Insulin and Isoproterenol Differentially Regulate
Mitogen-Activated Protein KinaseⴚDependent
Naⴙ-Kⴙ-2Clⴚ Cotransporter Activity in
Skeletal Muscle
Aidar R. Gosmanov and Donald B. Thomason
Recent studies have demonstrated that p44/42MAPK
extracellular signalⴚregulated kinase (ERK)1 and
-2ⴚdependent Naⴙ-Kⴙ-2Clⴚ co-transporter (NKCC) activity may contribute to total potassium uptake by
skeletal muscle. To study the precise mechanisms regulating NKCC activity, rat soleus and plantaris muscles
were stimulated ex vivo by insulin or isoproterenol
(ISO). Both hormones stimulated total uptake of the
potassium congener 86Rb by 25–70%. However, only ISO
stimulated the NKCC-mediated 86Rb uptake. Insulin
inhibited the ISO-stimulated NKCC activity, and this
counteraction was sensitive to the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 in the
predominantly slow-twitch soleus muscle. Pretreatment
of the soleus muscle with the phosphatidylinositol (PI)
3-kinase inhibitors wortmannin and LY294002 or with
SB203580 uncovered an insulin-stimulated NKCC activity and also increased the insulin-stimulated phosphorylation of ERK. In the predominantly fast-twitch plantaris
muscle, insulin-stimulated NKCC activity became apparent only after inhibition of PI 3-kinase activity,
accompanied by an increase in ERK phosphorylation. PI
3-kinase inhibitors also abolished insulin-stimulated
p38 MAPK phosphorylation in the plantaris muscle and
Akt phosphorylation in both muscles. These data demonstrated that insulin inhibits NKCC-mediated transport in skeletal muscle through PI 3-kinaseⴚsensitive
and SB203580-sensitive mechanisms. Furthermore, differential activation of signaling cascade elements after
hormonal stimulation may contribute to fiber-type specificity in the control of potassium transport by skeletal
muscle. Diabetes 51:615– 623, 2002
S
keletal muscle is generally considered to be the
predominant tissue responsible for glucose disposal and regulation of plasma potassium (1–3).
In patients with type 1 and type 2 diabetes,
skeletal muscle has been shown to have a blunted insulinFrom the Department of Physiology, College of Medicine, University of
Tennessee Health Science Center, Memphis, Tennessee.
Address correspondence and reprint requests to Donald B. Thomason, PhD,
Department of Physiology, College of Medicine, University of Tennessee
Health Science Center, 894 Union Ave., Memphis, TN 38163. E-mail: thomason@
physio1.utmem.edu.
Received for publication 16 August 2001 and accepted in revised form 11
December 2001.
ERK, extracellular signal⫺regulated kinase; IL, interleukin; ISO, isoproterenol; MAPK, mitogen-activated protein kinase; MEK; MAPK kinase; NKCC,
Na⫹-K⫹-2Cl⫺ cotransporter; PI, phosphatidylinositol.
DIABETES, VOL. 51, MARCH 2002
mediated glucose transport and insufficient potassium
uptake (4 – 6). Because potassium participates in glycogen
synthesis (7), it is possible that potassium entering the cell
might also be a significant physiological modulator of
glucose uptake. Despite this important role in glucose
metabolism, the mechanisms regulating muscle potassium
transport have not been established.
Both insulin and catecholamines are important hormonal mediators of potassium uptake by skeletal muscle
and other tissues, and Na-K ATPase is thought to be a main
effector (8,9). Several animal and clinical studies aimed at
unraveling the mechanism of altered potassium uptake in
diabetes have shown depression of sodium pump activity
(10,11). However, other transport mechanisms in skeletal
muscle also appear to be present. Recently, the role of the
Na⫹-K⫹-2Cl⫺ co-transporter (NKCC) in potassium transport by skeletal muscle has been studied (12). Our previous data indicated that NKCC activity can be stimulated by
catecholamines or electrical stimulation to contribute as
much as 33% of the potassium uptake (13). However, we
did not find that insulin significantly stimulated NKCC
activity in isolated rat skeletal muscle (13).
The basic mechanisms for the regulation of total and
NKCC-mediated potassium uptake by catecholamines and
insulin are not fully understood. The extracellular
signal⫺regulated kinase (ERK1/ERK2 or p44/42MAPK) and
p38 mitogen-activated protein kinase (MAPK) are components of MAPK signaling cascades that have been implicated in the regulation of cellular potassium uptake
(14,15). We and others have shown that ␣- and ␤-adrenergic stimulation of NKCC in striated muscle is dependent
on ERK activation (13,14). It is of interest that insulin
receptor stimulation is recognized as an activator of the
ERK pathway, but that insulin has no effect on NKCC
activity. Activation of the p38 MAPK pathway has been
characterized in response to stressors (such as hyperosmolarity), insulin, and, recently, ␤-adrenergic stimulation
(16 –19). However, hyperosmolarity and catecholamines
are also known to stimulate inwardly directed NKCC,
whereas insulin is not able to activate the cotransporter in
skeletal muscle (20 –22).
Therefore, the current study was undertaken to delineate the MAPK signaling pathways that stimulate total and
NKCC-mediated potassium uptake by skeletal muscle in
response to insulin and the ␤-adrenergic receptor agonist
615
REGULATION OF MUSCLE NAⴙ-Kⴙ-2CLⴚ COTRANSPORT
isoproterenol (ISO). Furthermore, we investigated whether
these pathways were specific to a certain fiber type.
RESEARCH DESIGN AND METHODS
Animal care and muscle preparation. Female SD rats (100 –150 g) were
randomly assigned to experimental groups and housed in light- and temperature-controlled quarters, where they received food and water ad libitum. The
rats were anesthetized with pentobarbital sodium (45 mg/kg i.p.) for tissue
removal. Soleus and plantaris muscles were used for the experiments. Soleus
(predominantly slow-twitch fibers, ⬃87% type I and 13% type II) and plantaris
muscles (predominantly fast-twitch fibers, ⬃9% type I and 91% type II) (23)
were removed by carefully dissecting the proximal and distal tendons. The
muscles were placed in oxygenated Krebs-Ringer buffer at 25°C in preparation
for further treatment. The Animal Care and Use Committee of the University
of Tennessee approved all procedures.
Materials. 86RbCl was from obtained from Du Pont-NEN (Boston, MA).
Insulin, ISO, and bumetanide were obtained from Sigma (St. Louis, MO). The
kinase inhibitors SB203580, wortmannin, and LY294002 were obtained from
CalBiochem (La Jolla, CA). Monoclonal phospho-specific mouse antibodies to
ERK and p38 MAPK and polyclonal anti-ERK2 and anti-p38 MAPK antibodies
were obtained from Santa Cruz Biotechnology (Santa Cruz, CA), and polyclonal rabbit antibodies to phosho-Akt-Ser (473) and Akt were obtained from
New England Biolabs (Beverly, MA).
Muscle incubation and 86Rb/K influx constant calculation. Each muscle
was attached with a 4-0 surgical silk suture to glass wands for rapid transfer
among solutions. The muscles were preincubated for 15 min at 30°C in
preincubation media (oxygenated Krebs-Ringer containing bumetanide [10⫺5
mol/l] or vehicle [DMSO] for the contralateral muscle, and kinase inhibitor as
necessary). For hormonal stimulation, after preincubation muscles were
taken directly to incubation medium (oxygenated Krebs-Ringer containing 1
␮Ci/ml 86Rb, either bumetanide or vehicle, and the appropriate kinase
inhibitor or vehicle) at 30°C, which contained 100 ␮U/ml insulin and/or 30
␮mol/l ISO. Incubation was for 10 min. Muscles were immediately washed
with ice-cold 0.9% saline solution. After being washed, the muscles were
blotted, weighed, and homogenized in 2 ml of 0.3 mol/l trichloroacetic acid.
86
Rb uptake by the muscle was measured by Cerenkov counting. 86Rb
transport was expressed as a rate constant, as previously described (13).
NKCC activity. The bumetanide-sensitive 86Rb rate constant was used as an
index of NKCC activity (13). The bumetanide-sensitive portion of 86Rb uptake
was calculated by subtracting the bumetanide treatment value for the muscle
of one hindlimb from the vehicle treatment value of the contralateral muscle.
Kinase inhibitors. The 86Rb uptake and phosphorylation of the ERK1/ERK2
isoforms and p38 MAPK were evaluated by adding inhibitors of intracellular
signaling pathways to the preincubation and incubation media. Wortmannin
(0.5 ␮mol/l) and LY294002 (15 ␮mol/l) were used for phosphatidylinositol (PI)
3-kinase inhibition. SB203580 (5 ␮mol/l) was used to inhibit p38 MAPK
activity.
Analysis of ERK1/ERK2 and p38 MAPK phosphorylation. The muscles
were preincubated and incubated as described, except for inclusion of 86Rb.
After incubation with kinase inhibitors and stimulation by insulin and ISO, the
muscles were placed in ice-cold lysis buffer (10 mmol/l Tris-HCl [pH 7.4], 5
mmol/l EDTA, 50 mmol/l NaCl, 30 mmol/l Na4P2O7, 50 mmol/l NaF, 100 ␮mol/l
Na3VO4, 1% Triton X-100, 1 mmol/l phenylmethylsulfonyl fluoride, 2 ␮g/ml
aprotinin, 2 ␮g/ml leupeptin, 2 ␮g/ml antipain, and 1 ␮g/ml pepstatin A),
homogenized with a Teflon pestle, and centrifuged at 4°C for 5 min at 5,000g.
The protein concentration of the supernatant was measured by the bicinchoninic acid assay (Pierce, Rockford, IL). Then, 25 ␮g protein was denatured in
SDS-PAGE buffer at 95°C for 5 min and electrophoresed on a 10% SDS-PAGE
gel. The gels were electroblotted onto polyvinylidine fluoride membranes. The
membranes were incubated overnight at 4°C in blocking buffer (1.5 mmol/l
NaH2PO4, 8 mmol/l Na2HPO4, 0.15 mol/l NaCl and 0.3% Triton X-100 [pH 7.4])
supplemented with 3% BSA. Immunologic reactions were performed at room
temperature for 1.5 h in blocking buffer containing 1% BSA and the specific
antibody. Primary antibodies were used in at a 1:1,000 dilution. Incubation for
45 min in 1% BSA blocking buffer with horseradish peroxidase⫺conjugated
secondary antibodies allowed visualization by chemiluminescence (ECL Plus;
Amersham, Piscataway, NJ). Bands were quantitated by video densitometry.
Protein phosphorylation was calculated as the ratio of phospho-to-total
protein expression, normalized to the basal level value (taken as 1.0).
Statistics. Comparisons within and among treatments for the rate constant
data were made by ANOVA. Differences between treatments were considered
significant at P ⬍ 0.05. Data are reported as means ⫾ SE. For differences that
were not significant, the power of these tests was typically ⬎0.85.
616
FIG. 1. Effect of hormonal stimulation on total (A) and bumetanidesensitive (B) 86Rb uptake in skeletal muscle. After preincubation with
bumetanide or vehicle for 15 min, isolated muscles were stimulated
with ISO (30 ␮mol/l) and/or insulin (100 ␮U/ml) during 10 min of
radiolabel uptake. NKCC-mediated 86Rb uptake was determined as the
difference between vehicle-treated (total uptake) and bumetanidetreated muscles. Results are means ⴞ SE of 10 –12 experiments. *P <
0.05 compared with nonstimulated muscle (basal); †P < 0.05 compared
with ISO-stimulated muscle.
RESULTS
Muscle total 86Rb uptake. Both ISO and insulin significantly increased skeletal muscle 86Rb uptake by 35–70 and
24 –29%, respectively (Fig. 1A). The most prominent effect
was detected in the predominantly slow-twitch soleus
muscle; stimulated K-transport was greater by 45–70%
compared with the predominantly fast-twitch plantaris
muscle. Further, ISO-stimulated 86Rb influx was considerably greater in both soleus and plantaris muscles compared with insulin-stimulated muscles (P ⬍ 0.01).
Surprisingly, the stimulation of total 86Rb influx by the
combination of ISO and insulin was not different from
insulin stimulation alone (P ⬎ 0.05).
NKCC-mediated 86Rb uptake. To determine whether
the ISO- and insulin-mediated increase in 86Rb influx might
involve activation of NKCC, isolated muscles were incubated in the presence of bumetanide, a specific inhibitor of
NKCC activity (24). Curiously, incubation of unstimulated
muscle with bumetanide slightly enhanced the rate of 86Rb
transport relative to vehicle-treated muscle (Fig. 1B).
There was a significant increase in the bumetanide-sensiDIABETES, VOL. 51, MARCH 2002
A. GOSMANOV AND D.B. THOMASON
FIG. 2. Effect of kinase inhibitors on ␤-adrenoreceptorⴚmediated
activation of total (A) and bumetanide-sensitive (B) 86Rb uptake.
Isolated muscles were preincubated in the absence or presence of 15
␮mol/l LY294002, 0.5 ␮mol/l wortmannin, or 5 ␮mol/l SB203580 before
ISO stimulation. Results are given as means ⴞ SE of six to eight
experiments. Kinase inhibitor treatments were part of the experimental design that included the groups from Fig. 1. Therefore, basal and
ISO-treated data are the same as reported in Fig. 1. *P < 0.05 compared
with nonstimulated muscle (basal); †P < 0.05 compared with ISOstimulated muscle, respectively.
tive portion of 86Rb influx in ISO-stimulated muscle,
contributing as much as 40% of the stimulated 86Rb influx
in the soleus muscle and 45% of the stimulated influx in the
plantaris muscle (Fig. 1B). In contrast, insulin did not
stimulate bumetanide-sensitive transport in either muscle,
in agreement with our previous report (13). Of particular
note, insulin abrogated the ISO-stimulated, bumetanidesensitive portion of 86Rb uptake regardless of muscle fiber
type (Fig. 1B).
Effect of kinase inhibitors on stimulated 86Rb uptake.
Previously, we showed that ERK activation is required for
NKCC upregulation (13). Because PI 3-kinase and p38
MAPK are known to modulate ERK activity and are
stimulated by ISO and insulin, we evaluated changes in
total and NKCC-mediated 86Rb uptake in response to
pharmacological inhibitors of these kinases (17,25–29).
The PI 3-kinase inhibitor wortmannin reduced the ISOstimulated total 86Rb influx in the slow-twitch soleus
muscle, in part through a 50% inhibition of NKCC-mediated transport (P ⬍ 0.01) (Fig. 2). Transport by the
DIABETES, VOL. 51, MARCH 2002
FIG. 3. Effect of kinase inhibitors on the insulin-mediated total (A) and
bumetanide-sensitive (B) 86Rb uptake. Isolated muscles were preincubated in the absence or presence of 15 ␮mol/l LY294002, 0.5 ␮mol/l
wortmannin, or 5 ␮mol/l SB203580 before insulin stimulation. Results
are given as means ⴞ SE of six to eight experiments. Kinase inhibitor
treatments were part of the experimental design that included the
groups from Fig. 1. Therefore, basal and insulin-treated data are the
same as reported in Fig. 1. *, †P < 0.05 compared with nonstimulated
(basal) and insulin-stimulated muscle, respectively.
fast-twitch plantaris muscle was not affected. Another PI
3-kinase inhibitor, LY294002, also reduced both total and
bumetanide-sensitive 86Rb transport in the soleus muscle
in a fashion similar to that of wortmannin (Fig. 2). These
data indicated that NKCC activation by ISO is partially PI
3-kinase⫺dependent only in slow-twitch oxidative muscle.
We next investigated whether p38 MAPK activation was
involved in ISO-stimulated NKCC activity. The p38 MAPK
inhibitor SB203580 (30) reduced the ISO-stimulated total
86
Rb uptake by 20% (P ⬍ 0.01) in the soleus muscle but did
not affect activation of NKCC in slow- or fast-twitch
skeletal muscle (Fig. 2).
Wortmannin abolished the insulin-mediated stimulation
of total 86Rb influx by the soleus muscle (Fig. 3A). Remarkably, wortmannin and LY294002 increased NKCC-mediated 86Rb transport by the soleus and plantaris muscles in
response to insulin (Fig. 3B). This stimulation was ⬃50%
the ISO-stimulated value in the soleus muscle and equiva617
REGULATION OF MUSCLE NAⴙ-Kⴙ-2CLⴚ COTRANSPORT
lent to the ISO-stimulated value in the plantaris muscle.
Treatment with the SB203580 reduced the insulin-stimulated total 86Rb transport by 10% in the soleus muscle (P ⬍
0.05) and prevented activation of 86Rb influx in the plantaris muscle. Interestingly, NKCC activation in the soleus
muscle was significantly increased by insulin in the presence of SB203580 to an extent similar to that of ␤-adrenergic stimulation (Fig. 3B). In the absence of stimulation
by insulin, there were no significant effects of SB203580 or
the PI 3-kinase inhibitors on either total or NKCC-mediated 86Rb influx (not shown). These findings indicated that
PI 3-kinase and p38 MAPK activity regulate NKCC function
in response to insulin through a potent inhibitory mechanism that is fiber-type specific. Therefore, we hypothesized
that activation of these pathways could be implicated in
the downregulation of the NKCC detected by the combination of ISO and insulin.
We repeated the above experiments by stimulating
muscle 86Rb influx with a combination of ISO and insulin.
Treatment with SB203580 resulted in a marked increase in
NKCC activity in the fast-twitch plantaris muscle (Fig. 4B).
This effect contributed to a significant elevation of total
86
Rb influx to the same degree as with ISO alone (Fig. 1).
We found that neither wortmannin or LY294002 had any
effect on NKCC-mediated 86Rb influx in either the soleus
or plantaris muscles (Fig. 4B). These results indicated that
inhibition of p38 MAPK precludes downregulation of
NKCC activity by insulin in fast-twitch skeletal muscle.
Isoproterenol-stimulated ERK and p38 MAPK phosphorylation. ERK activation is necessary for NKCC activation by ISO (13). Also, it was recently shown that p38
MAPK activity in striated muscle is altered upon catecholaminergic stimulation (19). To further determine the
intracellular signaling pathways responsible for ␤-adrenergic signal transduction in skeletal muscle, ERK and p38
MAPK phosphorylation were assessed by immunoblot analysis.
Isoproterenol significantly increased ERK phosphorylation in the soleus and plantaris muscles (Fig. 5). The
magnitude of ISO-stimulated ERK phosphorylation was
greater in the slow-twitch soleus muscle than in the
fast-twitch plantaris muscle. Fiber-type specificity in agonist-mediated ERK phosphorylation might be attributable
to differences in the total amount of ERK. Video densitometry of Western blots with equal loading of samples
showed that expression of ERK protein was 38% greater in
the soleus muscle than in plantaris muscle (P ⬍ 0.05; data
not shown). To determine the mechanism of ISO-mediated
ERK phosphorylation, muscles were incubated with PI
3-kinase inhibitors before and during ISO stimulation.
Consistent with our results in the 86Rb uptake assay,
wortmannin and LY 294002 significantly diminished ERK
phosphorylation by 35% (P ⬍ 0.05) in the soleus muscle
but had no effect on the plantaris muscle (Fig. 5). Short
exposure to hormone or inhibitors had no effect on total
ERK protein expression (not shown), indicating that alterations in ERK phosphorylation were a direct result of
intracellular signaling.
In the next set of experiments, we found that ISO led to
a twofold increase (P ⬍ 0.05) in p38 MAPK phosphorylation in the plantaris muscle, but not in the soleus muscle
(Fig. 6). The p38 MAPK upregulation in plantaris muscle
was not PI 3-kinase⫺sensitive because neither wortman618
FIG. 4. Effect of kinase inhibitors on the combined ISO and insulin
(INS)-mediated total (A) and bumetanide-sensitive (B) 86Rb uptake.
Isolated muscles were preincubated in the absence or presence of 15
␮mol/l LY294002, 0.5 ␮mol/l wortmannin, or 5 ␮mol/l SB203580 before
combined ISO and insulin stimulation. Results are given as means ⴞ SE
of six to eight experiments. Kinase inhibitor treatments were part of
the experimental design that included the groups from Fig. 1. Therefore, basal and ISO ⴙ INS data are the same as reported in Fig. 1.
*,†-P < 0.05 compared with nonstimulated (basal) and ISO ⴙ INSstimulated muscle, respectively.
nin nor LY294002 affected p38 MAPK phosphorylation
(Fig. 6). Similar to ERK protein expression, the total
amount of p38 MAPK protein was twofold greater in
slow-twitch soleus muscle than in fast-twitch plantaris
muscle (P ⬍ 0.01; data not shown).
Insulin-stimulated ERK and p38 MAPK phosphorylation. To assess the regulation of MAPK pathways by
insulin, homogenates soleus and plantaris muscles were
analyzed for ERK and p38 MAPK phosphorylation. Stimulation of the muscles with 100 ␮U/ml insulin resulted in
70% stimulation of ERK phosphorylation (P ⬍ 0.05),
regardless of fiber type (Fig. 7A). To explore whether p38
MAPK was regulated by insulin, we measured the levels of
phosphorylated p38 MAPK. Insulin had no effect on p38
MAPK phosphorylation in the soleus muscle but led to a
significant increase in p38 MAPK phosphorylation (1.5fold) in the plantaris muscle (Fig. 7B).
We showed that pharmacological inhibition of PI 3-kinase or p38 MAPK activity during insulin stimulation led to
DIABETES, VOL. 51, MARCH 2002
A. GOSMANOV AND D.B. THOMASON
FIG. 5. Phosphorylation of ERK1/ERK2 by ISO. Isolated soleus and
plantaris muscles were treated with 30 ␮mol/l ISO for 10 min in the
absence or presence of 15 ␮mol/l LY294002 (A) or 0.5 ␮mol/l wortmannin (B). Equal amounts of protein from whole cell lysates were
immunoblotted with an antibody to phospho-ERK1/ERK2. Representative blots are shown. The immunoblots then were stripped and reprobed with anti-ERK2 antibody. The intensity of each band was
quantified by densitometry and the phospho/total ratio was calculated.
Data are expressed relative to the basal level of phosphorylation
(taken as 1.0). Data were obtained from four to six different muscles
and are given as means ⴞ SE. *P < 0.05 compared with nonstimulated
(basal) state; †P < 0.05 compared with ISO-stimulated state.
FIG. 6. Phosphorylation of p38 MAPK by ISO. Isolated soleus and
plantaris muscles were treated with 30 ␮mol/l ISO for 10 min in the
absence or presence of 15 ␮mol/l LY294002 (A) or 0.5 ␮mol/l wortmannin (B). Then 25 ␮g protein from whole cell lysates were immunoblotted with an antibody to phospho-p38 MAPK. Representative blots are
shown. The immunoblots were then stripped and reprobed with antip38 MAPK antibody. The intensity of each band was quantified by
densitometry and the phospho/total ratio was calculated. Band intensity was linear up to at least 50 ␮g protein loaded. Values are expressed
relative to the basal level of phosphorylation (taken as 1.0). Data were
obtained from four to six different muscles and represent means ⴞ SE.
*P < 0.05 compared with nonstimulated (basal) state.
the upregulation of NKCC-mediated 86Rb uptake in a
fiber-type–specific manner. Because ERK phosphorylation
is required for NKCC stimulation (13), we examined
insulin-stimulated ERK activation in PI 3-kinase and p38
MAPK-inhibited muscles. In muscle treated with insulin in
the presence of wortmannin, ERK phosphorylation was
significantly greater compared with insulin stimulation
alone (Fig. 7). Likewise, insulin-stimulated muscle pretreated with LY294002 had significantly greater ERK phosphorylation, supporting the involvement of PI 3-kinase in
the negative regulation of ERK MAPK stimulation by
insulin. We also measured the insulin-mediated ERK phosphorylation after inhibition of p38 MAPK activity by
SB203580. This compound dramatically increased insulinstimulated phosphorylation of ERK in the soleus muscle
compared with insulin alone, indicating that activation of
p38 MAPK by insulin could stimulate a negative feedback
mechanism for ERK phosphorylation in the soleus muscle
(Fig. 7A).
There is little evidence regarding upstream regulators of
insulin-mediated p38 MAPK activation, though p38 MAPK
is thought to have a significant effect on the regulation of
muscle GLUT4 function (17,18). Stimulation of PI 3-kinase
activity by insulin is a critical step in a myriad of signal
cascades activated by this hormone (31). We hypothesized
that PI 3-kinase inhibition might result in alteration of p38
MAPK activation. Insulin caused p38 MAPK phosphorylation in the fast-twitch plantaris muscle but not in the
slow-twitch soleus muscle (Fig. 7B). As noted before,
protein expression of p38 MAPK was not altered by the
10-min stimulation. Incubation of the soleus muscle with
PI 3-kinase inhibitors did not change phospho-p38 MAPK
DIABETES, VOL. 51, MARCH 2002
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REGULATION OF MUSCLE NAⴙ-Kⴙ-2CLⴚ COTRANSPORT
FIG. 8. Phosphorylation of Akt on Ser473 by insulin (INS). Isolated
soleus and plantaris muscles were treated with 100 ␮U/ml INS for 10
min in the absence or presence of 15 ␮mol/l LY294002, 0.5 ␮mol/l
wortmannin, or 5 ␮mol/l SB203580. Then 100 ␮g protein from whole
cell lysates were immunoblotted with anti-phospho-Ser473 Akt antibody. Representative blots are shown. The immunoblots then were
stripped and reprobed with anti-Akt antibody. The intensity of each
band was quantified by densitometry and the phospho/total ratio was
calculated. Values are expressed as relative to the basal level of
phosphorylation (taken as 1.0). Data were obtained from four to six
different muscles and are means ⴞ SE. *P < 0.05 compared with
nonstimulated (basal) state; †P < 0.05 compared with insulin-stimulated state.
FIG. 7. Phosphorylation of ERK and p38 MAPK by insulin (INS).
Isolated soleus and plantaris muscles were treated with 100 ␮U/ml INS
for 10 min in the absence or presence of 15 ␮mol/l LY294002, 0.5 ␮mol/l
wortmannin, or 5 ␮mol/l SB203580. Then 25 ␮g protein from whole cell
lysates were immunoblotted with antibody to phospho-ERK1/ERK2 (A)
or p38 MAPK (B). Representative blots are shown. The immunoblots
were then stripped and reprobed with anti-ERK2 or anti-p38 MAPK
antibody, respectively. The intensity of each band was quantified by
densitometry and the phospho/total ratio was calculated. For P-p38
MAPK, band intensity was linear up to at least 50 ␮g protein loaded.
Values are expressed as relative to the basal level of phosphorylation
(taken as 1.0). Data were obtained from four to six different muscles
and are means ⴞ SE. *P < 0.05 compared with nonstimulated (basal)
state; †P < 0.05 compared with insulin-stimulated state.
immunoreactivity in response to insulin (Fig. 7B). However, when plantaris muscles were incubated with wortmannin and LY294002, the 1.5-fold increase in insulinstimulated p38 MAPK phosphorylation was abrogated,
clearly indicating that insulin mediates activation of p38
MAPK through a PI 3-kinase⫺dependent mechanism in the
fast-twitch plantaris muscle (Fig. 7B). Recently, several
investigators proposed that SB203580 was able to inhibit
stimulus-induced phosphorylation of p38 MAPK, although
the primary mechanism of action of the compound is to
inhibit enzymatic activity of p38 MAPK (18,30). Exposure
to the SB203580 did not alter either the basal or insulinstimulated p38 MAPK phosphorylation in the soleus or
plantaris muscles (Fig. 7B).
620
In this study we stimulated muscle with insulin in a
postprandial concentration (100 ␮U/ml). Others have used
much higher concentrations (40,43). We verified that this
concentration stimulated the classical pathway of PI 3-kinase/Akt. As shown in Fig. 8, insulin treatment of soleus
and plantaris muscles stimulated Akt Ser (473) phosphorylation. Akt activation was blocked by treatment with the
PI 3-kinase inhibitors but not the p38 MAPK inhibitor.
DISCUSSION
Diabetes and insulin resistance are among the conditions
known to reduce tissue potassium transport (4 – 6,10,11).
In addition, ␤-blocker treatment inhibits stimulation of
potassium uptake in skeletal muscle and reduces glucose
tolerance (32). However, the cellular mechanisms that
regulate potassium transport by hormones, insulin, and
ISO are poorly understood. The major finding of this study
was the counterregulatory mechanism of insulin on the
␤-adrenergic⫺stimulated kinase uptake in isolated rat
skeletal muscle. The counterregulatory action of insulin
appeared to operate through the inhibition of NKCCmediated potassium uptake. Regulation of NKCC-mediated K-uptake by insulin or catecholamines has been
previously documented, mostly in 3T3-L1 adipocytes, fibroblasts, vascular smooth muscle cells, and myocardium
(14,15,22,23,33,34). This study is the first to evaluate
hormone interaction in the regulation of NKCC activity in
skeletal muscle. Although bumetanide is a specific inhibitor of NKCC activity at 10 ␮mol/l, and muscle demonstrates ion dependence of transport consistent with NKCC
DIABETES, VOL. 51, MARCH 2002
A. GOSMANOV AND D.B. THOMASON
activity (12), bumetanide also slightly increased an apparently compensatory transport in the basal, unstimulated
muscle (Fig. 1B). We previously reported that insulin does
not stimulate bumetanide-sensitive potassium uptake by
skeletal muscle (13), and have now shown that insulin
inhibits this transport mechanism. Further, the data demonstrated that the effect of insulin on NKCC-mediated
potassium uptake was attributable to active control by the
hormone on the signaling mechanisms that regulate potassium transport in skeletal muscle, as described below.
To evaluate the signaling pathways responsible for the
counterregulatory action of insulin on the ␤-adrenergic⫺
stimulated NKCC activity, we examined the role of PI
3-kinase and p38 MAPK. Both the insulin and ␤-adrenergic
receptor can stimulate the ERK MAPK cascade, and this
stimulation is modulated through simultaneous stimulation of PI 3-kinase and p38 MAPK (25,26,35,36). We have
previously shown that activation of ERK is necessary for
NKCC activity in skeletal muscle (13), so PI 3-kinase and
p38 MAPK were candidate control points for interaction
between signals originating at the insulin and ␤-adrenergic
receptors. The data presented demonstrate several novel
mechanistic details regarding ␤-adrenergic regulation of
potassium transport. First, ISO-stimulated PI 3-kinase activity increased NKCC activity only in the predominantly
slow-twitch soleus muscle by activating the ERK pathway
(Figs. 2 and 5). To our knowledge, these data are the first
to demonstrate the dependence of ␤-adrenergic receptormediated ERK phosphorylation on PI 3-kinase activity in
skeletal muscle. However, although an inhibitor of p38
MAPK altered total ISO-stimulated K-transport in both
muscles, NKCC activity was not affected (Fig. 2).
In contrast to the stimulation of NKCC activity by
␤-adrenergic receptor⫺stimulated PI 3-kinase activity in
slow-twitch muscle, insulin receptor stimulation appears
to have inhibited NKCC activity through activation of PI
3-kinase (Fig. 3). The PI 3-kinase activation apparently
inhibited the phosphorylation of the ERK necessary for
NKCC activity (Fig. 7). The inhibitory action of insulinstimulated PI 3-kinase on NKCC activity may be stronger
than the excitatory action of ␤-adrenergic receptor⫺
stimulated PI 3-kinase activity because insulin inhibited
the ISO-stimulated NKCC activity (Fig. 1). Recently, insulinstimulated PI 3-kinase activity has been proposed as a site
of counterregulatory action on ISO effects (37). However,
our findings indicated that inhibition of PI 3-kinase activity
during treatment with both hormones was not sufficient to
restore ISO-stimulated NKCC activity (Fig. 4). Fiber-type
specificity of the signaling pathways became apparent
with the inhibition of insulin-stimulated p38 MAPK activity. It is known that activation of p38 MAPK inhibits some
cellular responses (16). For example, an interleukin (IL)1⫺mediated increase in p38 MAPK phosphorylation leads
to ␤-adrenergic hyporesponsiveness in cultured airway
smooth muscle cells, and SB203580 significantly reduces
the IL-1 effect (38). Only in the slow-twitch soleus muscle
did SB203580 allow a large stimulation of NKCC activity
and ERK phosphorylation by insulin alone (Figs. 3 and 7).
In contrast, only in the fast-twitch plantaris muscle was
the inhibition of p38 MAPK sufficient to overcome the
inhibition of NKCC activity by insulin and ISO combined
(Fig. 4). Despite the apparent implication of p38 MAPK
DIABETES, VOL. 51, MARCH 2002
activity for inhibition of NKCC activity in the soleus
muscle, insulin, like ISO, increased p38 MAPK phosphorylation only in the plantaris muscle (Fig. 6). There are
several explanations for this. It may be that the effect of
SB203580 is not related to inhibition of p38 MAPK activity.
Several lines of evidence have suggested that SB203580
can activate the Raf/MAPK kinase/ERK pathway independent of its action on p38 MAPK activity (28,39). Such an
effect of SB203580 would be consistent with the combination of insulin and SB203580 stimulating NKCC (Fig. 3)
and ERK phosphorylation in soleus muscle (Fig. 7). However, a stimulatory effect of SB203580 on the ERK pathway
that is independent of p38 MAPK activation is not consistent with the lack of stimulation of NKCC activity and ERK
observed when SB203580 was used alone. Another explanation of the effect of insulin in combination with
SB203580 in the soleus muscle is that there is a tonic level
of activation of p38 MAPK that does not inhibit NKCC
activity (consistent with the lack of effect of SB203580
alone). A tonic level of activation could make it difficult to
observe further activation by insulin, and this small,
necessarily compartmental p38 MAPK activation inhibits
NKCC activation such that the inhibition is relieved by
SB203580. Thus, although it is not clear from the SB203580
action whether p38 MAPK exerts an inhibitory effect on
NKCC activity, it is clear that a phenotypic difference
exists for the affected pathways, in addition to those
discussed previously for the PI 3-kinase pathway.
Our current findings are in contrast with a previous
report indicating that PI 3-kinase inhibition abolishes the
activation of ERK by insulin (40). In that study, the
investigators used a supraphysiological concentration of
insulin (20,000 ␮U/ml), whereas we obtained significant
stimulation of 86Rb uptake and MAPK and Akt activation
with a physiological concentration of 100 ␮U/ml (⬃1
nmol) of hormone. One possibility is that high dosages of
insulin may also activate the IGF-I receptors (41,42).
Activation of IGF-I receptors may compete with insulinmediated cellular events and could cross-talk with insulinsignaling pathways. This possibility is consistent with our
data and the report of Shepherd et al. (43), in which
inhibition of PI 3-kinase activity did not affect the phosphorylation of ERK in human skeletal muscle in response
to 1,000 ␮U/ml insulin. Moreover, in our study we measured insulin-stimulated protein kinase B/Akt Ser (473)
phosphorylation as a suitable reporter of hormone-mediated PI 3-kinase activity. In both muscles, insulin action
resulted in a two- to fivefold stimulation of Akt phosphorylation (Fig. 8). Furthermore, inhibition of PI 3-kinase
activity abolished insulin-mediated Akt phosphorylation,
in agreement with previously reported data (43,44). There
are several reports that have shown PI 3-kinase/Akt pathway inhibition of Raf-1/MEK/ERK in myocytes and human
embryonic kidney 293 cells (29,45). In this regard, we
propose that Akt activity may be a crucial element in
insulin-dependent inhibition of NKCC activity in skeletal
muscle.
In conclusion, we have presented data showing the
inhibitory influence of insulin-stimulated PI 3-kinase activity on ERK MAPK-dependent NKCC activity in skeletal
muscle. Our data also indicated that insulin inhibition of
NKCC activity can be relieved in a muscle phenotype⫺
621
REGULATION OF MUSCLE NAⴙ-Kⴙ-2CLⴚ COTRANSPORT
specific manner by the p38 MAPK inhibitor SB203580.
However, it is not clear that p38 MAPK is an active site of
this inhibition. Implicit in these data are a broader physiological consequence of the phenotypic differences because of the greater overall potassium transport and
NKCC activity of the slow-twitch soleus muscle in response to hormonal stimulation. A growing body of evidence indicates that changes in skeletal muscle fiber
phenotype toward a decrease in the highly insulin-sensitive slow-twitch fibers may facilitate insulin resistance and
diabetes (46 – 48). Therefore, the differences in skeletal
muscle phenotype⫺specific characteristics of potassium
uptake may contribute to the hyperkalemia that appears in
patients with diabetes and insulin resistance. Further, we
suggest that skeletal muscle, as a major site of insulin
action, might be a prominent locus of the hormone’s
regulatory effects on catecholamine action with respect to
potassium uptake.
ACKNOWLEDGMENTS
The research was supported by an American Diabetes
Association research award to D.B.T.
The authors are grateful to L.A. Malinick for assistance
with publication graphics.
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