Hyperinsulinemic-euglycemic Clamp Analysis Research Articles

Phloridzin Improves Hyperglycemia But Not Hepatic Insulin Resistance in a Transgenic Mouse Model of Type 2 Diabetes

The chronic hyperglycemia that occurs in type 2 diabetes may cause deterioration of beta-cell function and insulin resistance in peripheral tissues. Mice that express a dominant-negative IGF-1 receptor, specifically in skeletal muscle (MKR mice), exhibit severe insulin resistance, hyperinsulinemia, dyslipidemia, and hyper-glycemia. To determine the role of hyperglycemia in the worsening of the diabetes state in these animals, MKR mice were treated with phloridzin (PHZ), which inhibits intestinal glucose uptake and renal glucose reabsorption. Blood glucose levels were decreased and urine glucose levels were increased in response to PHZ treatment in MKR mice. PHZ treatment also increased food intake in MKR mice; however, the fat mass was decreased and lean body mass did not change. Serum insulin, fatty acid, and triglyceride levels were not affected by PHZ treatment in MKR mice. Hyperinsulinemic-euglycemic clamp analysis demonstrated that glucose uptake in white adipose tissue was significantly increased in response to PHZ treatment. Despite the reduction in blood glucose following PHZ treatment, there was no improvement in insulin-stimulated whole-body glucose uptake in MKR mice and neither was there suppression of endogenous glucose production by insulin. These results suggest that glucotoxicity plays little or no role in the worsening of insulin resistance that occurs in the MKR mouse model of type 2 diabetes.

Syntaxin 4 transgenic mice exhibit enhanced insulin-mediated glucose uptake in skeletal muscle.

Insulin-stimulated translocation of GLUT4 vesicles from an intracellular compartment to the plasma membrane in 3T3L1 adipocytes is mediated through a syntaxin 4 (Syn4)- and Munc18c-dependent mechanism. To investigate the impact of increasing Syn4 protein abundance on glucose homeostasis in vivo, we engineered tetracycline-repressible transgenic mice to overexpress Syn4 by fivefold in skeletal muscle and pancreas and threefold in adipose tissue. Increases in Syn4 caused increases in Munc18c protein, indicating that Syn4 regulates Munc18c expression in vivo. An important finding was that female Syn4 transgenic mice exhibited an increased rate of glucose clearance during glucose tolerance tests that was repressible by the administration of tetracycline. Insulin-stimulated glucose uptake in skeletal muscle was increased by twofold in Syn4 transgenic mice compared with wild-type mice as assessed by hyperinsulinemic-euglycemic clamp analysis, consistent with a twofold increase in insulin-stimulated GLUT4 translocation in skeletal muscle. Hepatic insulin action was unaffected. Moreover, insulin content and glucose-stimulated insulin secretion by islets isolated from Syn4 transgenic mice did not differ from that of wild-type mice. In sum, these data suggest that increasing the number of Syn4-Munc18c "fusion sites" at the plasma membrane of skeletal muscle increases the amount of GLUT4 available to increase the overall rate of insulin-mediated glucose uptake in vivo.

Peroxisome proliferator-activated receptor-alpha agonist treatment in a transgenic model of type 2 diabetes reverses the lipotoxic state and improves glucose homeostasis.

Abnormalities in insulin action are the characteristics of type 2 diabetes. Dominant-negative muscle-specific IGF-I receptor (MKR) mice exhibit elevated lipid levels at an early age and eventually develop type 2 diabetes. To evaluate the role of elevated lipids in the progression of the diabetic state, MKR mice were treated with WY14,643, a peroxisome proliferator-activated receptor (PPAR)-alpha agonist. WY14,643 treatment markedly reduced serum fatty acid and triglyceride levels within a few days, as well as muscle triglyceride levels, and subsequently normalized glucose and insulin levels in MKR mice. Hyperinsulinemic-euglycemic clamp analysis showed that WY14,643 treatment enhanced muscle and adipose tissue glucose uptake by improving whole-body insulin sensitivity. Insulin suppression of endogenous glucose production by the liver of MKR mice was also improved. The expression of genes involved in fatty acid oxidation was increased in liver and skeletal muscle, whereas gene expression levels of hepatic gluconeogenic enzymes were decreased in WY14,643-treated MKR mice. WY14,643 treatment also improved the pattern of glucose-stimulated insulin secretion from the perfused pancreata of MKR mice and reduced the beta-cell mass. Taken together, these findings suggest that the reduction in circulating or intracellular lipids by activation of PPAR-alpha improved insulin sensitivity and the diabetic condition of MKR mice.

Increased β-Oxidation but No Insulin Resistance or Glucose Intolerance in Mice Lacking Adiponectin

Previous reports showed that recombinant fragments of adiponectin (adipo) displayed pharmacological effects when injected into rodents, but the relevance of these observations to the physiological function of adipo is unclear. We generated Adipo(-/-) mice by gene targeting. Adipo(-/-) mice are fertile with normal body and fat pad weights. Plasma glucose and insulin levels of Adipo(-/-) and Adipo(+/+) mice are similar under fasting conditions and during an intraperitoneal glucose tolerance test (GTT). Insulin tolerance test (ITT) also produces similar plasma glucose and insulin levels in the two groups of mice. Hyperinsulinemic-euglycemic clamp analysis showed that Adipo(-/-) and Adipo(+/+) mice have similar glucose infusion rates to maintain a similar serum glucose. High-fat diet feeding for 7 months led to similar weight gain and similar GTT and ITT responses. We next measured beta-oxidation and found it to be significantly increased in muscle and liver of Adipo(-/-) mice. In conclusion, our study indicates that absence of adipo causes increased beta-oxidation but does not cause glucose intolerance or insulin resistance in mice.