AJP - Endo Add DOIs to your references at manuscript stage!
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Endocrinol Metab 288: E148-E154, 2005. First published September 7, 2004; doi:10.1152/ajpendo.00225.2004
0193-1849/05 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
288/1/E148    most recent
00225.2004v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (5)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Cruciani-Guglielmacci, C.
Right arrow Articles by Magnan, C.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Cruciani-Guglielmacci, C.
Right arrow Articles by Magnan, C.

Early changes in insulin secretion and action induced by high-fat diet are related to a decreased sympathetic tone

C. Cruciani-Guglielmacci, M. Vincent-Lamon, C. Rouch, M. Orosco, A. Ktorza, and C. Magnan

Laboratoire de Physiopathologie de la Nutrition, Centre National de la Recherche Scientifique UMR 7059, Université Paris 7, Paris, France

Submitted 27 May 2004 ; accepted in final form 2 September 2004


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
To evaluate the relationship between the development of obesity, nervous system activity, and insulin secretion and action, we tested the effect of a 2-mo high-fat diet in rats (HF rats) on glucose tolerance, glucose-induced insulin secretion (GIIS), and glucose turnover rate compared with chow-fed rats (C rats). Moreover, we measured pancreatic and hepatic norepinephrine (NE) turnover, as assessment of sympathetic tone, and performed hypothalamic microdialysis to quantify extracellular NE turnover. Baseline plasma triglyceride, free fatty acid, insulin, and glucose concentrations were similar in both groups. After 2 days of diet, GIIS was elevated more in HF than in C rats, whereas plasma glucose time course was similar. There was a significant increase in basal pancreatic NE level of HF rats, and a twofold decrease in the fractional turnover constant was observed, indicating a change in sympathetic tone. In ventromedian hypothalamus of HF rats, the decrease in NE extracellular concentration after a glucose challenge was lower compared with C rats, suggesting changes in overall activity. After 7 days, insulin hypersecretion persisted, and glucose intolerance appeared. Later (2 mo), there was no longer insulin hypersecretion, whereas glucose intolerance worsened. At all times, HF rats also displayed hepatic insulin resistance. On day 2 of HF diet, GIIS returned to normal after treatment with oxymetazoline, an {alpha}2A-adrenoreceptor agonist, thus suggesting the involvement of a low sympathetic tone in insulin hypersecretion in response to glucose in HF rats. In conclusion, the HF diet rapidly results in an increased GIIS, at least in part related to a decreased sympathetic tone, which can be the first step of a cascade of events leading to impaired glucose homeostasis.

insulin secretion and hepatic sensitivity

OVERWEIGHT AND OBESITY are known to be among the main environmental factors responsible for the metabolic syndrome that could lead to type 2 diabetes in predisposed subjects (7, 33, 34). Among early events characterizing premetabolic syndrome states, glucose-induced insulin oversecretion associated with insulin resistance is frequently observed (15, 33). This period of insulin hypersecretion may be followed by a further deterioration of insulin secretion in response to glucose (a pancreatic failure and the setting of type 2 diabetes; see Refs. 14 and 15). These abnormalities could be related to impaired free fatty acid (FFA) metabolism (33), which is known to contribute to the deterioration of both insulin secretion (47) and action (13, 46).

From studies performed on different animal models, it appeared that high-fat (HF) diet-related overweight induces an increase in plasma triglyceride (TG) concentration (6, 10, 11, 30) and/or TG ectopic storage (5, 10), a situation that may produce lipotoxicity at both pancreatic (3) and peripheral levels (10). On the other hand, it has been proposed that diet-induced dysfunction of the autonomic nervous system could also be an important component of premetabolic syndrome states (reviewed in Refs. 23 and 24). For example, Levin (28) showed that there was a reduced norepinephrine (NE) turnover in organs, including the brain, in obesity-prone rats before any increase in body weight. Furthermore, the high prevalence of obesity and type 2 diabetes in the Pima Indian community might be the consequence of a "thrifty genotype," including a low sympathetic activity (40).

In the present study, normal Wistar rats were submitted to a HF diet to study its short- and long-term effects on insulin secretion and action. We found that HF rats rapidly displayed insulin hypersecretion in response to glucose and hepatic insulin resistance (after 2 days of diet), followed by glucose intolerance (7 days) and the loss of pancreatic hypersecretion of insulin (2 mo). Furthermore, we aimed to test whether the HF diet could induce alterations in central nervous system (CNS) activity, focusing only on the very beginning of the diet (2nd day) when the alteration of insulin secretion and action appeared. Indeed, dyslipidemia is often associated with autonomic nervous system dysfunction (29, 47), and it has been previously shown that a decreased sympathetic tone could be partially responsible in insulin hypersecretion in response to glucose (31).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals. The experimental protocol was approved by the institutional animal care and use committee of University Paris 7. Male Wistar rats were randomized at 6 wk of age into the following two groups: one group received standard laboratory chow (113; UAR, Epinay sur Orge, France) ad libitum and the other a lard-based HF diet (231H, UAR) ad libitum (HF rats). The HF diet contained 40% fat (32.5% as lard and 7.5% as maize oil) vs. 4.5% for the chow diet, and the total energetic values were, respectively, 5,100 and 3,000 kcal/kg. Rats were housed individually in stainless steel cages in a room maintained at 24 ± 3°C with lights on from 7:00 AM to 7:00 PM; they had free access to water.

Food intake. Daily food intake was measured by weighing the pellets and the HF diet between 9:00 and 10:00 AM.

Glucose-induced insulin secretion. Insulin secretion in response to glucose was investigated every day after the beginning of the diet during the 1st wk, then every week during the remaining 7 wk. Briefly, a single dose of glucose was injected intraperitoneally (1 g/kg body wt) in rats deprived of food for 4 h. Blood samples were drawn from caudal vessels at time 0, 5, 10, 15, 20, 30, and 60 min after glucose injection. Glycemia was immediately measured using a glucose analyzer (Roche Diagnostics, Meylan, France). Plasma was then removed and stored at –20°C until RIA of insulin.

To appreciate the possible involvement of changes in sympathetic activity in the control of insulin secretion after 2 days of diet, glucose- induced insulin secretion (GIIS) was also studied in the presence of an {alpha}2A-adrenoreceptor agonist, oxymetazoline (Sigma Chemical, St. Louis, MO). The insulin response was tested in the presence of 0.1, 1, 10, and 1,000 pmol oxymetazoline/kg body wt, injected intraperitoneally 5 min before glucose loading. We measured the insulinogenic index from time 0 (glucose load) to time 60 min (end of the test). The time points were 0, 5, 10, 15, 20, 30, and 60 min. Each concentration corresponded to a different group of rats.

Glucose turnover rate. Measurements were performed in the basal state and during euglycemic-hyperinsulinemic clamps. Rats deprived of food for 4 h were anesthetized with pentobarbital sodium (50 mg/kg ip). A catheter was inserted in the right jugular vein for blood sampling. Infusions (insulin, labeled and unlabeled glucose) were carried out using butterfly needles that were inserted in the saphenous vein.

A priming dose of [3-3H]glucose (5 µCi) was injected through the saphenous vein at –50 min, followed by a continuous infusion at a rate of 0.2 µCi/min throughout the study. Blood samples were drawn at –15, –10, and 0 min during the basal period and at time 70, 80, and 90 min during the steady-state euglycemic clamp for glucose disposal rate evaluation.

Euglycemic-hyperinsulinemic clamps. Before insulin infusion, two sets of blood samples were drawn for determinations of basal blood glucose, plasma insulin, and nonesterified fatty acid (NEFA) levels. A priming dose of insulin (20 mU Actrapid; Novo, Copenhagen, Denmark) dissolved in isotonic saline was injected through a saphenous vein followed by a continuous infusion of insulin (0.4 U·kg–1·h–1) at a constant rate of 20 µl/min.

During clamps, blood was sampled by caudal vessels every 5 min to determine glycemia and to adjust the rate of unlabeled glucose infusion to maintain euglycemia (glycemia between 4.5 and 6 mM). The euglycemic clamp was attained within 30–40 min and maintained for 40 min thereafter. Steady-state specific glucose radioactivity and plasma glucose and insulin concentrations were determined during the last 20 min of the clamp.

Analytic methods. Blood glucose was determined using a glucose analyzer (Roche Diagnostics). For the assay of [3-3H]glucose radioactivity, blood samples were deproteinized with Ba(OH)2 and ZnSO4, and the supernatant was evaporated to dryness at 50°C to remove tritiated water. The dry residue was dissolved in 0.5 ml of water to which 10 ml of Aqualuma plus scintillation solution were added (Lumac LSC, Groningen, The Netherlands), and radioactivity was determined in a Packard Tri-Carb 460C liquid scintillation system. Plasma insulin was determined by an RIA kit (DiaSorin, Les Ullis, France). NEFA concentrations were measured using an enzymatic assay (NEFA-C test; Wako Chemicals).

Glucose kinetics and expression of the data. In the basal steady state, the rate of glucose appearance (Ra, rate of appearance, reflecting hepatic glucose production) was equal to the glucose disposal rate (Rd, rate of disappearance, reflecting glucose utilization). Rd was calculated according to the formula Rd = Ra = [3-3H]glucose infusion rate [disintegrations·min–1 (dpm)·min–1] divided by blood glucose specific activity (dpm/mg), during the last 20 min of the glucose clamp (50–70 min after the onset of insulin infusion). Results are given as means ± SE.

Pancreas, liver, and brain NE turnover. We used the synthesis inhibition method by Brodie et al. (8). At 2 h after lights on, rats were injected with {alpha}-methyl-DL-p-tyrosine, a tyrosine hydroxylase inhibitor (250 mg/kg ip; Sigma) that prevents the conversion of tyrosine to dihydroxyphenylalanine (DOPA), the rate-limiting step of catecholamine synthesis. Next, 3 h after this injection, one group of rats (time 3 h) was killed, and pancreas, liver, and whole brain were rapidly removed and frozen in liquid nitrogen. The other group (time 6 h) received an additional 125 mg/kg ip {alpha}-methyl-DL-p-tyrosine. These rats were killed 3 h later, and the organs were also removed and frozen. Another group (time 0 h) of rats was not injected with the inhibitor; these rats were killed at the beginning of the experiment to determine the basal level of NE concentration, NE0, expressed as nanograms per gram of organ. Because the concentration of NE ([NE]) in tissues declined exponentially after blockade of NE synthesis, we could obtain from the slope of the line of log[NE] vs. time k, the rate constant of NE efflux (expressed in h–1). The product of k and NE0 will give the rate of NE efflux, which is equal to the rate of NE synthesis and will be called turnover rate (TR, expressed in ng·g–1·h–1; for details, see Ref. 8) . Organ NE levels were determined using HPLC with a C18 reverse-phase column (150 x 2 mm Beckman column packed with Ultrasphere ODS C18, 5 µm average particle size) and electrochemical detection (Kontron 402) after alumina extraction. Calibration curves were made with spiked plasma or homogenate. Recoveries were calculated on the basis of peak heights measured by an integrator (Shimazu, Kyoto, Japan).

Liver analysis: glycogen and TG contents. Liver (~0.2 g) was ground in distilled water with Ultraturax (ice-cold procedure), and then TG were assayed in the homogenate using the commercial kit "GPO trinder" (Sigma Diagnostic). Glycogen was also measured in the homogenate using the method of Roehrig and Allred (42); it was hydrolyzed by amyloglucosidase treatment without preliminary extraction, and glucose was dosed by a colorimetric method using glucose oxidase.

Microdialysis procedure. A detailed description of surgical and technical procedures has been given previously (43). Briefly, at least 5 days before the experiment, the animal was placed in a stereotaxic frame (Kopf Instruments). A guide cannula was implanted to insert the microdialysis probe with its upper part in the paraventricular nucleus (PVN) of the hypothalamus and the lower part in the ventromedial hypothalamus (VMH). The coordinates of the guide tip were (according to the atlas of Paxinos and Watson; Ref. 37a) –1.9 mm posterior to bregma, 0.5 mm lateral, and 7 mm ventral to the dura. The dialysis probe protruded 2 mm beyond the guide tube, and its tip reached a point 9 mm ventral to bregma. The guide cannula and the venous catheter were fixed to the skull with stainless steel screws and dental cement.

On the day of the experiment, a system of tubing connected to a two-channel swivel placed on the beam allowed infusion of fluid in the probe and sampling. The flow rate (2 µl/min) allowed the collection of 30-µl samples every 15 min. The dialysis probe was inserted through the guide at 9:00 AM, and the infusion was performed with a Ringer-type solution containing 147 mM Na+, 2.3 mM Ca2+, 4 mM K+, and 155.6 mM Cl–. The first microdialysis samples were taken 4 h after insertion of the probe; this delay is necessary to achieve stable catecholamine levels. After four 15-min baseline samples were collected, the rats were subjected to an intraperitoneal glucose injection (1 g/kg body wt). Sampling was carried on every 15 min after the injection during at least 3 h.

Measurement of hypothalamic NE. The dialysates were analyzed by means of reverse-phase liquid chromatography with electrochemical detection (Decade; Antec) at a potential of 750 mV. The chromatographic system consisted of a 20-µl sample loop leading to a 10-cm column (Colochrom) with a 3.2-mm internal diameter and 3 µm C18 packing. The mobile phase consisted of an acetate buffer containing 100 µM EDTA, 1 mM octanesulfonic acid, and 4% vol/vol acetonitrile at pH 3.1. This system allowed detection of NE at a sensitivity of 0.1 nA and separation of it from the other monoaminergic compounds.

The mean NE levels of the four baseline samples (before the glucose injection) were calculated for each animal. The percentage of variation relative to the basal levels was calculated for each animal ateach time point. The results were then expressed as means of percentage of baseline ± SE.

Statistical analysis. Statistical analyses were performed using a two-factor repeated-measures ANOVA. Comparison between control and HF rats was assessed by a nonpaired Student's t-test. P < 0.05 was chosen as statistical significance.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Body weight, food intake, and basal plasmatic parameters. Rats submitted to the HF diet showed significant overweight after 5 wk of diet (data not shown), and overweight persisted throughout after the diet (Table 1). The energetic intake during24 h was doubled in HF rats vs. chow-fed rats whatever the time of the measurement (Table 1), whereas daily food intake, as expressed in grams per gram body weight, was similar between groups (data not shown). Basal plasma glycemia, insulinemia, NEFA, and TG concentrations did not change throughout the study, and the HF diet had no effect on any of these parameters. We observed a decrease in plasma ghrelin concentration after 2 and 7 days of diet and an increase in leptinemia that reached significance after 7 days of diet (Table 1).


View this table:
[in this window]
[in a new window]
  		Table 1. Body weight, food intake, and basal plasma parameters 2 days, 1 wk, and 8 wk after the beginning of the diet

 
GIIS. As shown in Fig. 1, the time course of glycemia after glucose loading was similar in both groups after 2 days of diet (Fig. 1A). In contrast, the increase in plasma insulin concentration was much higher in HF rats than in control rats (Fig. 1B). After 1 wk of diet, insulin hypersecretion persisted (Fig. 1D), and the increase in plasma glucose concentration was significantly higher in HF rats than in control rats (Fig. 1C), indicating glucose intolerance. After 8 wk of diet, GIIS was not significantly different from controls (Fig. 1F), whereas glucose intolerance worsened (Fig. 1E). Indeed, the area under the curve (AUC) of glycemia in HF rats after 8 wk of diet is significantly higher than the AUC after 1 wk of diet (342 ± 22 vs. 185 ± 7, P < 0,001).



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 1. Time course of plasma glucose and insulin concentrations in response to glucose load in chow-fed (dotted line) and high fat- fed (solid line) rats after 2 days of diet (A and B), 1 wk of diet (C and D), and 8 wk of diet (E and F). Values are means ± SE; n = 9 rats/group. *P < 0.05, significantly different from controls.

 
GIIS in control and HF rats after 2 days of diet in the presence of oxymetazoline. In the absence of oxymetazoline (Fig. 2), the insulinogenic index ({Delta}I/{Delta}G) was 2.5-fold higher in HF rats vs. control after 2 days of diet, which expresses insulin hypersecretion in response to glucose (also shown in Fig. 1, A and B). In HF rats, the injection of oxymetazoline significantly reduced insulin secretion in response to glucose in a dose-dependant manner: {Delta}I/{Delta}G was reduced by 24% vs. the basal value in the presence of 0.1 pmol/kg oxymetazoline and significantly reduced by 65, 74, and 93% in the presence of 1, 10 and 1,000 pmol/kg, respectively. In contrast, in control rats, {Delta}I/{Delta}G remained unchanged when animals were given 0.1, 1, and 10 pmol/kg oxymetazoline, whereas it was reduced by 74% in the presence of 1,000 pmol/kg.



View larger version (13K):
[in this window]
[in a new window]
  		Fig. 2. Insulinogenic index ({Delta}I/{Delta}G) in response to glucose loading in the presence of increasing amounts of oxymetazoline in control (filled bars) and high fat-fed (open bars) rats. *P < 0.02, significantly different from control rats. #P < 0.03, significantly different from basal value (without oxymetazoline).

 
Glucose TR under basal and euglycemic-hyperinsulinemic clamp conditions. Figure 3 shows the glucose TR under both basal and hyperinsulinemic-euglycemic clamp conditions in HF rats and chow-fed rats after 2 days of diet. Basal turnover was similar in both groups. During hyperinsulinemic clamps, plasma insulin concentration was increased by about six times above the basal value in both groups with insulin infusions of 0.4 U·kg–1·h–1. Under this condition, hyperinsulinemia was similar in both groups and reached ~1,500 pM (basal plasma insulin concentration in both groups was ~250 pM). The insulin-induced increase in glucose utilization (i.e., Rd) was similar in chow-fed and HF rats (Fig. 3A); in contrast, the decrease in hepatic glucose production (i.e., Ra) was attenuated in HF rats compared with controls (Fig. 3B). Therefore, hepatic insulin resistance was present as early as after 2 days of HF diet and persisted throughout the study (data not shown).



View larger version (13K):
[in this window]
[in a new window]
  		Fig. 3. Glucose turnover rate in the basal state (filled bars) and during euglycemic-hyperinsulinemic clamps (open bars) in rats fed chow or a high-fat diet. A: rate of glucose disappearance (Rd), i.e., glucose utilization. B: rate of glucose appearance (Ra), i.e., hepatic glucose production. *P < 0.01 vs. controls.

 
Hepatic aspects. After 2 days of HF diet, there was a significant increase in hepatic TG content (50.3 ± 3 mg/g liver vs. 31.7 ± 2.4 mg/g in chow-fed rats, P < 0.01) and a decrease in glycogen content (17.8 ± 1.4 vs. 28.5 ± 3.8 mg/g, P < 0.01).

Tissue NE turnover. In pancreas and brain, there was a significant increase in NE0 in HF rats and a slight increase (not significant) in liver (Table 2). The fractional turnover constant (k) was twofold decreased in pancreas and liver of HF rats, which indicated a twofold increase in NE half-life in these organs, whereas it remained constant in the whole brain. The TR was calculated by multiplying NE0 and k. It reached significance only in brain, since in liver and pancreas these parameters were varying in opposite ways (Table 2).


View this table:
[in this window]
[in a new window]
  		Table 2. NE0 in pancreas, liver, and brain, k, and TR of NE in HF rats and controls

 
Hypothalamic NE. Basal hypothalamic extracellular NE levels were similar in both groups (~20 pg/20 µl). During the 30 min after the intraperitoneal glucose injection (1 g/kg body wt), we observed a drastic decrease (–78 ± 3%) in extracellular NE concentration in the chow-fed group (Fig. 4), whereas the decrease was much smaller in HF rats (–26 ± 5%). This discrepancy between groups could indicate changes in nervous activity of neurons in the VMH/PVN area.



View larger version (8K):
[in this window]
[in a new window]
  		Fig. 4. Percentage of baseline extracellular norepinephrine concentration 30 min after an ip glucose injection (1 g/kg body wt) in high fat-fed rats (open bar) and controls (filled bar). NA, numerical aperture. *P < 0.01 vs. controls.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This study demonstrates that HF diet rapidly results in an increased GIIS (2 days) and a hepatic insulin resistance, followed by a glucose intolerance (1 wk). After 8 wk, insulin hypersecretion in response to glucose is not observed anymore, which can be explained by the inability of the pancreas to maintain hypersecretion for facing long-standing insulin resistance. Rats submitted to the HF diet became massively glucose intolerant, but they did not display basal hyperglycemia or hyperinsulinemia. Moreover, 60 min after glucose injection, their glycemia came back to the level of control rats (Fig. 1), indicating that insulin was still efficient enough to maintain basal normoglycemia.

Because some studies showed that exaggerated insulin secretion can be the first step of a cascade of events leading to an impaired insulin secretion and glucose homeostasis (3, 16), and finally to type 2 diabetes (15), we then decided to focus on day 2 of the HF diet, when insulin hypersecretion in response to glucose and hepatic insulin resistance appeared, i.e., the key period for the further deterioration of glucose homeostasis. Increased TG storage in the liver was probably the main change responsible for this feature, since accumulation of lipid esterification products has been shown to play an important role in the setting of hepatic insulin resistance (25, 27, 48), and several studies showed that hepatic insulin resistance could be reversed by a decrease in TG content (27). We observed a lower glycogen storage in our model, which is consistent with studies indicating that glycogen storage is inversely correlated with TG storage in liver of insulin-resistant HF rats (25). Because there was no increase in circulating FFA in HF rats throughout the diet, insulin hypersecretion observed after 2 days could be interpreted as a response to an increased insulin demand, caused by hepatic insulin resistance (49), rather than a direct effect of fatty acid on the {beta}-cell. Modifications of basal NE content in pancreas and k in both liver and pancreas could also account for alterations in insulin secretion and efficiency on the liver (2), as a reflection of sympathetic nervous changes. Moreover, an increase in basal NE content is often associated with diabetes (1). Another interesting finding from our study was a change in central noradrenergic activity in HF rats, put in evidence by a lower decrease in NE extracellular concentration in response to glucose stimulus (microdialysis study) and an increase in brain NE turnover, after 2 days of diet. Because both the pancreas (2) and target organs of insulin are richly innervated and controlled by the autonomic nervous system (reviewed in Refs. 36 and 37), whose fibers are emerging from the VMH area (26), we postulated that the modifications of PVN/VMH noradrenergic activity could result in a decreased sympathetic tone involved in {beta}-cell hyperresponsiveness. Consistent with this hypothesis, it has already been shown that a decrease in the sympathetic tone was responsible, at least in part, for insulin hypersecretion in rats infused with lipids (31). Because it is well established that the NE released by the postsynaptic nerve exerts its inhibitory effect on insulin secretion through {alpha}2A-adrenoreceptors (2, 4), we used oxymetazoline, an {alpha}2A-adrenoreceptor agonist, to overcome the postulated decrease in sympathetic tone. With the use of increasing amounts of oxymetazoline, {beta}-cell hyperresponsiveness was progressively blunted in HF rats; the dose of 1 pmol/kg oxymetazoline completely normalized their GIIS compared with controls, clearly indicating the involvement of a low sympathetic activity in the high insulin response in HF rats. Because {alpha}2A-adrenoreceptors are present not only on the {beta}-cell membrane but also in some hypothalamic areas (12), it can be hypothesized that this low activity may be of peripheral and/or central origin. The normalization of insulin secretion with oxymetazoline was associated with a higher glycemia profile, thus attesting that the higher GIIS in HF rats was necessary to compensate liver insulin resistance. In control rats, the lower doses of oxymetazoline had no detectable effect on GIIS. The likely decrease in sympathetic tone in HF rats could per se account for the islet hypersensitivity to oxymetazoline in HF rats, consistent with studies reporting interrelationships between autonomic nervous system activity and the expression of adrenergic receptors in humans (19, 45, 53) and dogs (50).

Because there was no increase in plasma FFA or TG, a local increase in the hypothalamic area was also unlikely. To explain central nervous modifications, we could hypothesize that high dietary fat intake triggered modifications of nervous and/or hormonal signals from peripheral organs to brain (17, 21–23). Indeed, it has been shown that afferent inputs from the liver and the gastrointestinal tract, as well as from peptides such as CCK, are transmitted through the vagus nerve and sympathetic fibers to the nucleus of the solitary tract and hypothalamus where they are integrated (32, 44). Because such a mechanism is implicated in the regulation of feeding, one could hypothesize that it could also be more broadly implicated in the regulation of insulin secretion and action. In addition, glucose sensors have been detected in the portal vein (9, 18), lipid sensors could exist and mediate to the brain the modifications induced by the HF diet. Consistent with this idea, it has been shown that a jejunum administration of linoleic acid increased the activity of neurons in the PVN of the hypothalamus (38) through the increase of hepatic vagal afferent activity (39). Among the hormonal signals from periphery to brain that could also trigger CNS modifications, ghrelin, a gut hormone acting on the hypothalamus to regulate food intake (20, 35, 52) and reported to inhibit insulin secretion (41, 51), could be a candidate. We found that the concentration of plasma ghrelin was decreased in HF rats after 2 and 7 days of diet, when insulin hypersecretion in response to glucose was observed; in contrast, after 2 mo of diet, there was no difference in plasmatic ghrelin concentration between HF rats vs. controls, and no more insulin hypersecretion. These findings are thus consistent with the involvement of ghrelin on the regulation of pancreatic insulin secretion.

In conclusion, Wistar rats submitted to a HF diet rapidly displayed a dysregulation of insulin secretion and action, accompanied with CNS activity modifications and independently of a peripheral increase of TG or FFA. Glucose intolerance set in during the 1st wk of the diet and worsened, while insulin hypersecretion in response to glucose decreased. Thus this model could be convenient for the study of diet-induced insulin resistance and glucose intolerance, especially to evaluate by which mechanisms a dietary lipid overload could act on CNS activity and dysregulate glucose homeostasis by this indirect way.


   FOOTNOTES
 

Address for reprint requests and other correspondence: C. Cruciani-Guglielmacci, Université Paris 7, CNRS UMR 7059, case 7126, 2, place Jussieu, 75251 Paris cedex 05, France (E-mail: cruciani{at}paris7.jussieu.fr)

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.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Adeghate E, Ponery AS, and Sheen R. Streptozotocin-Induced diabetes mellitus is associated with increased pancreatic tissue levels of noradrenaline and adrenaline in the rat. Pancreas 22: 311–316, 2001.[CrossRef][Web of Science][Medline]
  2. Ahrén B. Autonomic regulation of islet hormone secretion-implication for health and disease. Diabetologia 43: 393–410, 2000.[CrossRef][Web of Science][Medline]
  3. Ahren B, Gudbjartsson T, Al-Amin AN, Martensson H, Myrsen-Axcrona U, Karlsson S, Mulder H, and Sundler F. Islet perturbations in rats fed a high-fat diet. Pancreas 18: 75–83, 1999.[CrossRef][Web of Science][Medline]
  4. Angel I, Niddam R, and Langer SZ. Involvement of alpha-2 adrenergic receptor subtypes in hyperglycemia. J Pharmacol Exp Ther 254: 877–882, 1990.[Abstract/Free Full Text]
  5. Bays H, Mandarino L, and DeFronzo RA. Role of the adipocyte, free fatty acids, and ectopic fat in pathogenesis of type 2 diabetes mellitus: peroxisomal proliferator-activated receptor agonists provide a rational therapeutic approach. J Clin Endocrinol Metab 89: 463–478, 2004.[Free Full Text]
  6. Berk PD and Stump DD. Mechanisms of cellular uptake of long chain free fatty acids. Mol Cell Biochem 192: 17–31, 1999.[CrossRef][Web of Science][Medline]
  7. Boden G. Effects of free fatty acids (FFA) on glucose metabolism: significance for insulin resistance and type 2 diabetes. Exp Clin Endocrinol Diabetes 111: 121–124, 2003.[CrossRef][Web of Science][Medline]
  8. Brodie B, Costa E, Dlabec A, Neff N, and Snookler H. Application of steady state kinetics to the estimation of synthesis rate and turnover time in tissue catecholamines. J Pharmacol Exp Ther 154: 493–498, 1966.[Abstract/Free Full Text]
  9. Burcelin R, Dolci W, and Thorens B. Portal glucose infusion in the mouse induces hypoglycemia: evidence that the hepatoportal glucose sensor stimulates glucose utilization. Diabetes 49: 1635–1642, 2000.[Abstract]
  10. Chalkley SM, Hettiarachchi M, Chisholm DJ, and Kraegen EW. Long-term high-fat feeding leads to severe insulin resistance but not diabetes in Wistar rats. Am J Physiol Endocrinol Metab 282: E1231–E1238, 2002.[Abstract/Free Full Text]
  11. Chavez M, Seeley RJ, Havel PJ, Friedman MI, Matson CA, Woods SC, and Schwartz MW. Effect of a high-fat diet on food intake and hypothalamic neuropeptide gene expression in streptozotocin diabetes. J Clin Invest 102: 340–346, 1998.[Web of Science][Medline]
  12. Chong W, Li LH, Lee K, Lee MH, Park JB, and Ryu PD. Subtypes of alpha1- and alpha2-adrenoceptors mediating noradrenergic modulation of spontaneous inhibitory postsynaptic currents in the hypothalamic paraventricular nucleus. J Neuroendocrinol 16: 450–457, 2004.[CrossRef][Web of Science][Medline]
  13. DeFronzo RA. Pathogenesis of type 2 (non-insulin dependent) diabetes mellitus: a balanced overview. Diabetologia 35: 389–397, 1992.[CrossRef][Web of Science][Medline]
  14. Felber JP. From obesity to diabetes. Pathophysiological considerations. Int J Obes Relat Metab Disord 16: 937–952, 1992.[Web of Science][Medline]
  15. Felber JP and Golay A. Pathways from obesity to diabetes. Int J Obes Relat Metab Disord 26, Suppl 2: S39–S45, 2002.
  16. Hansen BC and Bodkin NL. Beta-cell hyperresponsiveness: earliest event in development of diabetes in monkeys. Am J Physiol Regul Integr Comp Physiol 259: R612–R617, 1990.[Abstract/Free Full Text]
  17. Havel PJ. Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis. Exp Biol Med (Maywood) 226: 963–977, 2001.[Abstract/Free Full Text]
  18. Hevener AL, Bergman RN, and Donovan CM. Novel glucosensor for hypoglycemic detection localized to the portal vein. Diabetes 46: 1521–1525, 1997.[Abstract]
  19. Hogikyan RV and Supiano MA. Arterial {alpha}-adrenergic responsiveness is decreased and SNS activity is increased in older humans. Am J Physiol Endocrinol Metab 266: E717–E724, 1994.[Abstract/Free Full Text]
  20. Horvath TL, Diano S, Sotonyi P, Heiman M, and Tschop M. Minireview: ghrelin and the regulation of energy balance–a hypothalamic perspective. Endocrinology 142: 4163–4169, 2001.[Abstract/Free Full Text]
  21. Konturek SJ, Zabielski R, Konturek JW, and Czarnecki J. Neuroendocrinology of the pancreas; role of brain-gut axis in pancreatic secretion. Eur J Pharmacol 481: 1–14, 2003.[CrossRef][Web of Science][Medline]
  22. Korner J and Leibel RL. To eat or not to eat: how the gut talks to the brain. N Engl J Med 349: 926–928, 2003.[Free Full Text]
  23. Kreier F, Kalsbeek A, Ruiter M, Yilmaz A, Romijn JA, Sauerwein HP, Fliers E, and Buijs RM. Central nervous determination of food storage-a daily switch from conservation to expenditure: implications for the metabolic syndrome. Eur J Pharmacol 480: 51–65, 2003.[CrossRef][Web of Science][Medline]
  24. Kreier F, Yilmaz A, Kalsbeek A, Romijn JA, Sauerwein HP, Fliers E, and Buijs RM. Hypothesis: shifting the equilibrium from activity to food leads to autonomic unbalance and the metabolic syndrome. Diabetes 52: 2652–2656, 2003.[Abstract/Free Full Text]
  25. Kusunoki M, Tsutsumi K, Hara T, Ogawa H, Nakamura T, Miyata T, Sakakibara F, Fukuzawa Y, Suga T, Kakumu S, and Nakaya Y. Correlation between lipid and glycogen contents in liver and insulin resistance in high-fat-fed rats treated with the lipoprotein lipase activator NO-1886. Metabolism 51: 792–795, 2002.[CrossRef][Web of Science][Medline]
  26. la Fleur SE, Kalsbeek A, Wortel J, and Buijs RM. Polysynaptic neural pathways between the hypothalamus, including the suprachiasmatic nucleus, and the liver. Brain Res 871: 50–56, 2000.[CrossRef][Web of Science][Medline]
  27. Lam TK, Carpentier A, Lewis GF, van de Werve G, Fantus IG, and Giacca A. Mechanisms of the free fatty acid-induced increase in hepatic glucose production. Am J Physiol Endocrinol Metab 284: E863–E873, 2003.[Abstract/Free Full Text]
  28. Levin B. Reduced norepinephrine turnover in organs and brains of obesity-prone rats. Am J Physiol Regul Integr Comp Physiol 268: R389–R394, 1995.[Abstract/Free Full Text]
  29. Levin B, Triscari J, and Sullivan A. Relationship between sympathetic activity and diet-induced obesity in two rat strains. Am J Physiol Regul Integr Comp Physiol 245: R364–R371, 1983.[Abstract/Free Full Text]
  30. Levin BE and Dunn-Meynell AA. Reduced central leptin sensitivity in rats with diet-induced obesity. Am J Physiol Regul Integr Comp Physiol 283: R941–R948, 2002.[Abstract/Free Full Text]
  31. Magnan C, Collins S, Berthault M, Kassis N, Vincent M, Gilbert M, Pénicaud L, Ktorza A, and Assimacopoulos-Jeannet F. Lipid infusion lowers sympathetic nervous activity and leads to increased {beta}-cell responsiveness to glucose. J Clin Invest 103: 413–419, 1999.[Web of Science][Medline]
  32. Marx J. Cellular warriors at the battle of the bulge. Science 299: 846–849, 2003.[Abstract/Free Full Text]
  33. McGarry JD. Banting lecture 2001: dysregulation of fatty acid metabolism in the etiology of type 2 diabetes. Diabetes 51: 7–18, 2002.[Free Full Text]
  34. Meigs JB. Epidemiology of the metabolic syndrome, 2002. Am J Manag Care 8: S283–S286, 2002.[Web of Science][Medline]
  35. Nakazato M, Murakami N, Date Y, Kojima M, Matsuo H, Kangawa K, and Matsukura S. A role for ghrelin in the central regulation of feeding. Nature 409: 194–198, 2001.[CrossRef][Medline]
  36. Niijima A. Neural mechanisms in the control of blood glucose concentration. J Nutr 119: 833–840, 1989.[Abstract/Free Full Text]
  37. Nonogaki K. New insights into sympathetic regulation of glucose and fat metabolism. Diabetologia 43: 533–549, 2000.[CrossRef][Web of Science][Medline]
  38. Paxinos G and Watson C. The Rat Brain in Stereotaxic Coordinates. New York: Academic, 1986.
  39. Randich A, Chandler PC, Mebane HC, Turnbach ME, Meller ST, Kelm GR, and Cox JE. Jejunal administration of linoleic acid increases activity of neurons in the paraventricular nucleus of the hypothalamus. Am J Physiol Regul Integr Comp Physiol 286: R166–R173, 2004.[Abstract/Free Full Text]
  40. Randich A, Spraggins DS, Cox JE, Meller ST, and Kelm GR. Jejunal or portal vein infusions of lipids increase hepatic vagal afferent activity. Neuroreport 12: 3101–3105, 2001.[CrossRef][Web of Science][Medline]
  41. Ravussin E. Energy metabolism in obesity. Studies in the Pima Indians. Diabetes Care 16: 232–238, 1993.[Abstract]
  42. Reimer MK, Pacini G, and Ahren B. Dose-dependent inhibition by ghrelin of insulin secretion in the mouse. Endocrinology 144: 916–921, 2003.[Abstract/Free Full Text]
  43. Roehrig KL and Allred JB. Direct enzymatic procedure for the determination of liver glycogen. Anal Biochem 58: 414–421, 1974.[CrossRef][Web of Science][Medline]
  44. Rouch C, Meile MJ, and Orosco M. Extracellular hypothalamic serotonin and plasma amino acids in response to sequential carbohydrate and protein meals. Nutr Neurosci 6: 117–124, 2003.[CrossRef][Web of Science][Medline]
  45. Schwartz MW, Woods SC, Porte D Jr, Seeley RJ, and Baskin DG. Central nervous system control of food intake. Nature 404: 661–671, 2000.[Medline]
  46. Supiano MA, Hogikyan RV, Stoltz AM, Orstan N, and Halter JB. Regulation of venous {alpha}-adrenergic responses in older humans. Am J Physiol Endocrinol Metab 260: E599–E607, 1991.[Abstract/Free Full Text]
  47. Taylor SI, Accili D, and Imai Y. Insulin resistance or insulin deficiency. Which is the primary cause of NIDDM? Diabetes 43: 735–740, 1994.[Web of Science][Medline]
  48. Unger R. Lipotoxicity in the pathogenesis of obesity-dependent NIDDM. Genetic and clinical implications. Diabetes 44: 863–870, 1995.[Abstract]
  49. Unger RH. Weapons of lean body mass destruction: the role of ectopic lipids in the metabolic syndrome. Endocrinology 144: 5159–5165, 2003.[Abstract/Free Full Text]
  50. Unger RH and Orci L. Diseases of liporegulation: new perspective on obesity and related disorders. FASEB J 15: 312–321, 2001.[Abstract/Free Full Text]
  51. Valette H, Deleuze P, Syrota A, Delforge J, Crouzel C, Fuseau C, and Loisance D. Canine myocardial beta-adrenergic, muscarinic receptor densities after denervation: a PET study. J Nucl Med 36: 140–146, 1995.[Abstract/Free Full Text]
  52. Wierup N, Yang S, McEvilly RJ, Mulder H, and Sundler F. Ghrelin is expressed in a novel endocrine cell type in developing rat islets and inhibits insulin secretion from INS-1 (832/13) cells. J Histochem Cytochem 52: 301–310, 2004.[Abstract/Free Full Text]
  53. Yoshihara F, Kojima M, Hosoda H, Nakazato M, and Kangawa K. Ghrelin: a novel peptide for growth hormone release and feeding regulation. Curr Opin Clin Nutr Metab Care 5: 391–395, 2002.[CrossRef][Web of Science][Medline]
  54. Zoukos Y, Thomaides T, Pavitt DV, Leonard JP, Cuzner ML, and Mathias CJ. Up-regulation of beta-adrenoceptors on circulating mononuclear cells after reduction of central sympathetic outflow by clonidine in normal subjects. Clin Auton Res 2: 165–170, 1992.[CrossRef][Medline]




This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
288/1/E148    most recent
00225.2004v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (5)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Cruciani-Guglielmacci, C.
Right arrow Articles by Magnan, C.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Cruciani-Guglielmacci, C.
Right arrow Articles by Magnan, C.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2005 by the American Physiological Society.