Insulin Resistance With Enhanced Insulin Signaling in High-Salt Diet–Fed Rats

Abstract

Previous clinical studies showed an apparent correlation between hypertension and insulin resistance, and patients with diabetes are known to have increased blood pressure responsiveness to salt loading. To investigate the effect of high salt intake on insulin sensitivity and the insulin signaling pathway, a high-salt diet (8% NaCl) or a normal diet was given to 7-week-old SD rats for 2 weeks. High salt-fed rats developed slightly but significantly higher systolic blood pressure than controls (133 ؎ 2 vs. 117 ؎ 2 mmHg, P < 0.001), with no change in food intake or body weight. High salt-fed rats were slightly hyperglycemic (108.5 ؎ 2.8 vs. 97.8 ؎ 2.5 mg/dl, P ‫؍‬ 0.01) and slightly hyperinsulinemic (0.86 ؎ 0.07 vs. 0.61 ؎ 0.06 ng/ml, P ‫؍‬ 0.026) in the fasting condition, as compared with controls. Hyperinsulinemic-euglycemic clamp study revealed a 52.7% decrease in the glucose infusion rate and a 196% increase in hepatic glucose production in high salt-fed rats, which also showed a 66.4% decrease in 2-deoxyglucose uptake into isolated skeletal muscle and a 44.5% decrease in insulin-induced glycogen synthase activation in liver, as compared with controls. Interestingly, despite the presence of insulin resistance, high salt-fed rats showed enhanced insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1, IRS-2 (liver and muscle), and IRS-3 (liver only). Phosphatidylinositol (PI) 3-kinase activities associated with IRS and phosphotyrosine in the insulin-stimulated condition increased 2.1-to 4.1-fold, as compared with controls. Insulin-induced phosphorylation of Ser-473 of Akt and Ser-21 of glycogen synthase kinase-3 also increased 2.9-and 2-fold, respectively, in the liver of the high salt-fed rats. Therefore, in both the liver and muscle of high salt-fed rats, intracellular insulin signaling leading to PI 3-kinase activation is enhanced and insulin action is attenuated. The hyperinsulinemic-euglycemic clamp study showed that decreased insulin sensitivity induced with a high-salt diet was not reversed by administration of pioglitazone. The following can be concluded: 1) a high-salt diet may be a factor promoting insulin resistance, 2) the insulin-signaling step impaired by high salt intake is likely to be downstream from PI 3-kinase or Akt activation, and 3) this unique insulin resistance mechanism may contribute to the development of diabetes in patients with hypertension. Diabetes 50:573-583, 2001 R ecently, much interest has focused on the association between diabetes and hypertension. A previous pioneering study showed an association of hypertension with insulin resistance (1). A recent large clinical study, the U.K. Prospective Diabetes Study, showed that Ͼ30% of patients recently diagnosed with type 2 diabetes had hypertension, underscoring the importance of treating hypertension in patients with diabetes (2). Although the molecular mechanism underlying the coexistence of hypertension and diabetes remains unclear, multiple lines of evidence indicate that high blood pressure is among the...

Serum concentrations of resistin-like molecules β and γ are elevated in high-fat-fed and obese db/db mice, with increased production in the intestinal tract and bone marrow

“…Preparation of the antibodies An antibody against the whole mouse resistin molecule was prepared by immunising rabbits with the recombinant mouse resistin protein, obtained as described previously [21]. Sequences corresponding to nucleotides 11-173 of RELMβ/FIZZ2 and 47-227 of RELMγ/FIZZ4 were amplified by PCR and cloned into the pGEX-3T expression vector (Pharmacia, Piscataway, NJ, USA).…”

“…Blood samples were centrifuged at 3,000 rev/min for 20 min and sera containing equal amounts of protein were used for immunoprecipitation. Immunoblotting against these immunoprecipitates was performed as previously described [21]. Animal care and procedures of the experiments were approved by the Animal Care Committee of the University of Tokyo.…”

Adipose-specific Expression, Phosphorylation of Ser794 in Insulin Receptor Substrate-1, and Activation in Diabetic Animals of Salt-inducible Kinase-2

“…The results shown are the representative data from triplicate experiments. 47). Recently, Qiao et al (35) reported the presence in the hepatic cytosols of a genetically diabetic rat of a kinase responsible for the Ser 789 phosphorylation of IRS-1, which was distinct from AMPK.…”

Oxidative stress induces insulin resistance by activating the nuclear factor-?B pathway and disrupting normal subcellular distribution of phosphatidylinositol 3-kinase

“…The rats were then killed by intracardiac injection of pentobarbital. Isolated soleus muscle was incubated for 20 min with or without 1.44×10 −8 mol/l human insulin (this concentration is equivalent to 2 mU/ml), as described previously [17]. 2-Deoxy glucose uptake into the isolated soleus muscle strips was measured using 2-deoxy-D-[ 3 H]glucose and [ 14 C]manitol as described previously [21].…”

“…Rats fasted overnight were anaesthetised by intraperitoneal injection of pentobarbital sodium (60 mg/kg body weight) and the left jugular and femoral veins were catheterised for blood sampling and infusion respectively. Hyperinsulinaemic-euglycaemic clamp analysis was performed as described previously [17]. The glucose utilisation rate, hepatic glucose production and an estimate of muscle glucose uptake during the clamp (defined as the glucose metabolic index) were calculated as previously described [20].…”