1 Department of Medicine and Research Center, 2 Department of Vascular Surgery, Huddinge Hospital, Karolinska Institute, S-141 86 Huddinge, Sweden
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ABSTRACT |
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Little is known about the regulation of
catecholamine-stimulated lipolysis in human skeletal muscle. Therefore,
-adrenergic regulation of lipolysis and blood flow was investigated
in healthy subjects in vivo by use of microdialysis of the
gastrocnemius muscle. First, during a hypoglycemic, hyperinsulinemic
clamp, which induces a lipolytic response in skeletal muscle tissue, the muscle was locally perfused with
-adrenoceptor blocking agents. Perfusion with nonselective (propranolol) and
2-selective (ICI-118551) blocking agents counteracted the hypoglycemia-induced lipolysis (P < 0.01), but perfusion with
metoprolol (
1-blocker) did not affect the glycerol response. Second, selective
-adrenoceptor agonists were perfused in situ into skeletal muscle during resting conditions.
2-Adrenoceptor
stimulation with terbutaline induced a concentration-dependent increase
in skeletal muscle glycerol levels and in tissue blood flow, whereas
perfusion with
1- or
3-adrenoceptor agonists
(dobutamine or CGP-12177) did not influence the glycerol concentration
or blood flow. In conclusion, in skeletal muscle tissue, only the
2-subtype is of importance
among
-adrenoceptors for regulation of lipolysis and blood flow.
This is in contrast to adipose tissue, where
1- and
3-adrenoceptors are also involved.
-adrenoceptors; insulin; glycerol; microdialysis; hypoglycemia
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INTRODUCTION |
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MOBILIZATION OF LIPIDS from the fat stores is a key event in the regulation of endogenous energy supply to the body. Several factors are rate limiting for lipid mobilization, in particular, the rate of hydrolysis (lipolysis) of triglycerides (TG) to the end products glycerol and free fatty acids (FFA) and the blood flow in lipolytically active tissues. Skeletal muscle is the major site for utilization of FFA during exercise. Although most of these FFA originate from the circulation, it has been calculated that as much as 50% of the FFA oxidized during exercise is derived from hydrolysis (lipolysis) of intracellular TG in muscle (see Refs. 14 and 30 for reviews). Indeed, muscle tissue contains significant amounts of TG, between 10 and 50 µmol/g muscle in humans (14).
However, our knowledge about the regulation of TG metabolism in muscle
is very limited. It has been observed that rodent diaphragm muscle and
rodent heart muscle can release the two end products of lipolysis, FFA
and glycerol (14, 30). In rat and human skeletal muscle, a lipase very
similar to the adipose tissue hormone-sensitive lipase has been found
(11, 21). These data indicate that a lipolytic process is present in
skeletal muscle. Recent studies suggest that this process can be
stimulated by catecholamines (8, 15) and inhibited by insulin (11, 15)
in vivo in humans, thus hormonally regulated in the same fashion as in
adipose tissue. In fat cells, catecholamines accelerate lipolysis
through three -adrenoceptor subtypes:
1,
2, and
3 (see Refs. 1, 9, 26 for
reviews). Concerning the function of
-adrenoceptor subtypes in human
skeletal muscle, no information is available. However, radioligand
binding studies have demonstrated a dominant expression of the
2-subtype in a tissue
homogenate that could be of both muscular and vascular origin (27).
The present study was undertaken to characterize the -adrenoceptor
subtypes involved in the in vivo regulation of lipolysis in muscle by
use of the microdialysis technique. The major question was whether they
differ from those of adipose tissue regulation. In one set of
experiments, skeletal muscle was microdialyzed with selective
-adrenoceptor blockers before and during endogenous catecholamine
activation by hypoglycemia. In another set of experiments, the muscle
tissue was locally stimulated with receptor subtype-selective
-adrenergic agonists. Local tissue blood flow is an important regulatory factor for lipolysis in adipose tissue (1), and it has been
suggested that vascular factors may modify the metabolic events in
skeletal muscle (10). We therefore added a flow marker (ethanol) to the
microdialysis solvent to monitor changes in tissue nutritive blood flow
during the experiments (19). We report on
-adrenoceptor regulation
of skeletal muscle lipolysis, which is markedly different from that of
adipose tissue lipolysis.
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MATERIALS AND METHODS |
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Subjects
Twenty-four healthy, drug-free volunteers were investigated (11 men, 13 women, 29 ± 1 yr; body mass index, 23.4 ± 0.5 kg/m2). Each subject participated in only one of the three studies except for one subject who participated, on separate occasions (4 mo apart), in both one of the clamp studies and theMicrodialysis Device
The principles of the microdialysis technique for lipolysis studies (25) and the microdialysis device (32) have been described in detail. The microdialysis catheter consists of a semipermeable polyamide membrane (30 × 0.62 mm, molecular weight cutoff 20,000), which is glued to the end of a double-lumen polyurethane tube. The microdialysis catheter is inserted percutaneously and continuously perfused. The perfusate solution enters the device through the outer lumen and streams by the membrane, where an exchange of metabolites takes place. The dialysate then leaves the catheter through the inner lumen, and samples are collected.Estimates of blood flow variations can be made qualitatively by adding a flow marker (e.g., ethanol) to the perfusate solution (26). Ethanol is not locally degraded and does not affect tissue metabolism. The ethanol concentration is determined in the in- and outgoing perfusate, respectively, and changes in the ethanol concentration ratio (out- vs. ingoing ethanol concentration) will reflect changes in the local blood flow. This technique has been validated against the 133Xe clearance technique in skeletal muscle (19), and it can detect changes in local blood flow that are >50% (18). One cannot use the 133Xe clearance technique in the present type of experiments, because it cannot detect the changes in tissue flow that occur in the area immediately surrounding the microdialysis catheter.
Study Protocol
All subjects were investigated in the supine position after an overnight fast. See the schematic figure of the study procedure (Fig. 1).
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Hypoglycemic insulin clamp. A
retrograde catheter was inserted in a dorsal hand vein. The hand was
placed in a heated box (63°C) for sampling of arterialized venous
blood (29), a method for arterialization shown not to induce reflectory
changes in blood flow or metabolism (7, 23). Another catheter was
placed in the contralateral cubital vein for infusion of insulin and glucose. After superficial skin anesthesia (EMLA, Astra,
Södertälje, Sweden), four microdialysis catheters were
inserted percutaneously with a steel guide cannula in the medial
portion of the gastrocnemius muscle, 1-2 in each leg. The
penetration of the muscle fascia could easily be recognized during
insertion, and the intramuscular location was additionally proved by
the presence of involuntary muscular twitches during insertion. The
distance between two catheters always exceeded 30 mm. The microdialysis
catheters were continuously perfused with Ringer solution (147 mmol/l
Na+, 4 mmol/l
K+, 2.20 mmol/l
Ca2+, 156 mmol/l
Cl) with or without the
addition of adrenoceptor blocking agents in two subsets of experiments.
First, in eight subjects, the catheters were perfused with Ringer
solution without (catheter
1) or with the addition of the
nonselective
-adrenoceptor blocking agent propranolol
(10
4 mol/l;
catheter
2), respectively. In the second set
of experiments, in eight other subjects, the perfusate was Ringer
solution without (catheter
1) or with the addition of either
the
1-selective adrenoceptor blocker metoprolol (10
8
mol/l; catheter
2) or the
2-selective blocker ICI-118551
(10
8 mol/l;
catheter
3). These concentrations of
metoprolol and ICI-118551 have, in previous microdialysis experiments
on human adipose tissue, been shown to block effectively and
selectively the designated receptors (2), whereas a stronger
concentration would lead to a mixed blockade of more than one receptor.
Furthermore, all perfusate solutions contained ethanol (50 mmol/l) for
the estimation of tissue blood flow variations. The perfusate flow rate
was 2 µl/min. In addition, one microdialysis catheter was perfused at 0.3 µl/min with Ringer solution alone to determine the true tissue levels of glycerol during the experiment. At this flow rate and with
the presently used dialysis membrane length, recovery of glycerol has
recently been shown to exceed 90%, which means that the almost true
tissue levels are measured (15). The perfusate was collected in 15-min fractions.
After 45 min of baseline sampling, an intravenous insulin infusion (0.15 U/kg body wt; Isohuman Rapid, Hoechst, Germany) was infused during 60 min. After ~25-30 min, the concentration of arterialized blood glucose had fallen to <2.5 mmol/l, and a variable glucose infusion (200 mg/ml) was started to maintain the blood glucose concentration at 2.5 mmol/l during 30 min. Then the insulin infusion was terminated, and blood glucose was allowed to recover gradually to the fasting level during the following 120 min. Blood glucose was monitored bedside for adjustments of the glucose infusion rate (HemoCue, Ängelholm, Sweden). Plasma samples were drawn in the middle of each dialysate sampling period for the analysis of plasma glycerol. Insulin and catecholamine concentrations were determined at regular intervals during the procedure.
-Adrenoceptor stimulation in situ.
Four microdialysis catheters were inserted in the calf muscle of nine
fasted subjects and perfused with the Ringer-ethanol solution as
described in Hypoglycemic insulin
clamp. Samples were collected in 15-min
fractions, and the perfusate flow rate was 2.0 µl/min. After a
baseline period of 45 min,
-adrenoceptor-specific agonists were
added to the perfusate of three of the microdialysis devices: the
1-selective agonist dobutamine
in one catheter, the
2-selective agonist terbutaline in a second catheter, and the
3-selective agonist CGP-12177
in the perfusate of a third catheter. The initial concentration was 10
6 mol/l. After 90 min,
the concentration was increased to
10
5 mol/l, and sampling was
continued for an additional 90-min period. The fourth microdialysis
catheter served as a control and was perfused with Ringer-ethanol
solution throughout the study period.
Chemical Analysis
Dialysate glycerol was determined with an enzymatic fluorometric method, with a tissue sample analyzer (CMA/60, CMA Microdialysis, Stockholm, Sweden). In uncharted experiments, this glycerol method gave almost identical values as a bioluminescence method (17). Dialysate ethanol was determined with an enzymatic spectrophotometric method (5), and the out- vs. ingoing perfusate ratio of ethanol was calculated. Plasma glycerol was determined with bioluminescence (17), insulin in serum was determined with a commercial RIA (Pharmacia, Uppsala, Sweden) after precipitation with polyethylene glycol (24), and catecholamines were determined by high-performance liquid chromatography (16). The coefficients of variation in the basal state (t =Statistics
Data are presented as means ± SE. Comparisons between the local glycerol responses to the different drugs were performed over time, with a two-factor analysis of variance (ANOVA). Differences between time segments were evaluated by comparing the areas under the glycerol curves (AUC) with the Student's paired t-test with Bonferroni correction of the P value. For statistical evaluation of changes in the ethanol outflow vs. inflow ratio variations over time in the individual, one-factor ANOVA for repeated measurements was used. The different time points were compared with post hoc analysis of the one-factor ANOVA (Fisher's protected least significant difference test). A software package was used for all statistical calculations (Statview SE + Graphics V. 1.05, Abacus Concepts, Berkeley, CA).Drugs and Chemicals
Ringer solution was obtained from Apoteksbolaget (Umeå, Sweden), propranolol from Zeneca (Gothenburg, Sweden), ICI-118551 from ICI Pharmaceutical (Cheshire, UK), metoprolol from Hässle (Gothenburg, Sweden), dobutamine from Eli Lilly (Indianapolis, IN), terbutaline from Draco (Lund, Sweden), and CGP-12177 from Ciba-Geigy (Basel, Switzerland). ![]() |
RESULTS |
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Hyperinsulinemic, Hypoglycemic Clamp
The plasma and near-true (i.e., >90% recovered) skeletal muscle tissue glycerol concentrations are shown in Fig. 2. In the basal, fasted state, the tissue glycerol concentration was almost twice that in plasma. During insulin infusion, there was an initial decrease in glycerol, followed by an increase starting after 30-45 min in plasma and muscle (ANOVA repeated measurement, P < 0.0001). The plasma glycerol concentration increased throughout the 3-h study, whereas the skeletal muscle tissue concentration peaked after 2 h and decreased toward basal levels at the end of the study. Throughout the experiment, the level of muscle tissue glycerol was higher than the respective plasma concentration (two-factor ANOVA, P = 0.0001). The plasma glucose and insulin concentrations and the catecholamine concentrations during the experiment are presented in Table 1. Plasma glucose concentrations decreased gradually during the first 30 min of the insulin infusion and remained at the hypoglycemic level during the infusion. After the insulin infusion was stopped, the glucose levels gradually returned to the initial concentrations during the remaining 2 h of the study. The peak value of norepinephrine and epinephrine occurred 50-70 min after the insulin infusion was started.
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The effects of local -adrenoceptor blockade are depicted in Figs.
3 and 4. The
addition of propranolol to the perfusate largely prevented the glycerol
increase after hypoglycemia, unlike in the control (no blocker in
perfusate) (two-factor ANOVA, P = 0.007, Fig. 3A). The baseline
glycerol levels in the experiments were 30 ± 4 and 31 ± 3 µmol/l (nonsignificant) for the perfusion with Ringer solution and
propranolol, respectively. These values are much lower than those in
Fig. 2 (~90 mmol/l). The difference is because of the perfusion rates
(i.e., the recovery of a metabolite from the tissue is dependent on the
perfusion speed). In Fig. 2, a perfusion rate of 0.3 µl/min was used,
leading to almost complete recovery of glycerol. In Fig. 3, a higher
perfusate flow rate (2 µl/min) was used, which led to incomplete
recovery (i.e., ~35%, as estimated from the ratio of muscle-derived
glycerol in Figs. 2 and 3).
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In the subset of experiments using selective -adrenoceptor blocking
agents, it was seen that the glycerol response when the
1-blocker metoprolol was added
did not differ from the control (Ringer solution alone). However, when
ICI-118551, the selective
2-blocker, was perfused, the
increase in glycerol after hypoglycemia was markedly attenuated (AUC
ICI 17,963 ± 1,412 µmol · l
1 · min
vs. AUC Ringer 21,768 ± 1,846 µmol · l
1 · min,
P = 0.02) (Fig.
3B).
The ethanol clearance during hypoglycemia and -adrenoceptor
stimulation is shown in Fig. 4. With the use of this technique, the
local skeletal muscle blood flow was found to be unaltered during
hypoglycemia (F = 0.7-1.4; P = 0.2-0.7 by one-factor ANOVA), because the ethanol ratio was almost
constant throughout the experiment in the control state (perfusion with
Ringer solution alone). However, the addition of propranolol or
ICI-118551 resulted in a significant increase in ethanol outflow vs.
inflow ratio during and after hypoglycemia, indicating a decrease in
blood flow (propranolol, P = 0.03;
ICI-118551, P = 0.007). Metoprolol did
not alter the ethanol ratio (P = 0.98, Fig. 4, A and
B).
Selective -Adrenoceptor Stimulation in Situ
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The data representing local tissue blood flow are shown in Fig.
6. The ethanol ratio showed a
concentration-dependent decrease when terbutaline was added to the
perfusate, indicating an increased tissue blood flow with increasing
concentrations of terbutaline (one-factor ANOVA,
P = 0.001, 106 vs.
10
5 mol/l,
P < 0.05, with post hoc analysis). The
1- and
3-adrenoceptor agonists did
not affect the ethanol ratio significantly compared with the control
solvent in which ethanol ratio remained almost constant throughout the
experiment.
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DISCUSSION |
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In this study, the -adrenergic regulation of lipolysis
in skeletal muscle has been investigated for the first time in vivo. Two different methods have been used, both employing microdialysis, which is an excellent tool for pharmacological investigation of lipolysis regulation, as discussed in detail (25).
In the first type of experiment, endogenous catecholamine production was enhanced by hypoglycemia. When muscle tissue concentrations of glycerol were monitored with the high recovery catheter (>90%), a marked increase in tissue glycerol was observed after hypoglycemia and endogenous elevation of catecholamines. The glycerol concentrations were much higher than in blood (twice as high at rest and thrice as high at the peak response levels to hypoglycemia). These data confirm earlier experiments (15) and indicate a marked basal lipolytic activity in muscle that is stimulated by endogenous catecholamines. Muscle tissue lipolysis has been evidenced previously by the finding of reduced TG content after norepinephrine infusion and exercise (14, 30).
Glycerol is an end product of lipolysis. In theory, it could be formed by extracellular hydrolysis of TG through the action of lipoprotein lipase and the lipolysis of intracellular TG through the action of hormone-sensitive lipase (or it could even be derived from some other glycerol-containing molecule, such as phospholipids). It is very unlikely that extracellular lipolysis is of importance in the present experiments, because microdialysis data with glycerol in adipose tissue and skeletal muscle show similar kinetic patterns during the present type of hypoglycemic provocation (15). The initial decrease in tissue glycerol in response to insulin infusion was most prominent in adipose tissue, indicating a more pronounced antilipolytic effect of insulin in fat cells compared with muscle tissue. Consequently, the subsequent lipolytic effect of catecholamines appeared earlier in muscle than in adipose tissue but was also more transient (15).
In vitro studies of experimental animals have pointed to
-adrenergic involvement in skeletal muscle lipolysis
(14), but which
-adrenoceptor subtype is responsible
for lipolysis in muscle after catecholamine stimulation? In the present
investigation, this question was answered by performing microdialysis
under conditions of local
-adrenoceptor blockade. We
used selective or nonselective
-adrenoceptor blockers
in concentrations known from previous microdialysis experiments in
human muscle to block effectively the designated receptor(s) (2).
Probably, the
2-adrenoceptor subtype is the only one of importance. First, a high concentration (10
4 mol/l) of propranolol
(nonselective
-blocker) almost completely prevented the
lipolytic response, and second, when selective
-adrenoceptor blockade was used, the lipolytic response
to hypoglycemia could be counteracted by a
2-adrenoceptor blocker
(ICI-118551) but not by a
1-adrenoceptor blocker
(metoprolol). These data clearly demonstrate the unique involvement of
2-adrenoceptors in the regulation of skeletal muscle lipolysis. Unfortunately, no selective
3-adrenoceptor blocker is
available for human use in vivo. However, this receptor is probably
absent in human skeletal muscle, because no mRNA for the
3-receptor gene has been
found (22).
Is glycerol in the muscle tissue dialysate during rest and hypoglycemia derived from the muscle cells or from adipocytes in the muscle tissue? The time course for glycerol in muscle and blood differed markedly. During hypoglycemia, the difference in glycerol kinetics between skeletal muscle and blood (the latter compartment probably reflects overall lipolysis in both fat and muscle), respectively, strongly suggests that the myocyte is the real source of glycerol. It is very unlikely that glycerol kinetics in fat cells of muscle tissue differ from the kinetics in fat cells in adipose tissue, because glycerol is a water-soluble molecule that easily passes through tissues.
The involvement of -adrenoceptor subtypes in muscle
lipolysis regulation was further examined in the second type of
experiments by perfusing the tissue with increasing concentrations of
dobutamine (
1-adrenoceptor
agonist), terbutaline
(
2-adrenoceptor agonist), and CGP-12177
(
3-adrenoceptor agonist),
respectively. These data show that only terbutaline influenced
lipolysis, which occurred in a concentration-dependent fashion. The
findings further support the view that the
2-adrenoceptor is the only
receptor subtype of importance for the adrenergic regulation of
lipolysis in skeletal muscle. This view is also strengthened by the
previous investigations of radioligand binding to extracts from human
skeletal muscle, showing a predominance of
2-receptors (27). The in
vivo findings with agonists differ markedly from lipolytic
microdialysis studies of human adipose tissue. With the use of the same
agonists and antagonists as in the present study at the same
concentrations, Arner et al. (2), Enoksson et al. (13), and others (4) found that all three
-adrenoceptor subtypes regulate
lipolysis in fat. It can be ruled out that the tissue
difference in response to
-agonist stimulation is
because of bioavailability. In earlier adipose tissue experiments, the
same type of microdialysis catheters were used as in the present muscle
tissue experiments. Furthermore, the drugs used mediate their action
through cell surface receptors and do not have to enter the cells.
We also examined the adrenergic regulation of the nutritive blood flow
in skeletal muscle tissue, because, in fat cells, it is evident that
tissue blood flow is involved in the regulation of lipolysis (1).
Vasoconstriction is usually accompanied by a decreased rate of
lipolysis in adipocytes, and the opposite is true for vasodilation.
Catecholamine stimulation may also induce vasodilation via vasoactive
lipolytic products and not only directly via adrenoceptors in the
vessels (1). During the hypoglycemic experiment, propranolol and
ICI-118551 inhibited
2-adrenoceptor-mediated vasodilation, leading to an increase in the ethanol outflow vs. inflow
ratio above baseline, indicating vasoconstriction. On the contrary,
metoprolol had no effect on the tissue flow. In the agonist
experiments, terbutaline caused a concentration-dependent vasodilation,
whereas dobutamine and CGP-12177 were ineffective regarding effects on
blood flow. These data indicate that the
2-adrenoceptor is the major
adrenoceptor subtype of importance for the regulation of blood flow in
human skeletal muscle. On the other hand, it must be pointed out that
the presently used ethanol-microdialysis technique cannot detect
changes in muscle blood flow that are <50% (18). Thus some
involvement of other
-adrenoceptor subtypes in the
regulation of muscle blood flow cannot be excluded. The
-adrenoceptor-mediated changes in blood flow cannot
explain the findings of attenuated glycerol during hypoglycemia,
because restricted vasodilation would instead retain the glycerol in
the tissue and, thus, increase the glycerol levels. It should be noted
that
-adrenoceptors also might be of importance for the
regulation of blood flow and lipolysis in skeletal muscle tissue,
because in adipose tissue the hypoglycemia-induced lipolysis seems to
be regulated by both
- and
-adrenoceptors (8, 20). However, the
-adrenoceptors were not the focus of this study. The
importance of local blood flow for the tissue glycerol level is
clarified by the findings in Figs. 5 and 6. When the terbutaline concentration was increased from 10
6
mol/l to 10
5 mol/l, there was only a
small (but statistically significant) increase in the glycerol
concentration from
125% to
150% of baseline, suggesting only a
minor effect of a 10-fold increase in terbutaline concentration on the
glycerol output from surrounding cells. However, at the same time, the
blood flow rate was further increased by the change in terbutaline
concentration. This increase in blood flow probably enhanced the
removal of glycerol from the interstitial space of muscle tissue, as in
the case of the effect of blood flow on glycerol levels in adipose
tissue (12). Therefore, the true effect of increased terbutaline
concentration on glycerol output is underestimated from mere
examination of the changes in glycerol concentration.
The dual-stimulatory role of the
2-receptor in lipolysis and
blood flow gives this receptor a particular role in the regulation of
energy metabolism in muscle. During catecholamine stimulation, lipids
can be rapidly mobilized in the muscle through
-adrenoceptor-mediated lipolysis, in combination with
-receptor-mediated vasodilation. With the microdialysis
technique, it is unfortunately not possible to measure FFA because of
their hydrophobic character. Hence, the present data cannot show the
proportion of FFA mobilized from the tissue to that remaining for
oxidation in the myocyte. It seems reasonable that FFA in some part are
mobilized from the tissue in the nonexercising state (14, 30), but the
role of this kind of energy mobilization from a tissue other than the major energy store, adipose tissue, remains unclear. It must, furthermore, be emphasized that our findings are based on examinations of the calf muscle. It is possible that lipolysis and blood flow are
regulated differently in other muscle groups. For example, studies
using the forearm muscle suggest involvement of
1-adrenoceptors in blood
flow regulation (6). Gender differences may also be of importance;
however, the number of subjects in the present studies was too small
for statistical evaluation of sex differences. Finally, the
2-adrenoceptor may also be
involved in other catecholamine-regulated metabolic events in skeletal
muscle, such as thermogenesis and glucose metabolism (3).
A number of studies suggest that the triglyceride content of muscle is
regulated by catecholamines under physiological situations, such as
exercise (14). A pathophysiological role of muscle TG has also been
suggested, because there is an inverse relationship between the muscle
TG content and the peripheral insulin sensitivity (31). The present
study shows that the -adrenergic stimulation of
lipolysis in muscle (presumably because of hydrolysis of intracellular TG in myocytes) involves only
2-adrenoceptors, as opposed
to
1-,
2-, and
3-adrenoceptors in adipose
tissue. Taken together, previous and present data point to an important
role of lipolysis in muscle for energy metabolism in normal and
pathophysiological conditions, which may be different from that in
adipose tissue.
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ACKNOWLEDGEMENTS |
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We gratefully acknowledge the excellent technical assistance of L. Dungner, K. Hertel, B.-M. Leijonufvud, E. Sjölin, and K. Wåhlén.
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FOOTNOTES |
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This study was supported by grants from the Swedish Medical Research Council, Karolinska Institute, Swedish Diabetes Association, Swedish Athletics Research Council, and the Foundations of Tore Nilsson, Thuring, Söderberg, Novo Nordisk, Bergwall, Einar Belvén, and Martin Rind. We thank CMA Microdialysis AB for generous support.
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. §1734 solely to indicate this fact.
Address for reprint requests: P. Arner, Center for Metabolism and Endocrinology, M63, MK-Division, Huddinge Hospital, S-141 86 Huddinge, Sweden
Received 11 March 1998; accepted in final form 21 August 1998.
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REFERENCES |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Arner, P.
Regulation of lipolysis in fat cells.
Diabetes Rev.
4:
450-463,
1996.
2.
Arner, P.,
E. Kriegholm,
and
P. Engfeldt.
In situ studies of catecholamine-induced lipolysis in human adipose tissue using microdialysis.
J. Pharmacol. Exp. Ther.
254:
284-288,
1990[Abstract].
3.
Astrup, A.,
L. Simonsen,
J. Bülow,
J. Madsen,
and
N. J. Christensen.
Epinephrine mediates facultative carbohydrate-induced thermogenesis in human skeletal muscle.
Am. J. Physiol.
257 (Endocrinol. Metab. 20):
E340-E345,
1989
4.
Barbe, P.,
L. Millet,
J. Galitzky,
M. Lafontan,
and
M. Berlan.
In situ assessment of the role of the beta1-, beta2- and beta3-adrenoceptors in the control of lipolysis and nutritive blood flow in human subcutaneous adipose tissue.
Br. J. Pharmacol.
117:
907-913,
1996[Abstract].
5.
Bernst, E.,
and
J. Gutman.
Determination of ethanol with alcohol dehydrogenase and NAD.
In: Methods of Enzymatic Analysis, edited by H. U. Bergmeyer. New York: Academic, 1974, chapt. 3, p. 1499-1505.
6.
Blaak, E. E.,
M. A. Van Baak,
G. J. Kemerink,
M. T. W. Pakbiers,
G. A. K. Heidendal,
and
W. H. M. Saris.
-Adrenergic stimulation of energy expenditures and forearm skeletal muscle metabolism in lean and obese men.
Am. J. Physiol.
267 (Endocrinol. Metab. 30):
E306-E315,
1994
7.
Blaak, E. E.,
M. A. Van Baak,
K. P. Kempen,
and
W. H. M. Saris.
Effect of hand heating by a warm air box on O2 consumption of the contralateral arm.
J. Appl. Physiol.
72:
2364-2368,
1992
8.
Bolinder, J.,
S. Sjöberg,
and
P. Arner.
Stimulation of adipose tissue lipolysis following insulin-induced hypoglycemia: evidence of increased beta-adrenoceptor-mediated lipolytic response in IDDM.
Diabetologia
39:
845-853,
1996[Medline].
9.
Coppack, S. W.,
M. D. Jensen,
and
J. M. Miles.
In vivo regulation of lipolysis in humans.
J. Lipid Res.
35:
177-193,
1994[Abstract].
10.
Delp, M. D.
Effects of exercise training on endothelium-dependent peripheral vascular responsiveness.
Med. Sci. Sports Exerc.
27:
1152-1157,
1995[Medline].
11.
Enoksson, S.,
E. Degerman,
E. Hagström-Toft,
V. Large,
and
P. Arner.
Various phosphodiesterase subtypes mediate the in vivo antilipolytic effect of insulin on adipose tissue and skeletal muscle in man.
Diabetologia
41:
560-568,
1998[Medline].
12.
Enoksson, S.,
J. Nordenström,
and
P. Arner.
Influence of local blood flow on glycerol levels in human adipose tissue.
Int. J. Obes.
19:
350-354,
1995.
13.
Enoksson, S.,
M. Schimizu,
F. Lönnquist,
J. Nordenström,
and
P. Arner.
Demonstration of an in vivo functional beta3-adrenoceptor in man.
J. Clin. Invest.
95:
2239-2245,
1995[Medline].
14.
Gorski, J.
Muscle triglyceride metabolism during exercise.
Can. J. Physiol. Pharmacol.
70:
123-131,
1992[Medline].
15.
Hagström-Toft, E.,
S. Enoksson,
E. Moberg,
J. Bolinder,
and
P. Arner.
Absolute concentrations of glycerol and lactate in human skeletal muscle, adipose tissue, and blood.
Am. J. Physiol.
273 (Endocrinol. Metab. 36):
E584-E592,
1997
16.
Hallman, H.,
L. E. Farnebo,
B. Hamberg,
and
G. Jonsson.
A sensitive method for determination of plasma catecholamines using liquid chromatography with electrochemical detection.
Life Sci.
23:
1049-1052,
1978[Medline].
17.
Hellmér, J.,
P. Arner,
and
A. Lundin.
Automatic luminometric kinetic assay of glycerol for lipolysis studies.
Anal. Biochem.
177:
132-137,
1989[Medline].
18.
Hickner, R. C.
Microdialysis in Skeletal Muscle: Development and Application of the Microdialysis Ethanol Technique (Thesis). Stockholm: Karolinska Institutet, 1995.
19.
Hickner, R. C.,
D. Bone,
U. Ungerstedt,
L. Jorfeldt,
and
J. Henriksson.
Muscle blood flow during intermittent exercise: comparison of the microdialysis ethanol technique and 133Xe clearance.
Clin. Sci. (Colch.)
86:
15-25,
1994[Medline].
20.
Hickner, R. C.,
U. Ungerstedt,
and
J. Henriksson.
Regulation of skeletal muscle blood flow during acute insulin-induced hypoglycemia in the rat.
Diabetes
43:
1340-1344,
1994[Abstract].
21.
Holm, C.,
P. Belfrage,
and
G. Fredrikson.
Immunological evidence for the presence of hormone-sensitive lipase in rat tissues other than adipose tissue.
Biochem. Biophys. Res. Commun.
148:
99-105,
1987[Medline].
22.
Krief, S.,
F. Lönnqvist,
S. Raimbault,
B. Baude,
A. Van Spronsen,
P. Arner,
A. D. Strosberg,
D. Ricquier,
and
L. J. Emorine.
Tissue distribution of beta3-adrenergic receptor mRNA in man.
J. Clin. Invest.
91:
344-349,
1993[Medline].
23.
Kurpad, A. V.,
K. Khan,
and
M. Elia.
The effect of arterialization of blood by hand warming on the interpretation of forearm metabolic studies.
Physiol. Meas.
15:
139-145,
1994[Medline].
24.
Kuzuya, M.,
P. M. Blix,
O. L. Horowitz,
D. F. Steiner,
and
A. H. Rubinstein.
Determinations of free and total insulin and C-peptide in insulin-treated diabetics.
Diabetes
26:
22-29,
1977[Abstract].
25.
Lafontan, M.,
and
P. Arner.
Application of in situ microdialysis to measure metabolic and vascular responses in adipose tissue.
Trends Pharmacol. Sci.
17:
309-313,
1996[Medline].
26.
Lafontan, M.,
and
M. Berlan.
Fat cell adrenergic receptors and the control of white and brown fat cell function.
J. Lipid Res.
34:
1057-1091,
1993[Abstract].
27.
Ligget, S. B.,
S. D. Shah,
and
P. E. Cryer.
Characterization of -adrenergic receptors of human skeletal muscle obtained by needle biopsy.
Am. J. Physiol.
254 (Endocrinol. Metab. 17):
E795-E798,
1988
28.
Maggs, D. G.,
R. Jacob,
F. Rife,
R. Lange,
P. Leone,
M. J. During,
W. V. Tamborlane,
and
R. S. Scherwin.
Interstitial fluid concentrations of glycerol, glucose, and amino acids in human quadriceps muscle and adipose tissue.
J. Clin. Invest.
96:
370-377,
1995[Medline].
29.
McGuire, E. A. H.,
J. H. Helderman,
J. D. Tobin,
R. Andres,
and
R. Bergman.
Effects of arterial versus venous sampling on analysis of glucose kinetics in man.
J. Appl. Physiol.
41:
65-73,
1976.
30.
Oscai, L. B.,
D. A. Essig,
and
W. K. Palmer.
Lipase regulation of muscle triglyceride hydrolysis.
J. Appl. Physiol.
69:
1571-1577,
1990
31.
Pan, D. A.,
S. Lillioja,
A. D. Kriketos,
M. R. Milner,
L. A. Baur,
C. Bogardus,
A. B. Jenkins,
and
L. H. Storlien.
Skeletal muscle triglyceride levels are inversely related to insulin action.
Diabetes
46:
983-988,
1997[Abstract].
32.
Tossman, U.,
and
U. Ungerstedt.
Microdialysis in the study of extracellular levels of amino acids in the rat brain.
Acta Physiol. Scand.
128:
9-14,
1986[Medline].