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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES

Objective: This study investigated the effect of different sodium content diets on rat adipose tissue carbohydrate metabolism and insulin sensitivity.

Methods and Procedures: Male Wistar rats were fed on normal- (0.5% Na+; NS), high- (3.12% Na+; HS), or low-sodium (0.06% Na+; LS) diets for 3, 6, and 9 weeks after weaning. Blood pressure (BP) was measured using a computerized tail-cuff system. An intravenous insulin tolerance test (ivITT) was performed in fasted animals. At the end of each period, rats were killed and blood samples were collected for glucose and insulin determinations. The white adipose tissue (WAT) from abdominal and inguinal subcutaneous (SC) and periepididymal (PE) depots were weighed and processed for adipocyte isolation and measurement of in vitro rates of insulin-stimulated 2-deoxy-d-[3H]-glucose uptake (2DGU) and conversion of -[U-14C]-glucose into 14CO2.

Results: After 6 weeks, HS diet significantly increased the BP, SC and PE WAT masses, PE adipocyte size, and plasma insulin concentration. The sodium dietary content did not influence the whole-body insulin sensitivity. A higher half-maximal effective insulin concentration (EC50) from the dose-response curve of 2DGU and an increase in the insulin-stimulated glucose oxidation rate were observed in the isolated PE adipocytes from HS rats.

Discussion: The chronic salt overload enhanced the adipocyte insulin sensitivity for glucose uptake and the insulin-induced glucose metabolization, contributing to promote adipocyte hypertrophy and increase the mass of several adipose depots, particularly the PE fat pad.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES

Hypertension is a great risk factor for cardiovascular and renal diseases. Elevated blood pressure (BP) often coexists with atherogenic dyslipidemia, glucose intolerance, and abdominal obesity, and it is one of the clinical criteria for the diagnosis of metabolic syndrome (1). Insulin resistance is considered the major metabolic abnormality underlying all these pathologic processes and type 2 diabetes mellitus (1). Moreover, insulin resistance and hyperinsulinemia may elevate BP through several mechanisms (2), increasing the propensity of diabetic patients to develop hypertension (3).

It has been recognized that some unidentified genes and acquired factors such as overweight, obesity, excessive salt intake, alcohol consumption, physical inactivity, and environmental stress are involved in the pathogenesis of hypertension in human subjects (4,5). Several epidemiological studies (6,7,8) and researches that evaluated small samples of normotensive (9,10) and hypertensive (10,11) individuals demonstrated a direct correlation between the sodium intake and the BP. In addition, reduction in systolic and diastolic BP levels was observed during low salt intake (12).

The effect of sodium consumption on insulin and carbohydrate metabolism has been investigated through the employment of different methods and models. Some euglycemic-hyperinsulinemic clamp studies revealed a strong relationship between high salt intake and insulin resistance in human (13,14) and animal (15,16) models. Salt-sensitive normotensive (17) and hypertensive (18) individuals, in comparison with salt-resistant counterparts, presented resistance to insulin-mediated glucose disposal and a hyperinsulinemic response to an oral glucose overload when submitted to a high-sodium (HS) diet. On the other hand, impairment in whole-body insulin sensitivity was also related to salt restriction (19,20,21,22,23), what raises doubts about the efficacy of low-sodium (LS) diets in the prevention of cardiovascular events by decreasing BP. Thus, faced with these controversial results, much remains unclear regarding the role of the salt dietary content to the development of insulin resistance.

In a previous report, it was shown that isolated periepididymal (PE) adipocytes from chronically (9 weeks from weaning) fed HS rats presented higher insulin-dependent glucose metabolism compared to LS animal cells, but without altering insulin sensitivity for glucose uptake in vitro (24). As a group receiving a normal-sodium (NS) diet was not included in this study and in order to get more insight into the effects of sodium intake on white adipose tissue (WAT) metabolic response, this study was planned to evaluate some aspects of the glucose metabolism and the insulin sensitivity of adipocytes from rats fed on HS and on LS diets, in comparison to those on NS diet and in different periods from weaning to adulthood.

Methods and Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES

Animals

Male Wistar rats from the Animal Resource of the Institute of Biomedical Sciences of the University of Sao Paulo were fed on either NS (0.5% Na+), LS (0.06% Na+), or HS (3.12% Na+) diets (RHOSTER Industry and Limited Commerce, Vargem Grande Paulista, Brazil) from weaning to adulthood. Rats were housed three per cage in a controlled temperature environment (25 ± 2 °C), under a 12/12-h light/dark cycle (lights on at 6:00 am) and given food and water ad libitum. Body weight was weekly measured. All experimental procedures reported here were in accordance with the Guidelines for Ethical Care of Experimental Animals and were approved by the Ethical Committee for Animal Research of the Institute of Biomedical Sciences of the University of Sao Paulo (No. 218/02). During the sixth week of the study period, tail-cuff systolic BP and heart rate (HR) were determined and an intravenous insulin tolerance test (ivITT) was performed. At the end of the third, sixth, and ninth weeks, the animals were killed (8:00 am) by decapitation under pentobarbital sodium anesthetic (4 mg/100 g body weight, intraperitoneally) after a 12-h fast. Trunk blood was collected and the serum used for glucose and insulin measurements. The subcutaneous (SC) WAT from abdominal and inguinal depots and both PE fat pads were excised, weighed, and processed for adipocyte isolation and measurement of in vitro rates of insulin-stimulated 2-deoxy-d-[3H]-glucose uptake (2DGU) and conversion of d-[U-14C]-glucose into 14CO2.

Tail-cuff BP and HR measurements

BP and HR were measured using a computerized tail-cuff system (Kent Scientific, Torrington, CT). For BP measurement, rats were placed in a warm rat restraining apparatus. A cuff was placed around the rat's tail and insufflated until blood flow was occluded, and then released until the first pulses of arterial flow could be detected and recorded (systolic BP) on a microcomputer (AT/CODAS, 100-Hz sampling rate, DataQ Instruments, Akron, OH). Three BP measurements were performed and BP was determined for each animal by averaging the obtained values. HR was obtained from BP pulse records.

ivITT

ivITT was performed in pentobarbital sodium (5 mg/100 g body weight, intraperitoneally) anesthetized animals after a 12-h fast. The regular insulin (Novo Nordisk, Montes Claros, Minas Gerais, Brazil) load was injected as a bolus via dorsal vein of the penis. The blood glucose levels were measured in samples obtained from tail vein using a glucometer (One Touch Ultra LifeScan, Milpitas, CA) at the following times: 0, 3, 6, 9, 12, 15, 20, 25, and 30 min after insulin infusion. The corresponding 3–20 min values were used to calculate the rate constant for plasma glucose disappearance (Kitt) according to the method of Bonora et al. (25).

Adipocyte isolation

SC and PE fat pads were minced using fine scissors, digested at 37 °C in Earle's salts, 25 mmol/l 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 4% bovine serum albumin, pH 7.4 (Earle-HEPES-bovine serum albumin (EHB) buffer 1) containing collagenase type II (0.8 mg/ml), and the adipocytes were isolated according to Rodbell (26). The isolated adipocytes (∼7–8 × 105 cells/ml) were suspended in EHB buffer 2 (Earle's salts, 20 mmol/l HEPES, 1% bovine serum albumin, 2 mmol/l sodium pyruvate, and 4.8 mmol/l NaHCO3), pH 7.4, at 37 °C. Cell size and number were determined as described previously (27).

Insulin-stimulated 2DGU rates

2DGU experiments were performed as described elsewhere with some modifications (28). Briefly, aliquots (40 μl) of isolated adipocytes (10% cell suspension) were transferred to 2-ml plastic test tubes with or without insulin (0.025, 0.1, 0.25, 0.5, 1.0, 2.5, and 10 nmol/l) diluted in EHB buffer 2 (pH 7.4), and the cells were then incubated for 15 min in a water bath at 37 °C. At the end of the incubation period, it was added a 10-μl aliquot of 2-deoxy-D-[3H]-glucose (3H-2DG, 0.4 mmol/l final concentration and 0.05 μCi/tube) was added to it and the uptake reaction was allowed to occur for exactly 3 min. 2DGU was interrupted by adding 250 μl of ice-cold phloretin (0.3 mmol/l in EHB and DMSO 0.05%). Next, 200 μl of this last mixture were transferred to microfuge tubes (450-μl capacity) layered with 200 μl of silicone oil (density = 0.963 mg/ml) and centrifuged (Microfuge E, Beckman Instruments, Palo Alto, CA) for 10 s at 11,000 g. The cell pellet on top of the oil layer was removed to 4-ml vials containing 3 ml of scintillation cocktail (EcoLume, ICN Pharmaceuticals, Costa Mesa, CA), and the trapped radioactivity was measured in a liquid scintillation counter (Tri Carb 2100TR, Packard Instrument, Meriden, CT). Unspecific 3H-2DG radiolabel trapping was determined in a parallel tube already prepared with 250 μl of ice-cold phloretin to stop transport reaction before adding the tracer. This value was discounted from the total trapping, and the resultant specific uptake was recalculated to be expressed as picomole per square centimeter of cell surface area.

Adipose cell sensitivity to insulin was evaluated by the determination of the half-maximal effective insulin concentration (EC50) from the 2DGU dose-response curve (29).

Conversion of d-[U-14C]-glucose into 14CO2

From a 10% adipocyte suspension in Krebs/Ringer/phosphate buffer pH 7.4, with 1% bovine serum albumin and 2 mmol/l glucose, at 37 °C and saturated with a gas mixture of CO2 (5%)/O2 (95%), 450-μl aliquots were transferred to polypropylene test tubes containing 5 μl (0.05 μCi/tube) of d-[U-14C]-glucose, in the presence or absence of insulin (10 nmol/l). These samples were then incubated (final volume = 500 μl) for 1 h at 37 °C in a water bath. The tubes had a rubber stopper and the atmosphere inside was enriched with CO2 (5%)/O2 (95%). At the end of incubation, the stopper was removed, 0.2 ml H2SO4 (8 N) was added and a 4-ml scintillation vial containing a 2 × 4 cm2 piece of filter paper moistened with 0.2 ml of ethanolamine, was immediately placed (mouth-to-mouth) on the top of the reaction tube and the point of connection between the tubes was sealed with a stripe of plastic film. The assembled tubes were incubated for additional 30 min. At the end of incubation, the scintillation vial with the filter paper was filled with 3 ml of scintillation cocktail (EcoLume, ICN Pharmaceuticals, Costa Mesa, CA) for measurement of adsorbed radioactivity. The results were expressed as nanomoles/106 cells/h.

Serum glucose and insulin measurements

Serum glucose determination was performed using the enzymatic glucose-oxidase/peroxidase method (30) available as a commercial kit (Glicose SL-e, CELM, Sao Paulo, Brazil). Serum insulin levels were quantified using a specific rat insulin radioimmunoassay kit (Linco Research, St. Charles, MO). The estimated intra-assay coefficient of variation was <5%.

Statistical analyses

Statistical procedures were performed using the one-way ANOVA, followed by Bonferroni post-tests for multiple comparisons among groups. Data are expressed as mean ± s.e. P values <0.05 were considered statistically significant. The analysis was performed by the statistical software package GraphPad Prism version 4.0 for Windows (GraphPad Software, San Diego, CA).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES

Body weight, fat pad weights, and adipocytes size

No significant difference (P > 0.05) was observed in body weight of NS (50.6 ± 1.2, n = 17), LS (51.3 ± 1.7, n = 16), and HS (50.3 ± 1.5, n = 17) groups at weaning (3 weeks of age) and after 3, 6, and 9 weeks of diet (Table 1).

Table 1.  Effect of chronic (3, 6, and 9 weeks) administration of LS, NS, and HS diets on body weight, fat pads weight, and adipocytes volume
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As indicated in Table 1, the SC and PE WAT weights were higher in HS rats than in NS and LS rats (P < 0.01) after 6 weeks of treatment. The volume of PE adipocytes was increased in HS rats (P < 0.05) compared with NS and LS rats at this same week of study, while the SC adipocytes size did not differ significantly among the three groups throughout the 9-week period.

BP and HR

In the sixth week of treatment, the systolic BP was higher (P < 0.001) in rats on HS (143 ± 2 mm Hg, n = 8) than in those on NS (120 ± 3 mm Hg, n = 7) or LS (115 ± 2 mm Hg, n = 7) diets. No significant difference was observed in the HR among the groups in this period (Figure 1).

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Figure 1. Effect of chronic (6 weeks) administration of low-sodium (LS), normal-sodium (NS), and high-sodium (HS) diets on (a) blood pressure (BP) and (b) heart rate (HR) of rats. Values are mean ± s.e., LS, n = 7–8 rats/group. *P < 0.001 vs. NS and LS.

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Glucose and insulin levels

As indicated in Figure 2a, no significant difference (P > 0.05) was observed in serum glucose concentration of NS, LS, and HS groups during the 9-week follow-up period of study.

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Figure 2. Effect of chronic (3, 6, and 9 weeks) administration of low-sodium (LS), normal-sodium (NS), and high-sodium (HS) diets on (a) glucose and (b) insulin serum levels. Values are mean ± s.e., LS,n = 9–19; NS, n = 9–19; HS, n = 6–19 rats. *P < 0.05 vs. NS and LS values of the same week.

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HS animals exhibited higher fasting serum insulin levels (P < 0.05) than NS and LS rats after 3 and 6 weeks of diet (Figure 2b).

ivITT

In the sixth week of diet, no differences were observed among groups in the glycemic curves (Figure 3a) and Kitt values (Figure 3b) during an ivITT, indicating that the sodium dietary content seemed not to influence the whole-body insulin sensitivity.

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Figure 3. Effect of chronic (6 weeks) administration of low-sodium (LS), normal-sodium (NS), and high-sodium (HS) diets on (a) glycemic curve and (b) Kitt during an intravenous insulin tolerance test in 12-h fasted animals. Values are mean ± s.e., LS, n = 7; NS, n = 6; HS, n = 7 rats.

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Insulin-stimulated 2DGU rates

The results from the in vitro studies on isolated adipocytes are shown in Figures 4,5,6.Figures 4 and 5 show the effect of LS, NS, and HS diets on 2DGU. In the SC adipocytes, both diets did not alter either the basal or the insulin-stimulated (maximal) rates of glucose transport compared to NS rats (Figure 4a). The PE adipocytes of HS animals presented the lowest basal 2DGU rate among the groups during the first period of observation (3 weeks, Figure 4b). The LS diet significantly increased the maximal 2DGU rate in PE adipocytes (P < 0.01 vs. NS) after 9 weeks of treatment (Figure 4b).

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Figure 4. Effect of chronic (3, 6, and 9 weeks) administration of low-sodium (LS), normal-sodium (NS), and high-sodium (HS) diets on 2-deoxy-d-[3H]-glucose uptake (2DGU) in subcutaneous (SC) and periepididymal (PE) isolated adipocytes from rats. Values are mean ± s.e., LS, n = 5–15; NS, n = 7–17; HS, n = 7–15 rats. *P < 0.05 vs. NS and LS; **P < 0.01 vs. NS values of the same week. Bs, basal; Mx, maximal.

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image

Figure 5. Effect of chronic (6 weeks) administration of low-sodium (LS), normal-sodium (NS), and high-sodium (HS) diets on 2-deoxy-d-[3H]-glucose uptake (2DGU) curve and half-maximal effective insulin concentration (EC50) in subcutaneous (SC) and periepididymal (PE) isolated adipocytes from rats. Values are mean ± s.e., LS, n = 6–8; NS, n = 7–10; HS, n = 7–9 rats. *P < 0.001 vs. NS; **P < 0.001 vs. LS;***P < 0.01 vs. NS and LS.

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image

Figure 6. Effect of chronic (6 weeks) administration of low-sodium (LS), normal-sodium (NS), and high-sodium (HS) diets on conversion of d-[U-14C]-glucose into CO2 in subcutaneous (SC) and periepididymal (PE) isolated adipocytes from rats. Values are mean ± s.e., LS, n = 8–14; NS, n = 9–14; HS, n = 9–12 rats. *P < 0.05 vs. NS and LS; **P < 0.05 vs. NS values of the same week. Bs, basal; Mx, maximal.

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Despite the similar values in the 2DGU dose-response curve after 6 weeks of diets, the EC50 was significantly higher in the PE adipocytes of HS animals in comparison with NS and LS cells, indicating the higher insulin sensitivity for this process (Figure 5). The EC50 value of SC adipocytes also differed significantly among the three experimental groups, being lower in LS animals (Figure 5).

Insulin-stimulated rates of d-[U-14C]-glucose conversion

As depicted in Figure 6a, the SC adipocytes isolated from HS group presented the highest basal and insulin-induced rates of glucose conversion to CO2 after 9 weeks of diet. HS animals also exhibited a significant increase in the PE adipocytes maximal ability to oxidize glucose to CO2 compared to NS rats during the sixth week of diet (Figure 6b). Thus, the increased glucose metabolic rate in SC and visceral adipocytes from HS rats also evidences the higher insulin responsiveness in this group.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES

The main aim of this study was to investigate the effect of sodium dietary content on metabolic response to insulin of WAT depots of Wistar rats in different occasions from their weaning to adulthood (12 weeks old). Our results demonstrated a rise in the insulin sensitivity for glucose uptake and in the insulin responsiveness for glucose conversion to CO2 in the isolated PE adipocytes from salt-overloaded animals, at the same period as they presented higher PE adipose mass.

As it was demonstrated previously (21,31,32), the sodium restriction (a 0.06% Na+ diet) did not influence the animals' systolic BP and HR when they were compared to those on an NS diet. On the other hand, the 3.12% Na+ (∼8% NaCl) diet consumed by our rats during 6 weeks raised the systolic BP in the same way that this high sodium dietary content did when administered to normotensive rodents for periods like 2, 8, 9, or as long as 69 weeks (15,24,31,32,33). In the Lima and colleagues study, the HS diet-induced rise in the tail-cuff BP occurred even after a previous long-standing low salt intake (33). Thus, these studies and our results agree in demonstrating the protective role of salt restriction to avoid the BP elevation and the hypertensive effect of salt overload when administered to healthy and normotensive rats.

The salt diets were introduced immediately after weaning and at the third, sixth, and ninth weeks of consumption, the glucose serum levels did not differ among the groups, confirming previous data demonstrated in 3-month-old rats fed on similar kind of diets (21,24).

The fasting insulin serum levels were significantly more elevated in HS animals until the second experimental period (sixth week), and then, they almost matched the NS and LS values in the last one (ninth week). A similar profile was also observed in 7-week-old Sprague-Dawley rats receiving an 8% NaCl diet for 2 (high plasma insulin concentration) or 8 (similar plasma insulin concentration) weeks in comparison with a control group fed on 0.3% NaCl diet (15). However, different from that study, which an euglycemic-hyperinsulinemic clamp test was realized (15), our chronic salt overload did not reduce the animals' systemic insulin sensitivity, as evaluated by the ivITT.

The scientific investigations into the effect of salt dietary content on the insulin action and glucose homeostasis have produced controversial results. While some reports revealed a strong relationship between high salt intake and insulin resistance in human (13,14) and animals (15) models, a great number of studies demonstrated an increase in the insulin and glucose blood concentrations and an accentuated impairment in whole-body insulin sensitivity related to salt restriction (19,20,21,22,23,31,34,35).

Although insulin is a hormone with pleiotrophic effects, the determination of insulin resistance or sensitivity is based on its ability to stimulate glucose uptake in the insulin-sensitive peripheral tissues (i.e., WAT, skeletal, and cardiac muscle) (36). As the WAT importantly contributes to glucose homeostasis, we decided to evaluate the influence of sodium dietary content on some aspects of its glucose cellular metabolism. Because the assay to determinate the 3H-2DG (a nonmetabolized glucose analog) uptake rates in the presence of insulin may provide results that correlate to measurements and definitions of insulin sensitivity of a specific cell type, we performed this in vitro biological test with adipocytes isolated from SC (abdominal and inguinal) and PE (representative of visceral) adipose depots.

During each one of the three distinct experimental periods, there was only a small difference in the basal and insulin-stimulated (maximal insulin concentration) 2DGU rates among the three salt diet groups. Although in the sixth week of diet the 2DGU dose-response curves of both SC and PE adipocytes presented similar values in the several insulin concentrations, the EC50 differed among the groups being significantly lower for PE and SC (only in relation to LS) adipocytes from HS group and higher for SC adipocytes from LS group. According to a previous study (29), the EC50 calculated from a 3H-2DG uptake dose-response curve is a cellular insulin sensitivity measurement parameter and its lower value in the adipocytes from HS animals indicates a salt overloaded-induced rise in insulin sensitivity for this process. It also indicates that at more physiological concentrations of this hormone, adipocytes from HS-fed animals are more responsive.

In the sixth week of diet, PE adipocytes from HS rats presented a significant rise in the insulin-stimulated rate of glucose conversion to CO2 indicating, therefore, their higher insulin responsiveness in this process. A slight increase in the basal rate of glucose oxidation was also observed in relation to NS group, but the difference was not statistically significant. At the end of the ninth week, both insulin-stimulated and nonstimulated rates were higher in SC adipocytes from HS rats. Similar results were obtained by Lima and coworkers (24), but only in comparison with visceral adipocytes from LS animals because they did not include a normal salt diet fed group. The higher insulin receptor density also demonstrated by these authors (24) and the study of Okamoto et al. (23) that demonstrated higher glucose transporter-4 gene expression, as well as basal and insulin-stimulated glucose transporter-4 translocation to the plasma membrane in PE WAT of HS rats might help to explain the higher insulin responsiveness and sensitivity for glucose oxidation and uptake, respectively.

Glucose is an important lipogenic substrate because it allows the synthesis of pyruvate and glycerol-3-phosphate that may be transformed in fatty acids through the action of several enzymes, including the acetyl-CoA carboxylase and fatty-acid synthase. These fatty acids may be stored as triacylglycerol into lipid droplets within the adipocytes. This anabolic pathway is mainly under the insulin control, a lipogenic hormone that among other actions elevates the mRNA expression and activity of lipogenic enzymes (37,38), promoting the WAT growth by cellular hypertrophy (increase in cell size).

Thus, the higher insulin sensitivity and responsiveness in PE adipocytes might have contributed to enhance the adipocyte volume and the adipose mass in HS rats. As the adipose cells of HS animals did not present insulin resistance, we can infer that their high insulin serum levels for a prolonged period of time (until the 6 weeks of diet) also might have elevated the glucose uptake and the triacylglycerol synthesis contributing for adipose mass increase. Moreover, in a previous study, we demonstrated a HS-induced higher capacity of adipocytes to incorporate glucose into lipids and an increase in the activity of some lipogenic enzymes (glucose-6-phosphate dehydrogenase and malic enzyme), factors that also could be involved in the more intense fat accumulation in this group (39).

Data from the present work do not clarify the mechanisms involved in the effects of changes in dietary salt intake on the measured parameters; nevertheless, some speculations are pertinent. Some studies showed that bradykinin, a kallikrein kinin system component, enhanced the insulin-stimulated glucose uptake in skeletal muscle and adipose tissues by raising the glucose transporter-4 translocation to the plasma membrane and the phosphorylation of the insulin receptor β-subunit and the insulin receptor substrate-1 (40,41,42,43,44). It was recently demonstrated that high salt intake has stimulated the kallikrein kinin system and elevated the bradykinin plasma concentration in human subjects as an attempt of attenuating the hypertensive effect of sodium retention (45). Thus, considering that this modulating answer has also been observed in rats, the bradykinin peptide might be involved in the higher insulin sensitivity and responsiveness of adipocytes from salt-overloaded animals. The higher bradykinin circulating levels might raise the insulin-induced plasma membrane glucose transporter-4 translocation and then, enhance the in vivo adipocyte glucose uptake contributing to increase the adipose mass in the HS-fed rats.

As the systemic insulin sensitivity evaluated in the ivITT was not changed by the sodium dietary content, the results allow us to infer that the alteration in the insulin sensitivity was WAT specific. Besides, the effect also depended on the origin of the adipose (SC or PE) cell. Metabolic differences between the SC and visceral adipose depots, such as lipolytic and lipogenic activities, gene expression, and/or protein secretion, were well documented previously (46,47,48,49). Then, it is possible that some metabolic steps that participate and interfere to promote the local heterogeneity between SC and PE adipocytes may have been affected by the sodium dietary content modifying the insulin sensitivity for the glucose transport system in relation to NS cells. Our data do not permit to identify such responsible factor, but we infer that the heterogeneity in the insulin sensitivity for glucose uptake between SC and PE adipocytes from HS animals might be explained by differences in the number of bradykinin B2 receptors (higher in PE adipocytes), as similar differences were also extensively demonstrated for α- and β-adrenergic receptors (46,47,48,49).

Although the SC adipocytes from LS group have presented insulin resistance for glucose uptake, as it was demonstrated by the EC50 value of the 2DGU dose-response curve, this defect seemed not to be spread systemically, affecting other tissues. Besides, the in vitro rates of glucose uptake and oxidation, both in the absence and presence of insulin and in the PE and SC adipocytes from this group, were not lower than those measured in the NS-fed rats cells, demonstrating that salt restriction did not reduce adipocyte insulin responsiveness to glucose metabolization and transportation.

In summary in this study, we demonstrated that prolonged administration of a HS diet increased the BP, SC, and visceral adipose masses, and the metabolic response to insulin of rat adipocytes. The higher insulin sensitivity for glucose transport through plasma membrane and insulin responsiveness for glucose metabolization may have promoted the adipocyte hypertrophy and contributed to fat accumulation. The salt dietary content, including the chronic sodium restriction did not alter the systemic insulin sensitivity evaluated by ivITT. This work provides an evidence that high sodium intake may interfere with glucose and insulin metabolism, specifically in the WAT, contributing to raise the body adiposity in rats.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES

This work was supported by Sao Paulo State Research Foundation, grant No. 03/04409-9. We thank Luciana C. Brito, Sidney B. Peres, Maria I. C. Alonso-Vale, Cecília E.M. Costa, Patricia C. Brum, and Katt C. Mattos for technical support.

REFERENCES

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. Acknowledgment
  8. Disclosure
  9. REFERENCES