β-Adrenergic activity preserves GLUT4 protein in glycolytic fibers in fasting

Regularly swimming exercise modifies opioidergic neuromodulation in rostral ventrolateral medulla in hypertensive rats

Moderate exercise reduces arterial pressure (AP) and heart rate (HR) in spontaneously hypertensive rats (SHR) and changes neurotransmission in medullary areas involved in cardiovascular regulation. We investigated if regularly swimming exercise (SW) affects the cardiovascular adjustments mediated by opioidergic neuromodulation in the RVLM in SHR and Wistar-Kyoto (WKY) rats. Rats were submitted to 6 wks of SW. The day after the last exercise bout, α-chloralose-anesthetized rats underwent a cannulation of the femoral artery for AP and HR recordings, and Doppler flow probes were placed around the lower abdominal aorta and superior mesenteric artery. Bilateral injection of endomorphin-2 (EM-2, 0.4 mmol/L, 60 nL) into the RVLM increased MAP in SW-SHR (20 ± 4 mmHg, N = 6), which was lower than in sedentary (SED)-SHR (35 ± 4 mmHg, N = 6). The increase in MAP in SW-SHR induced by EM-2 into the RVLM was similar in SED- and SW-WKY. Naloxone (0.5 mmol/L, 60 nL) injected into the RVLM evoked an enhanced hypotension in SW-SHR (-66 ± 8 mmHg, N = 6) compared to SED-SHR (-25 ± 3 mmHg, N = 6), which was similar in SED- and SW-WKY. No significant changes were observed in HR after EM-2 or naloxone injections into the RVLM. Changes in hindquarter and mesenteric conductances evoked by EM-2 or naloxone injections into the RVLM in SW- or SED-SHR were not different. Mu Opioid Receptor expression by Western blotting was reduced in SW-SHR than in SED-SHR and SW-WKY. Therefore, regularly SW alters the opioidergic neuromodulation in the RVLM in SHR and modifies the mu opioid receptor expression in this medullary area.

Carbenoxolone enhances peripheral insulin sensitivity and GLUT4 expression in skeletal muscle of obese rats: Potential participation of UBC9 protein

AIM

This study investigates the insulin sensitizer effect of carbenoxolone (CBX) and potentially involved peripheral mechanisms.

MAIN METHODS

Taking glucose transporter 4 (GLUT4) as a marker of glucose disposal, we investigated the CBX effects on whole-body insulin sensitivity and solute carrier 2a4 (Slc2a4)/GLUT4 expression in visceral (VAT) and subcutaneous (SAT) adipose tissues and soleus muscle of monosodium glutamate (MSG)-induced obese rats. Sterol regulatory element binding protein (SREBP1), an enhancer of Slc2a4 expression was analyzed through mRNA content and SREBP1-binding to Slc2a4 promoter. Finally, the small ubiquitin-modifier conjugating enzyme 9 (UBC9), whose low content indicates accelerated GLUT4 degradation was analyzed in soleus.

KEY FINDINGS

Hypercorticosteronemia, hyperinsulinemia and low glucose decay rate in the insulin tolerance test of obese rats were restored by CBX (P < 0.05). Slc2a4/GLUT4 increased in SAT (P < 0.05) and decreased in VAT (P < 0.01) of obese rats. In soleus, obesity increased Slc2a4 but decreased GLUT4 (P < 0.01), possibly by accelerating GLUT4 degradation, as suggested by decreased UBC9 (P < 0.01). CBX restored both UBC9 and GLUT4 contents. SREBP1 did not participate in the Slc2a4 transcriptional regulation.

SIGNIFICANCE

The insulin sensitizer effect of CBX involves the increase of GLUT4 expression in soleus, indicating an increased glucose disposal in skeletal muscle. This observation reinforces the skeletal muscle as the main site of insulin-induced glucose uptake and sheds new light on the metabolic effects of 11βHSD1 inhibitors, since most of the studies so far have focused on its effects on liver and adipose tissues.

Insulin acutely triggers transcription of Slc2a4 gene: participation of the AT-rich, E-box and NFKB-binding sites

AIMS

The insulin-sensitive glucose transporter protein GLUT4 (solute carrier family 2 member 4 (Slc2a4) gene) plays a key role in glycemic homeostasis. Decreased GLUT4 expression is a current feature in insulin resistant conditions such as diabetes, and the restoration of GLUT4 content improves glycemic control. This study investigated the effect of insulin upon Slc2a4/GLUT4 expression, focusing on the AT-rich element, E-box and nuclear factor NF-kappa-B (NFKB) site.

MAIN METHODS

Rat soleus muscles were incubated during 180 min with insulin, added or not with wortmannin (phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit gamma isoform (PI3K)-inhibitor), ML9 (serine/threonine protein kinase (AKT) inhibitor) and tumor necrosis factor (TNF, GLUT4 repressor), and processed for analysis of GLUT4 protein (Western blotting); Slc2a4, myocyte enhancer factor 2a/d (Mef2a/d), hypoxia inducible factor 1a (Hif1a), myogenic differentiation 1 (Myod1) and nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (Nfkb1) messenger ribonucleic acids (mRNAs) (polymerase chain reaction (PCR)); and AT-rich- (myocyte-specific enhancer factor 2 (MEF2)-binding site), E-box- (hypoxia inducible factor 1 alpha (HIF1A)- and myoblast determination protein 1 (MYOD1)-binding site), and NFKB-binding activity (electrophoretic mobility assay).

KEY FINDINGS

Insulin increased Slc2a4 mRNA expression (140%) and nuclear proteins binding to AT-rich and E-box elements (~90%), all effects were prevented by wortmannin and ML9. Insulin also increased Mef2a/d and Myod1 mRNA expression, suggesting the participation of these transcriptional factors in the Slc2a4 enhancing effect. Conversely, insulin decreased Nfkb1 mRNA expression and protein binding to the NFKB-site (~50%). Furthermore, TNF-induced inhibition of GLUT4 expression (~40%) was prevented by insulin in an NFKB-binding repressing mechanism. GLUT4 protein paralleled the Slc2a4 mRNA regulations.

SIGNIFICANCE

Insulin enhances the Slc2a4/GLUT4 expression in the skeletal muscle by activating AT-rich and E-box elements, in a PI3K/AKT-dependent mechanism, and repressing NFKB-site activity as well. These results unravel how post-prandial increase of insulin may guarantee GLUT4 expression, and how the insulin signaling impairment can participate in insulin resistance-induced repression of GLUT4.

Sulfonylurea induction of caffeine-enhanced insulin secretion and reduction of glycemic levels in diabetic rats

CONTEXT

Caffeine can stimulate insulin secretion by attenuating hyperglycemia in diabetes models with significant reduction of pancreatic functional β cells. Knowledge of these mechanisms could contribute to new strategies for treating diabetes.

OBJECTIVE

This study evaluated the effects of caffeine and physical exercise on glycemic and insulin responses in diabetic rats.

MATERIALS AND METHODS

The diabetes model was induced by intraperitoneal administration of 60 mg/kg of streptozotocin (STZ). Animals were divided into six groups: control, caffeine, STZ control, STZ caffeine, STZ sulfonylurea, and STZ caffeine + sulfonylurea. Acutely, control animals received 6 mg of caffeine and 10 mg/kg sulfonylurea or 10 mg/kg saline. Animals were sacrificed after physical exercise; blood samples were collected for glucose, glycerol, lactate, and insulin analyses. Cardiovascular responses were recorded before and after treatments. A one-way ANOVA and the post hoc Student-Newman-Keuls test were used to analyze statistical differences between treatments (p < 0.05).

RESULTS

About 6 mg/kg of caffeine did not alter cardiovascular responses, but promoted blood glucose reduction after 60 min of exercise when compared to animals in the control groups (387-187 mg/dL; p < 0.05). Insulin levels increased significantly (0.6-10 µU/mL; p < 0.05) in rats that received acute caffeine treatment associated with sulfonylurea compared to rats in the other groups.

DISCUSSION AND CONCLUSION

Acute caffeine intake with exercise can increase glucose uptake enhancing insulin secretion stimulated by sulfonylurea in β cells-deficient pancreas. The results indicate the potential use of caffeine as a strategy for glycemic and insulin control in diabetes.

Recovery of insulin sensitivity and Slc2a4 mRNA expression depend on T3 hormone during refeeding

OBJECTIVE

GLUT4 protein, encoded by the Slc2a4 gene, plays a key role in muscle glucose uptake, and its expression decreases in muscles under insulin resistance. Slc2a4/GLUT4 decreases with fasting and rapidly increases with refeeding and the same occurs to plasma glucose, amino acids, insulin and T3. Thus, they might be potential regulators of the Slc2a4 gene, which makes them promising targets for strategies to improve GLUT4 expression. Herein, we investigate the role of metabolic-hormonal parameters triggered by refeeding upon the Slc2a4 expression.

MATERIALS/METHODS

Plasma glucose/insulin/T3, and gastrocnemius Slc2a4 mRNA contents were measured in rats studied at the end of 48-h fasting, and subsequently at: i) 2-4h after spontaneous refeeding; ii) 2-4h after T3 injection, without refeeding; and iii) 0.5-2h after intravenous infusion of insulin, insulin+glucose and insulin+amino acids, without refeeding.

RESULTS

Refeeding increased plasma glucose/insulin/T3 and muscle Slc2a4 mRNA, reverting insulin resistance. Post-fasting infusions surprisingly induced a further Slc2a4 mRNA decrease (~20%, P<0.05 vs. fasting), but T3 injection induced a ~2-fold increase in Slc2a4 mRNA, 2-4h later (P<0.001). Moreover, T3 increased glycemia and insulinemia to the 2h-refed rats levels, suggesting that T3 elevation is a key factor to the mechanisms of metabolic balance during refeeding.

CONCLUSIONS

Refeeding induces a rapid increase in muscle Slc2a4 expression, not associated with increased plasma glucose, insulin or amino acids, but highly correlated to increased plasma T3 concentration. This result points out T3 hormone as a powerful Slc2a4 enhancer, an effect that may be acutely explored in situations of insulin resistance.

Decreased diabetes-induced glycemic impairment in WKY and SHR involves enhanced skeletal muscle Slc2a4/GLUT4 expression

BACKGROUND

Hypertension has been associated to diabetes, and participates in the development of diabetic complications. The spontaneously hypertensive rat (SHR) is the gold standard model for the study of hypertension, and experimental diabetes has been currently investigated in SHR. Wistar-Kyoto rat is usually taken as control for SHR, however, regarding the glycemic homeostasis, WKY may be similar to SHR, when compared to the standard Wistar rat, importantly affecting the interpretation of data. Slc2a4 gene, which encodes the GLUT4 protein, is expressed in insulin-sensitive tissues, such as muscle cells and adipocytes, and alteration in Slc2a4/GLUT4 expression is inversely related to glycemic levels. We investigated the effect of diabetes on the expression of Slc2a4/GLUT4 and glycemic control in Wistar-Kyoto and SHR.

FINDINGS

Slc2a4 mRNA (Northern-blotting) and GLUT4 protein (Western-blotting) were investigated in skeletal muscles (soleus and extensor digitorum longus) of Wistar, Wistar-Kyoto and SHR, rendered or not diabetic for 1 month. Non-diabetic SHR shows hyperinsulinemia, and unaltered GLUT4 expression. The hyperglycemia was significantly attenuated in diabetic Wistar-Kyoto and SHR, compared to that observed in diabetic Wistar, although all of them presented the same hypoinsulinemic levels. Besides, diabetes significantly reduced Slc2a4/GLUT4 in Wistar, as expected; however, that was not observed in diabetic Wistar-Kyoto and SHR.

CONCLUSIONS

Non-diabetic SHR is insulin resistant, despite unaltered GLUT4 expression. Diabetic Wistar-Kyoto and diabetic SHR presented high Slc2a4/GLUT4 expression in skeletal muscle, as compared to diabetic Wistar. This Slc2a4/GLUT4 regulation does not depend on insulin level and possibly protects the WKY and SHR from severe glycemic impairment.

Carbohydrate- and lipid-enriched meals acutely disrupt glycemic homeostasis by inducing transient insulin resistance in rats

Chronic intake of high-carbohydrate or high-lipid diets is a well-known insulin resistance inducer. This study investigates the immediate effect (1-6 h) of a carbohydrate- or lipid-enriched meal on insulin sensitivity. Fasted rats were refed with standard, carbohydrate-enriched (C), or lipid-enriched (L) meal. Plasma insulin, glucose, and non-esterified fatty acids (NEFA) were measured at 1, 2, 4, and 6 h of refeeding. The glucose-insulin index showed that either carbohydrates or lipids decreased insulin sensitivity at 2 h of refeeding. At this time point, insulin tolerance tests (ITTs) and glucose tolerance tests (GTTs) detected insulin resistance in C rats, while GTT confirmed it in L rats. Reduced glycogen and phosphorylated AKT and GSK3 content revealed hepatic insulin resistance in C rats. Reduced glucose uptake in skeletal muscle subjected to the fatty acid concentration that mimics the high NEFA level of L rats suggests insulin resistance in these animals is mainly in muscle. In conclusion, carbohydrate- or lipid-enriched meals acutely disrupt glycemic homeostasis, inducing a transient insulin resistance, which seems to involve liver and skeletal muscle, respectively. Thus, the insulin resistance observed when those types of diets are chronically consumed may be an evolution of repeated episodes of this transient insulin resistance.