Diabetes
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To better define the modifications of liver gluconeogenesis and citric acid cycle, or Krebs' cycle, activity induced by insulin deficiency and the effects of metformin on these abnormalities, we infused livers isolated from postabsorptive or starved normal and streptozotocin-induced diabetic rats with pyruvate and lactate (labeled with [3-13C]lactate) with or without the simultaneous infusion of metformin. Lactate and pyruvate uptake and glucose production were calculated. The 13C-labeling pattern of liver glutamate was used to calculate, according to Magnusson's model, the relative fluxes through Krebs' cycle and gluconeogenesis. ⋯ Metformin decreased glucose output from the liver of starved diabetic rats (P < 0.05). The flux through PC-PEPCK and also pyruvate kinase were decreased (P < 0.05) by metformin in both groups of diabetic rats. In conclusion, insulin deficiency increased in this model of diabetes gluconeogenesis through enhanced uptake of substrate and increased flux through PC-PEPCK; metformin decreased glucose production by reducing the flux through PC-PEPCK.
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UCP2 and UCP3 are two recently cloned genes with high sequence homology to the gene for uncoupling protein (UCP)-1, which regulates thermogenesis in brown adipose tissue. In the context of the current debate about whether UCP2 and UCP3 in the skeletal muscle may also function as mediators of thermogenesis or as regulators of lipids as fuel substrate, we have examined their mRNA expressions in rat gastrocnemius muscle in response to dietary manipulations known to differentially affect thermogenesis during the phase of weight recovery after starvation. Compared with ad libitum-fed control rats, the refeeding of isocaloric amounts of a low-fat (high-carbohydrate) diet resulted in lower energy expenditure and lower mRNA levels of muscle UCP2 and UCP3. ⋯ Regression analysis of gastrocnemius UCP mRNA levels against parameters that included body composition, energy expenditure, and plasma levels of free fatty acids (FFAs), insulin, and glucose as well as the increase in plasma glucose after a glucose load, revealed that only the latter (an index of insulin resistance) could explain the variability in muscle UCP2 and UCP3 mRNA expressions (r = 0.41, P < 0.02; r = 0.45, P < 0.01, respectively). Taken together, these data are at variance with a role for skeletal muscle UCP2 and UCP3 in dietary regulation (or modulation) of thermogenesis. However, they are consistent with the notion that these UCP homologs may function as regulators of lipids as fuel substrate and raise the possibility that high-fat induced upregulation of muscle UCP2 and UCP3 may be more closely linked to insulin resistance than to changes in circulating FFAs.
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We directly examined whether visceral fat (VF) modulates hepatic insulin action by randomizing moderately obese (body wt approximately 400 g) Sprague-Dawley rats to either surgical removal of epididymal and perinephric fat pads (VF-; n = 9) or a sham operation (VF+; n = 11). Three weeks later, total VF was fourfold increased (8.5 +/- 1.2 vs. 2.1 +/- 0.3 g, P < 0.001) in the VF+ compared with the VF- group, but whole-body fat mass (determined using 3H2O) was not significantly different. The rates of insulin infusion required to maintain plasma glucose levels and basal hepatic glucose production in the presence of hepatic-pancreatic clamp were markedly decreased in VF- compared with VF+ rats (0.57 +/- 0.02 vs. 1.22 +/- 0.19 mU x kg(-1) x min(-1), P < 0.001). ⋯ The improvement in hepatic insulin sensitivity in VF- rats was also supported by a approximately 70% decrease in the plasma levels of insulin-like growth factor binding protein-1, a marker of insulin's transcription regulation in the liver. The removal of VF pads also resulted in marked decreases in the gene expression of tumor necrosis factor-alpha (by 72%) and leptin (by 60%) in subcutaneous fat. We conclude that visceral fat is a potent modulator of insulin action on hepatic glucose production and gene expression.
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Uncoupling proteins 3 and 2 (UCP3 and UCP2) are two newly cloned genes that have been implicated in the regulation of lipids as fuel substrate in skeletal muscle on the basis that their mRNA expressions are upregulated during starvation (when fat stores are being rapidly mobilized) and downregulated during the early phase of refeeding (when fat stores are being rapidly replenished). To test the hypothesis that circulating free fatty acids (FFAs) may have a physiological role as an interorgan signal linking these dynamic changes in the fat stores to skeletal muscle expression of UCP3 and UCP2, the mRNA levels of these UCP homologs were examined in fed and fasted rats treated with the antilipolytic agent nicotinic acid. In 46-h fasted rats, we observed a threefold increase in serum FFA levels and increases in UCP3 and UCP2 mRNA levels that were more marked in the gastrocnemius and tibialis anterior muscles (predominantly fast-twitch fibers) than in the soleus muscle (predominantly slow-twitch fibers). ⋯ Furthermore, treatment of ad libitum-fed animals with nicotinic acid resulted in a twofold reduction in serum FFA levels (i.e., by a magnitude similar to that observed during early refeeding) and significant reductions in UCP3 and UCP2 mRNA levels in the soleus muscle, but not in the gastrocnemius or tibialis anterior muscles. These results revealed a muscle-type dependency in the way UCP2 and UCP3 gene expression in skeletal muscle is regulated, and suggest that the hypothesis that circulating FFAs function as an interorgan signal between fat stores and skeletal muscle UCP3 and UCP2 gene expression is adequate only for slow-twitch (oxidative) muscles. Consequently, a signal(s) other than circulating FFAs must be implicated in the link between dynamic changes in body fat stores and UCP expression in predominantly fast-twitch (glycolytic/oxidative-glycolytic) muscles, which constitute the major fiber type of the total skeletal muscle mass and which have high susceptibility to developing insulin resistance and impairment in substrate utilization in metabolic diseases.
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In the fasted rat, efficient glucose-stimulated insulin secretion (GSIS) is absolutely dependent on an elevated level of circulating free fatty acids (FFAs). To determine if this is also true in humans, nonobese volunteers were fasted for 24 h (n = 5) or 48 h (n = 5), after which they received an infusion of either saline or nicotinic acid (NA) to deplete their plasma FFA pool, followed by an intravenous bolus of glucose. NA treatment resulted in a fall in basal insulin concentrations of 35 and 45% and in the area under the insulin response curve (area under the curve [AUC]) to glucose of 47 and 42% in the 24- and 48-h fasted individuals, respectively. ⋯ The insulin AUC in response to glucose was unaffected by lowering of the FFA level in nonobese subjects, but fell by 29% in the obese group. The data clearly demonstrate that in humans, the rise in circulating FFA levels after 24 and 48 h of food deprivation is critically important for pancreatic beta-cell function both basally and during subsequent glucose loading. They also suggest that the enhancement of GSIS by FFAs in obese individuals is more prominent than that seen in their nonobese counterparts.