF, because L-NAME, but not
D-NAME, prevented the mechanical alterations triggered by
these mediators. Several lines of evidence demonstrate that NO
modulates skeletal muscle contraction. The study of the force-frequency relationship from different skeletal muscle types shows an inverse correlation between NOS activity and force development
(14). Moreover, treatment with NOS inhibitors enhanced
skeletal muscle contractility (10, 14, 20) and reduced the
decline of contractile force induced by endotoxin (8) or
ischemia and reperfusion injury (23). NO-producing
substances, such as SNAP (21) or sodium nitroprusside
(14), induce frequency-dependent reduction of contractile
force. The finding that both inhibitors of NO synthesis (10,
14) and a PAF receptor antagonist shift the force-frequency relationship and increase contractile force suggests that skeletal muscle in basal conditions produces both these mediators. Indeed, Balon
and Nadler (2) reported that resting skeletal muscle releases significant amounts of NO. Our experiments support the hypothesis that TNF- induces NO generation through the synthesis of
PAF rather than directly. These results are in agreement with the
finding that the angiogenic effect of TNF- also depends on the
production of PAF and on PAF-induced NO generation (18). Previous studies have shown that PAF contributes to the induction of
NOS by bacterial lipopolysaccharides (LPS). Indeed, it has been shown
that PAF receptor antagonists inhibit the induction of
calcium-independent NOS in the lungs of rats treated with LPS, but does
not interfere with the in vitro activity of the enzyme (27). These experiments suggest that the synthesis of PAF
induced by LPS stimulates subsequent production of NO. Moreover, it has been shown that the vasoactive and hypotensive effects of PAF are
dependent on NO generation (7, 27). In conclusion, the results of the present experiments indicate that TNF- acutely impairs skeletal muscle contractility, as a result of NO production, which may be largely dependent on the synthesis of PAF.
Perspectives
The results of the present study indicate that TNF- exerts a
negative effect on skeletal muscle contractility, via generation of
secondary mediators such as PAF and NO. In light of our knowledge that
TNF- is involved in several pathophysiological conditions, such as
endotoxic/septic shock, uremia, and cardiac failure, which include
symptoms related to muscle weakness, such studies can be designed to
inhibit some of the biological effects of this cytokine by blocking the
action of PAF and/or NO. Because it has been recognized that TNF-
also possesses beneficial properties, such as protection against
infection, and TNF- inhibition may be, in some cases, detrimental to
the organism, it may be useful to investigate therapeutic strategies
designed to interfere only with the negative effects of this cytokine.
This could be achieved with greater knowledge of the secondary
mediators involved in the different biological actions of TNF-.
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ACKNOWLEDGEMENTS |
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This study was supported by grants of the Ministero dell'Universitá e della Ricerca Scientifica, Istituto Nazionale per la Fisica della Materia, Consiglio Nazionale delle Ricerche (target project on Biotechnology) and Istituto Superiore di Sanitá (Pathology, Clinic and Therapy of AIDS, Grant 30.B.10).
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FOOTNOTES |
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Address for reprint requests and other correspondence: G. Alloatti, Dipartimento di Biologia Animale e dell'Uomo, Università degli Studi di Torino, Via Accademia Albertina 13 10123 Torino, Italy (E-mail: alloatti{at}dba.unito.it).
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.
Received 29 December 1999; accepted in final form 7 August 2000.
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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